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Dynamics of in-stream wood and its importance as fish habitat in a large tropical floodplain river
Abstract
The recruitment of wood from the riparian zone to rivers and streams provides a complex habitat for aquatic organisms and can influence both aquatic biodiversity and ecosystem function. The Daly River in the wet–dry tropics of northern Australia is a highly seasonal, perennially flowing sand-bed river where surveys of river wood aggregations at the reach scale (~2 km) in 2008 and 2009 recorded densities of 37–78 km À1 and identified distinct types of river wood aggregations: key pieces, standing trees, fallen trees, wrack and single pieces. After larger than average flows in the 2008/2009 wet season, between 46% and 51% of the surveyed river wood had moved. The distribution of wood age classes indicated con-tinual recruitment and slow turnover of wood within the river. Surveys of fish and habitat characteristics at the mesohabitat scale (~100 m) showed fish species richness; diversity and fish abundance were not correlated to the proportion of wood present. Fish assemblage structure was associated with wood cover as well as other environmental variables such as stream width and depth. The importance of in-stream wood also varied for dif-ferent species and age classes of fish. This study documents the dynamic nature of river wood aggregations and their complex and variable dis-tribution and suggests their importance as fish habitat in this tropical river.
... In this study, 15 taxa were associated with moderately deep and low velocity habitats or pools. Pool habitats may have higher species diversity relative to other habitat types, because: (i) they are likely to be more effectively buffered against changes in habitat structure, water level and water quality 58,62 , (ii) they contain higher structural diversity (submerged wood, undercut banks, root masses, etc.) for feeding and shelter 63 , and (iii) they usually cover a greater surface area compared to other mesohabitats, such as riffles and runs 9 . ...
... A number of previous studies have shown that riverine fish assemblages are influenced by the physical structural complexity of river channels, including the presence of large submerged woody debris and instream vegetation 66 . Pettit et al. 63 examined the distribution and movement of large woody debris and its importance as a fish habitat in the Daly River. Using a sub-set of the same fish data analysed in the current study, they demonstrated that fish species richness, diversity and abundance were not correlated with the proportion of wood present at a reach scale. ...
... Using a sub-set of the same fish data analysed in the current study, they demonstrated that fish species richness, diversity and abundance were not correlated with the proportion of wood present at a reach scale. However, juveniles and adults of L. calcarifer and adults of H. fuliginosus were strongly associated with wood cover 63 . Large submerged wood is considered to be an important contributor to habitat heterogeneity, through providing overhead cover which reduces predation risk, and also camouflage for predators to ambush their prey, among other functions 66 . ...
The modification of river flow regimes poses a significant threat to the world’s freshwater ecosystems. Northern Australia’s freshwater resources, particularly dry season river flows, are being increasingly modified to support human development, potentially threatening aquatic ecosystems and biodiversity, including fish. More information is urgently needed on the ecology of fishes in this region, including their habitat requirements, to support water policy and management to ensure future sustainable development. This study used electrofishing and habitat survey methods to quantify the dry season habitat use of 20 common freshwater fish taxa in the Daly River in Australia’s wet-dry tropics. Of twenty measured habitat variables, water depth and velocity were the two most important factors discriminating fish habitat use for the majority of taxa. Four distinct fish habitat guilds were identified, largely classified according to depth, velocity and structural complexity. Ontogenetic shifts in habitat use were also observed in three species. This study highlights the need to maintain dry season river flows that support a diversity of riverine mesohabitats for freshwater fishes. In particular, shallow fast-flowing areas provided critical nursery and refuge habitats for some species, but are vulnerable to water level reductions due to water extraction. By highlighting the importance of a diversity of habitats for fishes, this study assists water managers in future decision making on the ecological risks of water extractions from tropical rivers, and especially the need to maintain dry season low flows to protect the habitats of native fish.
... Native forest plays a vital role in physical channel heterogeneity by supplying large wood, branches, litter and roots to aquatic systems (Allan, 2004;Jackson et al., 2015). These structures are responsible for creating mesohabitats and microhabitats along stream channels, and for increasing marginal and instream habitat complexity (Harvey, Henshaw, Parker, & Sayer, 2018), which can provide habitat for the establishment of additional and different fish species (Angermeier & Karr, 1984;Pettit et al., 2013;Wright & Flecker, 2004). For instance, a coarse root inside of a channel can create backwater microhabitats, favouring the deposition of drifted material such as leaves, seeds and aquatic or terrestrial insects. ...
... For instance, while streams in the Machado River basin have been experiencing environmental filters associated with deforestation for~40 years (Alves et al., 1998), in the Alto Paraná, native forest removal took place more than 150 years ago (Victor et al., 2005). Indeed, in the Machado basin, wood structures used by several fish species (Pettit et al., 2013;Wright & Flecker, 2004) were still common even in highly deforested catchments. In the Alto Paraná streams, on the other hand, it is likely that functionally distinct fauna had already been lost following the loss of most of the allochthonous structures. ...
• Deforestation can modify stream habitat and the functional structure of fish assemblages. The aims of this study were (a) to identify whether deforestation has a similar effect on local habitats from two biogeographically distinct river basins in a tropical region; (b) to identify how fish trait–habitat relationships were influenced by deforestation; and (c) to compare functional redundancy patterns in these basins.
• Environmental and biological variables were obtained for 160 stream reaches, 85 located in the Alto Paraná River basin and 75 in the Machado River basin. Traits were associated with body size, habitat preference, food items and foraging period. Linear models were constructed to verify the environmental convergence of habitats across streams from the Alto Paraná and the Machado basins. An RLQ and a fourth‐corner analysis were conducted to investigate how deforestation affected habitat variables and trait–habitat relationships. The nearest relative index (NRI) was used as a functional redundancy measure.
• Deforestation led to a similar habitat gradient from streams with a higher proportion of coarse roots in more forested catchments to streams with streamside grasses, bare soil and unconsolidated substrate in more deforested catchments; however, a general pattern of fish trait–habitat relationships was not identified across basins. Functional redundancy was associated with a long history of habitat loss in the Alto Paraná streams, whereas functional complementarity was related to more recent and less intense habitat loss in Machado streams.
• We believe that differences in regional species pools and historical processes between the basins influenced the fish functional responses to deforestation. Nevertheless, the results highlighted the importance of stream habitat heterogeneity and the presence of preserved forest fragments in a region to prevent the loss of unique traits. Decision‐makers should therefore maintain large forest fragments and restore riparian forests to preserve stream habitat and the functional structure of fish assemblages.
... Over the last century almost all dead large wood in and along Western European rivers has been removed in order to improve water discharge and reduce risk of collision with ships and weirs. Recently, managers have tried to restore these original complex large wood habitats by installing large trees under water in the littoral zones (Nagayama et al. 2008;Pettit et al. 2013;Dossi et al. 2015;Pander and Geist 2016). In contrast to the large surface of stone habitats on the river bottom for which gobies have a strong preference, the complex structure of large wood habitats fills the whole water column with tree branches and roots, with only a small part of the structure resting on the bottom. ...
... Since the water velocity in the River Lek is very low because of the presence of weirs, these native fish species can easily reach the structure provided by large wood. It is also likely that the large wood habitat provides a large amount of food for zoobenthivorous and zooplanktivorous fish species, resulting in an attractive feeding habitat (Dollof andWarren 2003;Nagayama et al. 2008;Pettit et al. 2013;Dossi et al. 2015;Pander and Geist 2016). Since fish density is relatively high around the large wood structures, piscivorous fish species also use large wood as a feeding and/or shelter habitat type, as illustrated by the higher density of pike-perch at the large wood compared to reference habitats (Table 1). ...
Regulated rivers in Western Europe have rapidly been colonized by invasive alien Ponto-Caspian gobies. In particular, the round goby (Neogobius melanostomus) can reach high densities in habitats with hard substratum, such as groynes and dams made of basalt stones. High densities of Ponto-Caspian gobies negatively impact native benthic fishes. It is hypothesized that natural complex three-dimensional structures in Western European rivers, such as (pieces of) large wood (e.g., trees that fell into the river), are a less attractive habitat for Ponto-Caspian gobies. These bottom-dwelling fishes are strongly associated with sheltered places on the river bottom and may avoid the three-dimensional structure of large wood in the water column. The colonization of littoral zones provided with large wood (i.e., entire trees) by round goby and native fishes was studied in the River Lek (a distributary of the River Rhine) in the Netherlands during the period 2014–2016. The fish assemblage of four reference sites dominated by basalt stones was compared with that of four large wood sites. Counts of round goby in large wood habitats were significantly lower than in habitats dominated by basalt stones, while native fishes were more abundant in large wood habitats. In large wood habitats counts of native fishes were significantly higher than those of round goby, whereas the reverse was true in the reference habitat. Counts of the entire fish assemblage did not significantly differ between habitat types. These results suggest that large wood in regulated Western European rivers predominantly functions as a suitable habitat for native fishes whereas the invasive bottom-dwelling round goby only uses large wood habitats to a limited extent. Large wood may be applicable as a management tool to stimulate native fish fauna with minimal facilitation of the round goby.
... New data collected by the project team during field sampling involved quantitative sampling of fish communities at multiple sites throughout the catchment during the early and late dry season from 2006 (see Stewart-Koster et al. 2011 andChan et al. 2012 for a detailed description of sampling methods). This yielded information on the distribution, abundance, habitat use, diet, and influence of flow on fish communities (reported in Davis et al. 2010, 2012a, b, c, Cook et al. 2011, Chan et al. 2012, Linke et al. 2012, Pettit et al. 2013). ...
... In addition to enhancing the scientific understanding of fish biology and flow-ecology relationships (Stewart-Koster et al. 2011, Chan et al. 2012, Pettit et al. 2013, the research partnership generated a number of beneficial outcomes for indigenous people. First, field trips provided opportunities for people to exchange "stories about fish," including their cultural significance. ...
Studies that apply indigenous ecological knowledge to contemporary resource management problems are increasing globally; however, few of these studies have contributed to environmental water management. We interviewed three indigenous landowning groups in a tropical Australian catchment subject to increasing water resource development pressure and trialed tools to integrate indigenous and scientific knowledge of the biology and ecology of freshwater fish to assess their water requirements. The differences, similarities, and complementarities between the knowledge of fish held by indigenous people and scientists are discussed in the context of the changing socioeconomic circumstances experienced by indigenous communities of north Australia. In addition to eliciting indigenous knowledge that confirmed field fish survey results, the approach generated knowledge that was new to both science and indigenous participants, respectively. Indigenous knowledge influenced (1) the conceptual models developed by scientists to understand the flow ecology and (2) the structure of risk assessment tools designed to understand the vulnerability of particular fish to low-flow scenarios.
... New data collected by the project team during field sampling involved quantitative sampling of fish communities at multiple sites throughout the catchment during the early and late dry season from 2006 (see Stewart-Koster et al. 2011 andChan et al. 2012 for a detailed description of sampling methods). This yielded information on the distribution, abundance, habitat use, diet, and influence of flow on fish communities (reported in Davis et al. 2010, 2012a, b, c, Cook et al. 2011, Chan et al. 2012, Linke et al. 2012, Pettit et al. 2013). ...
... In addition to enhancing the scientific understanding of fish biology and flow-ecology relationships (Stewart-Koster et al. 2011, Chan et al. 2012, Pettit et al. 2013, the research partnership generated a number of beneficial outcomes for indigenous people. First, field trips provided opportunities for people to exchange "stories about fish," including their cultural significance. ...
Studies that apply indigenous ecological knowledge to contemporary resource management problems are increasing globally; however, few of these studies have contributed to environmental water management. We interviewed three indigenous landowning groups in a tropical Australian catchment subject to increasing water resource development pressure and trialed tools to integrate indigenous and scientific knowledge of the biology and ecology of freshwater fish to assess their water requirements. The differences, similarities, and complementarities between the knowledge of fish held by indigenous people and scientists are discussed in the context of the changing socioeconomic circumstances experienced by indigenous communities of north Australia. In addition to eliciting indigenous knowledge that confirmed field fish survey results, the approach generated knowledge that was new to both science and indigenous participants, respectively. Indigenous knowledge influenced (1) the conceptual models developed by scientists to understand the flow ecology and (2) the structure of risk assessment tools designed to understand the vulnerability of particular fish to low-flow scenarios.
... Wood is acknowledged to play an important role in the morphology and ecology of streams (Bisson et al., 1987;Montgomery, 2003;Wohl, 2013;Wohl et al., 2023). In-stream wood, markedly large wood (LW) pieces (classically defined as pieces longer than 1 m and thicker than 10 cm), can alter the channel morphology and hydraulics (Abbe and Montgomery, 1996;Bilby and Bisson, 1998;Faustini and Jones, 2003;Paula et al., 2011), increase nutrient cycling and sediment retention (Beckman and Wohl, 2014;Guyette et al., 2008;Harmon et al., 1986;Sutfin et al., 2016), and benefit aquatic biota directly and indirectly (Angermeier and Karr, 1984;Bryant, 1983;Bilby and Bisson, 1998;Frainer et al., 2018;Leal et al., 2016;Leitão et al., 2018;Macedo et al., 2016;Pettit et al., 2013;Sterling and Warren, 2018;Wright and Flecker, 2004). ...
The delivery of wood to streams varies in space and time and may occur continuously or episodically. Once in a stream, wood may be retained or transported due to the balance between driving (water flow) and resisting forces (channel features and wood characteristics). Despite there being few studies directly measuring wood mobility, notable variations in annual transport rates and mean travelled distances have already been detected worldwide. In the tropics, due to wetter and warmer conditions and consequently higher decay rates and peak discharge, wood is believed that more readily degraded and transported downstream. However, there exists a knowledge gap regarding wood dynamics in tropical streams and even more considering tropical non-forested environments. In the present study, we aimed to document large wood (LW) recruitment, retention, and mobilization over a period of one year in eight low-order streams of the Cerrado (the South American Savanna) by tagging and tracing LW pieces. We detected high LW transport and recruitment rates in Cerrado streams, such that the total wood amounts remained constant over time. However, there were important differences between streams due to their distinct channel morphologies and the occurrence of episodic events. The LW length and diameter, original location and orientation were the most important factors in the prediction of the LW travelled distance, which ranged from zero to almost 100 m in one year. Our findings provide unprecedented information regarding wood mobility in tropical savanna streams in South America. To further advance knowledge about the dynamics of wood in Cerrado streams, future studies should characterize the great diversity of wood, especially with regard to densities that may strongly affect wood mobility. Furthermore, it is essential to perform long-term studies to document the dynamics of wood over years in these environments.
... 55 GDV can provide a wide range of ecosystem services (Howard et al., 2023) including: (1) habitat for a 56 wide range of plants and animals; e.g. understorey vegetation (Sprecht and Sprecht, 1993), waterbird 57 nesting (Briggs and Thornton, 1995), fish (Pettit et al., 2013), pollinators (Chaplin-Kramer et al., 2011); 58 ...
... Deep pools in non-perennial rivers are considered unique ecosystems that support a rich diversity of fish species as they are buffered against physico-chemical changes and human interference, like fishing and cover more surface area with high structural diversity over other mesohabitats viz. run, riffles, etc. (Beesley & Prince, 2010;Pettit et al., 2013;Bose et al., 2019;Keller et al., 2019;Seaton et al., 2020). Spatial identification of deep pools and understanding their temporal dynamics are necessary to sustain the fisheries of any riverine ecosystem as they provide refuge to the resident species until the time of recolonisation during the subsequent monsoon season (Baird, 2006). ...
Rivers are dynamic ecosystems with diverse habitats that require ample connectivity to ensure the flow of ecosystem services, thus empowering the sustainable development of an entire basin. Geo-spatial tools offer powerful prospects for monitoring of aquatic ecosystems. The usefulness of Sentinel-2 datasets to assess river connectivity has been explored for an un-gauged seasonal river system. The present study was undertaken in five ecologically unique river reaches viz. Wainganga, Wardha, Pranhita, Godavari-mid and Manair in Godavari Basin in the Indian Deccan Plateau to map water spread dynamics at various time scales, i.e., fortnightly, monthly, seasonal, annual and demi-decadal during 2016–2021. The maximum value of perennial water spread per square kilometre of total floodplain area (2016–2021), determined using Sentinel-2 imageries, was observed in river Wardha (0.18) followed by Pranhita (0.12) and Wainganga (0.11). The water spread showed a decreasing trend, while the number of patches in the river corridor increased over time from post-monsoon to pre-monsoon season. The copious perennial habitat with relatively larger patches, incessant flow in river Pranhita and obstructed flow, large-sized patches reported in river Wardha during summer months, hold importance in terms of providing refuge to aquatic biota. This study provides evidence for the impact of water projects on spatio-temporal water spread dynamics in Godavari Basin. The demonstrated utility of Sentinel-2 imagery coupled with gauge station measurements for river continuity assessment and deep pool mapping would aid in enhancing our understanding on environmental flow at a spatial scale, which in turn would aid in effective river management to achieve the Sustainable Development Goals. The implications of this study for sustainable environmental management and limitations are also discussed.
... However, perceived longitudinal, elevational, and total discharge (Q) gradients in rivers function as surrogates for other factors (habitat size and stability, depth, velocity, substrate composition, energy availability, and habitat diversity) that are proximally more influential on patterns of species distribution and abundance (Hitt & Roberts, 2012). Lowland habitats may indeed contain a variety of microhabitats such as macrophyte beds, aggregations of in-stream wood and diverse habitat patches defined by depth, water velocity, and substrate composition and thus contain more species (Pettit et al., 2013). Despite the high richness of species with an estuarine dependence in northern Australian rivers (Pusey et al., 2017), the upstream/downstream gradient in richness detected within the Daly River was not driven by species turnover in this species set alone; rather it was more the result of the nested downstream accumulation of strictly freshwater species. ...
• Environmental water management seeks to balance competing demands between the water needed to sustain human populations and their economic activities and that required to sustain functioning freshwater ecosystems and the species they support. It must be predicated on an understanding of the environmental, hydrological, and biological factors that determine the distribution and abundance of aquatic species.
• The Daly River of the wet–dry tropics of northern Australia consists of a perennially flowing main stem and large tributaries, as well as many small to large naturally intermittent tributaries, and associated off‐channel wetlands. Increased groundwater abstraction to support irrigated agriculture during the dry season threatens to reduce dry‐season flows that maintain perenniality and persistence of freshwater fishes.
• Fish assemblages were surveyed at 55 locations during the dry season over a 2‐year period with the goal of establishing the key landscape‐scale and local‐scale (i.e. habitat) drivers of fish species distribution.
• Longitudinal (upstream/downstream) and lateral (river/floodplain) gradients in assemblage structure were observed with the latter dependent on the position in the river landscape. Underlying these gradients, stream flow intermittency influenced assemblage composition, species richness, and body size distributions. Natural constraints to dispersal were identified and their influence on assemblage structure was also dependent on position within the catchment.
• Eight distinct assemblage types were identified, defined by differences in the abundance of species within five groups differing in functional traits describing body size, spawning requirements, and dispersal capacity. These functional groups largely comprised species widely distributed in northern Australia.
• The results of the study are discussed with reference to the environmental flow needs of the Daly River and other rivers of northern Australia. The findings may also be applied to environmental flow management in savannah rivers elsewhere.
... We found that relationships between barramundi and flow shifted from primarily negative in the early dry to primarily positive in the late dry seasons. Wet season flows can act as significant disturbance events in the Daly River, causing significant changes in channel form (width and depth) and altering the availability of refuges for fishes in the form of submerged wood (Pettit et al., 2013). ...
• Predator–prey interactions are an inherently local‐scale phenomenon, but the intensity of these interactions can be mediated by abiotic conditions that can exert a multi‐scaled influence through space and time. Understanding how multi‐scale abiotic factors may influence local‐scale biotic processes has proven challenging; however, the hierarchical nature of riverine flow regimes makes these environments an ideal setting to test how predator–prey relationships may vary with multi‐scaled flow variation.
• We developed a series of Bayesian hierarchical models to explore how predator–prey relationships between barramundi Lates calcarifer and their prey may be influenced by multi‐scaled flow variables in the Daly River, northern Australia.
• We found that spatio‐temporal variation in barramundi abundance was strongly related to both antecedent flow and the abundance of prey fishes (predictive r² = 0.57), and that barramundi abundance is more likely to be influenced by bottom‐up, rather than top‐down predator–prey dynamics. We also found that the strength and direction of these relationships varied across the catchment and between seasons. We found stronger, positive relationships between barramundi abundance and prey abundance in the most downstream sites with higher mean annual flows, compared to upstream sites.
• These results indicate that the abundance of predatory fishes can be related to both recent abiotic (flow) conditions and the abundance of prey (biotic conditions), and provides strong support for the importance of bottom‐up trophic dynamics. Management of iconic predators such as barramundi should therefore consider both flow management and other key factors such as habitat maintenance to support their prey.
... Reduced tree density would reduce in-stream coarse wood inputs and in-stream habitat diversity. These changes in the riparian zone are significant because algal production is an important source of C in many aquatic foods webs in this region (Douglas et al. 2005, Jardine et al. 2012), and in-stream course wood is an important habitat for fish (Pettit et al. 2013). Despite the significant changes in vegetation in the riparian zone, some effects observed in other regions are unlikely to occur in these savanna streams. ...
... Reduced tree density would reduce in-stream coarse wood inputs and in-stream habitat diversity. These changes in the riparian zone are significant because algal production is an important source of C in many aquatic foods webs in this region (Douglas et al. 2005, Jardine et al. 2012), and in-stream course wood is an important habitat for fish (Pettit et al. 2013). Despite the significant changes in vegetation in the riparian zone, some effects observed in other regions are unlikely to occur in these savanna streams. ...
Intermittent streams drain much of the world's tropical savannas, yet these vitally important stream ecosystems are among the most poorly understood components of tropical savannas. Fire is a widely used management tool in tropical savannas, but the effects of prescribed burning on savanna streams or riparian zones have not been investigated experimentally. This study was undertaken within a catchment-scale, replicated experiment to determine the effect of prescribed burning on savanna riparian zones in northern Australia. Regardless of position along the stream, burning had dramatic effects on the composition and structure of the riparian vegetation. Woody species richness and total abundance, the abundance of small- and medium-sized trees, total basal area, canopy cover, and the richness and cover of vines were reduced by burning, whereas grass cover was much higher in burned areas. Fire also reduced seed production of the dominant riparian eucalypt. This study adds greatly to our understanding of the effects of fire management on savanna riparian zones and demonstrates that they are far more fire sensitive than the surrounding savanna.
... The roughness of a channel bed plays an important role in the evasion of CO 2 and other gases as it has impact on the turbulence of flows (Liss and Slater, 1974). Further, the shape of a channel and its substrate roughness are important ecohydraulic features for determining habitat suitability for various fish and biota (Allan, 1995;Moir and Pasternack, 2010;Pettit et al., 2012) and river management (Neeson et al., 2008). Estimation of bed roughness using measures (such as pebble countssee Kondolf, 1997) can be susceptible to local bias and are subjective to some extent as they depend on scale of observation 1 (e.g., Wolcott and Church, 1991;Crowder and Diplas, 1997;Whitacre et al., 2007). ...
This brief pilot study implements a camera-based laser scanning system that potentially offers a viable, cost-effective alternative to traditional terrestrial laser scanning (TLS) and LiDAR equipment. We adapted a low-cost laser ranging system (SICK LSM111) to acquire area scans of the channel and bed for a temporarily diverted stream. The 5 m × 2 m study area was scanned at a 4 mm point spacing which resulted in a point cloud density of 5,600 points/m2. A local maxima search algorithm was applied to the point cloud and a grain size distribution of the stream bed was extracted. The 84th and 90th percentiles of this distribution, which are commonly used to characterize channel roughness, were 90 mm and 109 mm, respectively. Our example shows the system can resolve both large-scale geometry (e.g., bed slope and channel width) and small-scale roughness elements (e.g., grain sizes between about 30 and 255 mm) in an exposed stream channel thereby providing a resolution adequate for the estimation of ecohydraulic roughness parameters such as Manning's n. While more work is necessary to refine our specific field-deployable system's design, these initial results are promising in particular for those working on a limited or fixed budget. This opens up a realm of laser scanning applications and monitoring strategies for water resources that may not have been possible previously due to cost limitations associated with traditional TLS systems.
... data), and have been successfully used in other studies in northern Australia (e.g. Chan et al. 2012; Pettit et al. 2013; Warfe et al. 2013). All fish collected in each electrofishing pass were immediately placed in a 100-L aerated holding tank, identified and counted before being released. ...
Permanent waterholes in intermittent rivers of northern Australia support a diverse piscifauna and are popular areas for customary and recreational fishing. The present preliminary study explored the perception that fishing reduces the abundance of targeted, large-bodied species that become restricted to disconnected waterholes during the distinct dry season. River sites in the Fitzroy River catchment, Western Australia, could be clearly classified as experiencing either high or low fishing pressure by using metrics of human ‘accessibility’. The abundance of Hephaestus jenkinsi and Lates calcarifer, targeted by both recreational and customary fishers was highly correlated with accessibility and showed a negative relationship with fishing pressure. Non-target species showed no discernible trend. We estimated that 38% of river length remains subject to relatively low fishing pressure. These preliminary relationships suggest that fish harvest can potentially alter the structure of fish assemblages in disconnected habitats. The potential impact of fishing on the sustainability of fish populations is, most likely, greatest for non-diadromous species and will become more apparent with increasing distance from recolonisation sources. Combining management techniques that maintain recolonisation and recruitment potential with traditional fisheries management strategies (e.g. bag and size limits) presents a suitable approach to mitigate the effects of fish harvesting from tropical intermittent rivers.
Extensive riparian vegetation clearance occurred in many rivers of southeastern Australia throughout the 19th and 20th century, post colonisation. With improvements in river management practices, coincident with a period of minimal flooding, vegetation recovery of riparian corridors has been occurring since the 1980s in all coastal catchments of New South Wales (NSW). However, the catchment‐by‐catchment spatial pattern and temporal trends, trajectories and rates of vegetative recovery remain unknown. We reconstruct a 70‐year time series of change in riparian vegetation coverage for all rivers in coastal catchments of NSW, utilising historical aerial imagery, Landsat and Sentinel data and a manual digitisation and machine learning method. We quantify and statistically analyse decadal trends in woody and non‐woody vegetation coverage along 19 485 km of stream length in 20 catchments. Across the region, there has been, on average, a 40% increase in vegetation coverage along riparian corridors since the 1950s, with some catchments increasing towards a current day coverage of between 60% and 80%. However, the trends vary by subregion and by catchment. An early logarithmic increase occurred up to 2004 on the North coast, and a recent and accelerated exponential increase since 1995 has occurred on the South coast. After a steep initial increase, riparian corridor vegetation coverage has fluctuated on the Central coast since about 1995. We used these linear, exponential and logarithmic regressions to undertake preliminary forecasts of possible future timeframes of riparian vegetation change, based on several climate, management and natural disaster disturbance scenarios. If the current trend continues, riparian vegetation coverage can reach 70% across the region by 2055. However, if management practices change or catastrophic disturbance events such as fire and flood occur, increases in riparian vegetation coverage could be set back by up to 30 to 40 years.
Data obtained by direct observation of organisms in their natural habitats can provide valuable contributions regarding intra- and inter-specific interactions, as well as spatial distribution and trophic relationships. However, relatively few such data are available for the Neotropical ichthyofauna relative to the great species diversity of this region. We present data on the ichthyofauna in karst environments of the Serra da Bodoquena, Upper Paraguay River basin regarding their categorization into functional trophic groups. We used direct observations that focused on the most frequent feeding tactics and their spatial (vertical and horizontal) distribution in the channel. We observed 83 fish species in 17 functional trophic groups. The greatest species richness was concentrated in the Characidae and Loricariidae in the categories Diurnal Backwater Drift Feeders and Grazers, respectively. Regarding horizontal depth stratification, we found a positive relationship between the association of small fish with the shallow marginal habitats. This work represents a contribution to a region known for its high tourist potential and to the important diversity of the entire ichthyofauna of the Upper Paraguay basin region that requires proper conservation strategies for sustaining these species.
Instream wood plays important chemical, physical and ecological functions in aquatic systems, benefiting biota directly and indirectly. However, human activities along river corridors have disrupted wood recruitment and retention, usually leading to reductions in the amount of instream
wood. In the tropics, where wood is believed to be more transient, the expansion of agriculture and infrastructure might be reducing instream wood stock even more than in the better studied temperate streams. However, research is needed to augment the small amount of information about wood in different biomes and ecosystems of neotropical streams. Here we present the
first extensive assessment of instream wood loads and size distributions in streams of the wet tropical Amazon and semi-humid-tropical Cerrado (the Brazilian savanna). We also compare neotropical wood stocks with those in temperate streams, first comparing against data from the literature, and then from a comparable dataset from temperate biomes in the USA. Contrary to our expectations, Amazon and Cerrado streams carried similar wood loads, which were lower than the world literature average, but similar to those found in comparable temperate forest and savanna streams in the USA. Our results indicate that the field survey methods and the wood metric adopted are highly important when comparing different datasets. But when properly compared, we found that most of the wood in temperate streams is made-up of a small number of large pieces, whereas wood in neotropical streams is made up of a larger number of small pieces that produce similar total volumes. The character of wood volumes among biomes is linked more to the delivery, transport and decomposition mechanisms than to the total number of pieces. Future studies should further investigate the potential instream wood drivers in neotropical catchments in order to better understand the differences and similarities here detected between biomes and climatic regions.
River wood-debris are a major hazard to lives and infrastructures, because tons of wood material can travel nearing the speed of the flood-flow. If post-event mapping, detection and numerical simulation have made important progress, it is still impossible to detect the wood-debris as they travel, due to poor visibility conditions (rain, night…). The present work aims to solve this issue by adapting Ground Penetrating Radar as an electromagnetic imaging method for in-flow wood de-bris detection. Laboratory test over a water circulation flume using a 800 MHz nominal frequency antenna sampling at 100 Hz a set of single wood logs of 20 cm length has shown that the method had the potential to detect moving wood debris, and that it could “see” underneath to the flume floor. The experiments resulted in the ability to count wood debris travelling underneath the an-tenna, and instantaneous velocity were obtained with velocities ranging from 0.307 to 0.352 m/s, which slightly higher than the averaged velocity measured from video, due to the acceleration time when wood was introduced in the flume.
Karst systems constitute unique environments in the world. Its geomorphology is based on carbonate rocks that act as a natural filter in the hydrology, resulting in highly transparent aquatic environments and extraordinary scenic beauty. In addition to their visual appeal, they directly contribute to water consumption by a large portion of the world's population. Despite these factors, karst environments are threatened by human action, either directly through the modification of the environment or contamination of beds by defensive agents and fertilizers from the agricultural system, or indirectly by global warming. In Brazil, karst environments are mostly represented in the Northeast and Southeast regions. In the Center-West region, close to the border with Paraguay, the only karst system in the state of Mato Grosso do Sul is the Serra da Bodoquena which consists of a plateau inserted in the southern edge of the Upper Paraguay River basin, whose tributaries drain into the plain of the Brazilian Pantanal.
It is a region known worldwide for ecological tourism, especially for activities involving its
crystalline rivers and resident fish fauna. Although the richness of fish species in the region has its consequent appeal for tourism and the maintenance of biodiversity as a whole, basic information about their biology still incipient. Furthermore, the region has undergone changes in terms of its main economic activities, with the replacement of pastures by crops, but the consequent impact of these changes on fish is still unknown. Therefore, the general objective of this Thesis was to understand how species use the environment (Chapter 1), the feeding resources (Chapter 2), and the effect of landscape on community composition (Chapter 3). To achieve these goals, we use the classification of species into functional trophic groups, which allow us to understand the use of the spatial dimension by fish within streams and how and where resources are obtained by them (Chapter 1). Knowing the use of space by fish, we determine which items are part of the species' diet and how they are related in the regionalfood web (Chapter 2). We close by investigating how elevation and land use act under the replacement of species and/or functional attributes (Chapter 3). In summary, we found that: (1.) the largest portion of fish in the region is small and contributes directly to the richness of trophic functional groups; (2.) the species' diet basically consists of items from outside the
aquatic environment and the trophic network of the community presents a high degree of
specialization, nestedness and modularity. In other words, the same resource can be shared among several species, although there are those that specialize in a certain item, which results in the compartmentalization of the community in relation to the type of resource consumed; (3.) The replacement of species/functional attributes was not related to land use, but rather to altitude, showing that, in the first case, stochastic factors are involved and, in the second, environmental ones. In conclusion, the set of fish species in Serra da Bodoquena presents unique patterns of habitat use, a reflection of the environmental imposition of the region; small fish mainly use margins and represent the largest portion of the richness of the community, evidencing a Neotropical pattern; resource sharing is high and dependent on allochthonous items; and, in the context of the Upper Paraguay basin, the region's unique altitude pattern is a fundamental factor in maintaining fish diversity.
The African cyprinid genus Labeobarbus is characterized by a high polymorphism in mouth morphology. The association between four species of this genus (two chiselmouths, one rubberlip with a well‐developed fleshy mental lobe on the lower jaw, i.e., a fleshy appendage on the chin, and one usually with a poorly developed one) and 26 environmental variables was investigated in four affluents of the Luhoho River in the Albertine Highlands of the Upper Congo basin. A combination of univariate and multivariate analyses suggested that differences exist in habitat composition at localities where different Labeobarbus species were reported, and within different mouth phenotypes. The association of L. longidorsalis with the presence of cobbles and pebbles and of L. brauni with the presence of boulders suggests that the occurrence of chiselmouths phenotype is associated with stony habitats, whereas phenotypes with a mental lobe, i.e., Labeobarbus longifilis and L. paucisquamatus, mostly occupy habitats without stones. Differences in the distribution between these two species appear to be linked to physico‐chemical parameters such as electrical conductivity and water temperature rather than substrate type. The findings of this study are relevant for developing guidelines that aim effective local preservation and conservation of these Labeobarbus species.
This article explores the character and role of wood in fluvial systems. Following descriptions of the key characteristics of wood that affect its role and how it is measured, the quantities and styles in which wood is retained in fluvial systems are considered. This leads to discussions of wood budgets and wood mobility in fluvial systems and, finally, to relationships between wood and landforms. Throughout, discussions and descriptions focus on “small,” “medium” and “large” rivers where size relates to the ratio of wood piece length to channel width. The article concludes by drawing together some common aspects of wood and geomorphology across rivers with different physical and wood characteristics and then highlighting some areas that require more research attention.
Fluvial riparian vegetation (RV) links fluvial and terrestrial ecosystems. It is under significant pressure from anthropogenic activities, and, therefore, the management and restoration of RV are increasingly important worldwide. RV has been investigated from different perspectives, so knowledge on its structure and function is widely distributed. An important step forward is to convert existing knowledge into an overview easily accessible-for example, for use in decision-making and management. We aim to provide an overview of ecosystem services provided by RV by adopting a structured approach to identify the ecosystem services, describe their characteristics, and rank the importance of each service. We evaluate each service within four main riparian vegetation types adopting a global perspective to derive a broad concept. Subsequently, we introduce a guided framework for use in RV management based on our structured approach. We also identify knowledge gaps and evaluate the opportunities an ecosystem service approach offers to RV management.
Riparian trees can reduce bank erosion rates, but once a tree falls into a river it can increase local bank erosion. However, the influence of multiple logs, that hydraulically interact, on near-bank velocities has not been investigated. This paper reports flume experiments of the near-bank velocity changes and water level changes produced by multiple in-stream logs with equal and unequal spacing. The results suggest that the near-bank velocity increase caused by a single log can be reduced, and even reversed, by multiple logs. This reduced near-bank velocity mainly results from wake interference between the logs, rather than from the effect of backwater, and it varies systematically with the spacing between the logs. Bank erosion potential can be reduced where logs are spaced under 17 root-plate diameters and where the root-plate is located close to the bank. By contrast, the logs are likely to increase bank erosion when they are within an intermediate distance from the bank and are closely spaced (under 3.3 root-plate diameters apart). The flume results allow us to explore the temporal changes of the potential bank erosion in a reach with various log distributions.
It is well established that riparian trees reduce bank erosion. However, fallen trees can increase bank erosion as the flow is deflected and accelerated between the log and its root plate and the bank face, increasing the boundary shear stress and eroding the bank. This erosion contributes to hydraulic variability but can also be a concern in restoring wood loads in rivers. In this first quantitative study into this phenomenon, we develop a theoretical hydraulic model to estimate the near-bank velocity around a single log using principles of continuity and energy loss for a range of log characteristics (morphology, angle, distance from the bank) and flow characteristics. Flume experiments support the theoretical model and suggest that a single log can almost double the near-bank velocity. The fractional channel area taken up by the log (blockage ratio) and the distance between the log and the bank are key factors governing the near-bank velocity. The smaller the angle of the log with the bank the lower the erosion rate, but also the further downstream the erosion extends.
As the morphology of the log changes after falling into the channel, bank erosion potentially develops quickly, but slows over time; and migrates downstream.
Understanding the role of interactions in influencing community structure and ecosystem function is a goal in ecology, and identifying biotic entities that are strong interactors is imperative for setting targeted conservation strategies. Several different mechanisms have been linked with strongly interacting species (e.g., predation, competition, abiotic habitat modification), but the most important organisms often influence ecosystems in multiple ways. We propose that these strong interactors share a broad common feature: They catalyze ecosystem processes, such as rates of primary productivity, species interactions, and/or physical disturbances. We provide a case study of Spanish cedar (Cedrela odorata), focusing on its influence as a provider of large woody debris (LWD) on food web dynamics in tropical floodplain rivers and associated oxbow lakes. Large woody debris has been subject to considerable attention because of its perceived importance in creating geomorphologically favorable conditions for target commercial species (e.g., distribution of pools and riffle for salmonids). However, in this study we suggest that LWD catalyzes a suite of ecological processes in addition to geomorphology that determines its important role within aquatic communities. Through a factorial experiment manipulating large and small fish access to treatments with and without LWD piles, we tested the role of Spanish cedar in modifying interactions between different-sized fishes, invertebrates, and primary producers in a tropical floodplain river and associated oxbow lake. Path analysis revealed that fishes influence particulate matter accumulation and invertebrate abundances more so in wood piles than outside of wood piles in both river and lake ecosystem contexts. In addition to providing the first experimental test of factors controlling trophic dynamics in an Amazonian river, we suggest that understanding the role of organisms through the ecological processes they catalyze provides an overarching conceptual framework to link single species and ecosystem-based management strategies.
Large wood is a key structural and functional component of rivers and it is reintroduced to provide physical habitat, encourage channel stability and influence in-channel hydraulic conditions at a range of scales. Typically, studies assessing the influence of reintroduced wood on in-channel hydraulic character have been undertaken at relatively small scales –at a site. Relatively little is known about how reintroduced wood influences in-channel hydraulic character at larger reach scales and over different discharges. In this study the hydraulic character or hydraulic landscape of multiple reference (wooded), control (un-wooded) and managed (wood reintroduced) reaches (average length = 430 m) at ten different discharges in the Barwon-Darling River, Australia, were analysed. Landscape scale hydraulic character was compared using spatial metrics describing the size, shape, and relative location of hydraulic patches in each reach. It was hypothesised that reintroducing wood would significantly influence hydraulic landscapes within the managed reaches, with those reaches becoming more similar to reference reaches. The reintroduction of wood into the Barwon-Darling River did not significantly influence the hydraulic landscapes of the managed reaches; instead, they remained more similar to control reaches across discharges. Discharge did significantly influence the hydraulic landscape, with stepwise changes in the character of the hydraulic landscape in response to increasing discharge. The limited influence of reintroduced wood on the hydraulic landscapes is hypothesised to be because reintroduced wood did not match naturally occurring wood in terms of the character of individual pieces or their spatial distribution within the reach.
Knowledge of the habitats occupied by species is fundamental for the development of effective conservation and management actions. The collapse of pelagic fish species in the Sacramento–San Joaquin Delta, California, has triggered a need to better understand factors that drive their distribution and abundance. A study was conducted in summer–fall 2014 in an attempt to identify physical and biological habitat conditions that drive the abundance and distribution of pelagic species in the northern region of the system. The study was conducted in the three largest channels in the northern Sacramento–San Joaquin Delta by dimension, volume, and flow capacity. The pelagic community was dominated by three nonnative species, Siberian prawn Exopalaemon modestus, which comprised 56% of the total number of organisms, and two fish species, Threadfin Shad Dorosoma petenense and Mississippi Silversides Menidia audens, which together comprised 43% of the total number of organisms. Total fish and total shrimp abundance were sensitive to the most extreme values of turbidity and temperature encountered and positively associated with total zooplankton biomass. The results suggested that habitat conditions in terminal channels, historically a common feature on the landscape, support higher abundances of pelagic species and zooplankton than open-ended channels. These results provide resource managers with useful information on the habitat associations of pelagic species and on how the future distribution and abundance of pelagic species will likely change in response to climate or other ecological factors.
Received March 8, 2016; accepted September 22, 2016
Flow regimes are fundamental to sustaining ecological characteristics of rivers worldwide, including their associated floodplains. Recent advances in understanding tropical river?floodplain ecosystems suggest that a small set of basic ecological concepts underpins their biophysical characteristics, especially the high levels of productivity, biodiversity and natural resilience. The concepts relate to (1) river-specific flow patterns, (2) processes ?fuelled? by a complex of locally generated carbon and nutrients seasonally mixed with carbon and nutrients from floodplains and catchments, (3) seasonal movements of biota facilitated by flood regimes, (4) food webs and overall productivity sustained by hydrological connectivity, (5) fires in the wet/dry tropical floodplains and riparian zones being major consumers of carbon and a key factor in the subsequent redistribution of nutrients, and (6) river?floodplains having inherent resilience to natural variability but only limited resilience to artificial modifications. Understanding these concepts is particularly timely in anticipating the effects of impending development that may affect tropical river?floodplains at the global scale. Australia, a region encompassing some of the last relatively undisturbed tropical riverine landscapes in the world, provides a valuable case study for understanding the productivity, diversity and resilience of tropical river?floodplain systems. However, significant knowledge gaps remain. Despite substantial recent advances in understanding, present knowledge of these highly complex tropical rivers is insufficient to predict many ecological responses to either human-generated or climate-related changes. The major research challenges identified herein (for example, those related to food web structure, nutrient transfers, productivity, connectivity and resilience), if accomplished in the next decade, will offer substantial insights toward assessing and managing ecological changes associated with human alterations to rivers and their catchments.
To effectively manage wood in rivers, we need a better understanding of wood mobility within river networks. Here, we review primarily field-based (and some numerical) studies of wood transport. We distinguish small, medium, large, and great rivers based on wood piece dimensions relative to channel and flow dimensions and dominant controls on wood transport. We suggest further identification and designation of wood transport regimes as a useful way to characterize spatial-temporal network heterogeneity and to conceptualize the primary controls on wood mobility in diverse river segments. We draw analogies between wood and bedload transport, including distinguishing Eulerian and Lagrangian approaches, exploring transport capacity, and quantifying thresholds of wood mobility. We identify mobility envelopes for remobilization of wood with relation to increasing peak discharges, stream size, and dimensionless log lengths. Wood transport in natural channels exhibits high spatial and temporal variability, with discontinuities along the channel network at bankfull flow and when log lengths equal channel widths. Although median mobilization rates increase with increasing channel size, maximum mobilization rates are greatest in medium-sized channels. Most wood is transported during relatively infrequent high flows, but flows under bankfull can transport up to 30% of stored wood. We use conceptual models of dynamic equilibrium of wood in storage and of spiralling wood transport paths through drainage networks, as well as a metaphor of traffic on a road, to explore discontinuous wood movement through a river network. The primary limitations to describing wood transport are inappropriate time scales of observation and lack of sufficient data on mobility from diverse rivers. Improving models of wood flux requires better characterization of average step lengths within the lifetime travel path of a piece of wood. We suggest that future studies focus on: (i) continuous or high-frequency monitoring of wood mobility; (ii) monitoring changes in wood storage; (iii) using wood characteristics to fingerprint wood sources; (iv) quantifying volumes of wood buried within river corridors; (v) obtaining existing or new data from unconventional sources, such as citizen science initiatives, and (vi) creating online interactive data platforms to facilitate data synthesis.
Nearly 50 years of research focused on large wood (LW) in rivers provide a basis for understanding how wood enters rivers; how wood decays, breaks, and is transported downstream; and how at least temporarily stable wood influences channel geometry, fluxes of water, sediment, and organic matter, and the abundance and diversity of aquatic and riparian organisms. Field-based studies have led to qualitative conceptual models and to numerical stimulations of river processes involving wood. Numerous important gaps remain, however, in our understanding of wood dynamics. The majority of research on wood in rivers focuses on small- to medium-sized rivers, defined using the ratio of wood piece size to channel width as channels narrower than the locally typical wood-piece length (small) and slightly narrower than the longer wood pieces present (medium). Although diverse geographic regions and biomes are represented by one or a few studies in each region, the majority of research comes from perennial rivers draining temperate conifer forests. Regional syntheses most commonly focus on the Pacific Northwest region of North America where most of these studies originate. Consequently, significant gaps in our understanding include lack of knowledge of wood-related processes in large rivers, dryland rivers, and rivers of the high and low latitudes. Using a wood budget as an organizing framework, this paper identifies other gaps related to wood recruitment, transport, storage, and how beavers influence LW dynamics. With respect to wood recruitment, we lack information on the relative importance of mass tree mortality and transport of buried or surficial downed wood from the floodplain into the channel in diverse settings. Knowledge gaps related to wood transport include transport distances of LW and thresholds for LW mobility in small to medium rivers. With respect to wood storage, we have limited data on longitudinal trends in LW loads within unaltered large and great rivers and on fluctuations in LW load over time intervals greater than a few years. Other knowledge gaps relate to physical and ecological effects of wood, including the magnitude of flow resistance caused by LW; patterns of wood-related sediment storage for diverse river sizes and channel geometry; quantification of channel-floodplain-LW interactions; and potential threshold effects of LW in relation to physical processes and biotic communities. Finally, knowledge gaps are related to management of large wood and river corridors, including understanding the consequences of enormous historical reductions in LW load in rivers through the forested portions of the temperate zone; and how to effectively reintroduce and manage existing LW in river corridors, which includes enhancing public understanding of the importance of LW. Addressing these knowledge gaps requires more case studies from diverse rivers, as well as more syntheses and metadata analyses.
We use data and insights from several inter-related but independent projects conducted over 6 years
(2006–11) in the Daly River catchment in Australia’s Northern Territory to explore the potential impacts
of ‘development’ on Indigenous and non-Indigenous residents. We do this by combining economic,
hydrological, and ecological data and models into an integrated model. We then use the model to assess
the effect of six different types of economic ‘development’ on water resources, the habitat of aquatic
resources and the incomes of Indigenous and non-Indigenous people. We combine this information with
estimates of the value of aquatic resources consumed by local Indigenous people to generate estimates of
the potential net impact of those development scenarios. We also discuss other social and cultural
impacts likely to be associated with the degradation of aquatic resources.
Our analysis shows that Indigenous people not only have more to lose from ‘development’ which
erodes natural capital than do non-Indigenous people, but they also have significantly less to gain. Under
current institutional arrangements it thus seems that, at best, ‘development’ may have a relatively benign
impact on their well-being. At worst, it may have a detrimental effect brought about by degradation of
local aquatic ecosystems.
There are negative spill-over effects from development for non-Indigenous people too. Evidently some
conservation land uses – which tend to align more closely with current Indigenous cultural prerogatives –
may be better able to effectively promote the well-being of both Indigenous and non-Indigenous people
than more conventional types of ‘development’.
The annual flood pulse in tropical lowland rivers creates a continually moving land-water margin that forces aquatic organisms of the littoral zone to relocate to new habitats at intervals ranging from days to weeks. Thus, species assemblages in patchy littoral-zone habitats have large potential to be influenced by stochastic colonization dynamics, whereby organisms may exhibit weak selection for mesohabitat features and arrive on a patch in no particular sequence. We examined species assemblages of fishes and macroinvertebrates occupying habitats of varying structural complexity through an annual flood cycle of the Cinaruco River, a species-rich floodplain river in the southern llanos of Venezuela. We collected 268 standardized samples that contained 54,596 individual fishes representing 156 species (23 families) and 6973 macroinvertebrates representing 8 families. Analysis of species-specific patterns of habitat occupancy based on a randomization procedure indicated greater species selectivity during low water (92% of taxa) relative to rising- and falling-water periods (60% and 63% of taxa, respectively). Community-wide correspondence analysis (CA) revealed similar patterns. Assemblages from similar habitat types clustered together in the CA ordination, and these nonrandom patterns were most apparent during low water. Results from partial canonical correspondence analysis revealed that assemblage structure was strongly influenced by habitat type (patches of different composition located within the shallow littoral zone of either the river main channel or lagoons), which accounted for 56% of the total explainable variation in the species 3 sample matrix. Mesohabitat parameters (e.g., water velocity, depth), sampling period (month), and location on the landscape explained 30%, 23%, and 17% of variation, respectively. As water level changes and the aquatic-terrestrial interface moves across the landscape, species assemblages in patchy littoral habitats, while highly variable, reconstitute with significant determinism. At the same time, a large proportion of unexplained variation in assemblage structure probably is associated with unmeasured components of landscape variation as well as a stochastic element in colonization dynamics.
This paper reviews studies of relationships between riverine fish and woody debris at micro- and meso-habitat scales, and discusses the potential functions of instream structure for lowland river fish. Experimental research, mainly in North America, has identified three main functions of woody debris as microhabitat for fish in upland streams: overhead cover that decreases predation risk both vertically and horizontally; horizontal visual isolation that reduces contact between fish; and velocity refuge which minimizes energetic costs. As with habitat features in other aquatic environments, increasing spatial complexity of woody debris may modify predator-prey interactions and provide greater surface areas for the growth of prey items. Woody debris may also provide spatial reference points for riverine fish to assist them in orienting within their surroundings. Lowland rivers differ from upland streams in terms of a number of physical variables, including turbidity, depth and water turbulence. Relationships between fish and woody debris in lowland rivers are likely to rely on mechanisms different to those in upland streams. Recent initiatives involving the reintroduction of woody debris into previously cleared lowland rivers to replace lost fish habitat are a positive development for lowland river restoration. However, if woody debris reintroduction is to maximally benefit lowland river fisheries, there is a requirement for better understanding of the ecological functions of woody debris in lowland rivers.
This study examined the temporal dynamics and longitudinal distribution of wood over a multi-decadal timescale at the river reach scale (36 km) and a meander bend scale (300–600 m) in the Ain River, a large gravel-bed river flowing through a forested corridor, and adjusting to regulation and floodplain land-use change.
At the 36 km scale, more wood was recruited by bank erosion in 1991–2000 than since the 1950s. The longitudinal distribution of accumulations was similar between 1989 and 1999, but in both years individual pieces occurred homogeneously throughout the reach, while jam distribution was localized, associated with large concave banks. A relationship between the mean number of pieces and the volume recruited by bank erosion (r2 = 0·97) indicated a spatial relationship between areas of wood production and storage.
Wood mass stored and produced and channel sinuosity increased from 1993 to 2004 at three meander bends. Sinuosity was related to wood mass recruited by bank erosion during the previous decade (r2 = 0·73) and both of these parameters were correlated to the mean mass of wood/plot (r2 = 0·98 and 0·69 respectively), appearing to control wood storage and delivery at the bend scale. This suggests a local origin of wood stored in channel, not input from upstream trapped by preferential sites.
The increase in wood since 1950 is a response to floodplain afforestation, to a change from braided to meandering channel pattern in response to regulation, and to recent large floods. We observed temporal stability of supply and depositional sectors over a decade (on a reach scale). Meander bends were major storage sites, trapping wood with concave banks, also delivering wood. These results, and the link between sinuosity and wood frequency, establish geomorphology as a dominant wood storage and recruitment control in large gravel-bed rivers. Copyright
Transfer of large woody debris (>10 cm diameter) from old-growth Douglas-fir (Pseudotsugamenziesii (Mirbel) Franco) forests into five first-to fifth-order stream reaches (drainage areas of 0.1 to 60.5 km2) has ranged from 2.0 to 8.8 Mg•ha−1•year−1 in 7- to 9-year study periods. Amounts of large debris in these streams range from 230 to 750 Mg•ha−1 with generally lower values in larger channels. The addition of woody debris is widely scattered in time and space and comes mainly from single trees rooted away from the streambank. We infer that wind is a major agent for entry of wood into these streams. Downstream movement of debris is strongly related to length of individual pieces; most pieces that moved were shorter than bankfull width.
The upstream extent of coastal cutthroat (Oncorhynchus clarki clarki) and rainbow (Oncorhynchus mykiss) trout distribution in logged and unlogged streams of the western Cascade Mountains appears to be primarily constrained by steep channel gradient and sparse pool habitat. Narrow or intermittent wetted channels are also important constraints in logged drainages. The upstream extent of trout distribution appears to be resilient to the combined impacts of historic and current forest management activities, in the absence of impassable road culverts. The probability of trout presence decreased with channel gradient and increased with pool abundance in both logged and unlogged streams, as indicated by logistic regression analysis of physical stream attributes flanking the trout distribution limit in 37 logged and 21 unlogged streams. Reductions in wetted channel width reduced the likelihood of trout presence in logged streams. Logistic regression models fit to data from logged drainages generated accurate predictions of trout presence or absence when applied to data from unlogged drainages. The pervasive extent of native trout in the channel networks of the Cascade Mountains emphasizes the ecological importance of small streams in watershed planning.
Introduction If you leave the Wet Tropics around Cairns, and head west by car for an hour or so, the road goes up and over the mountains that lie behind the coast. On the other side, the rainfall drops off quickly, and you enter the great ‘sea’ of savanna that stretches across Northern Australia. Still heading west, several long days of driving later, you will reach Broome on the edge of the Indian Ocean. For all this time, for all the 3000 or more kilometres of travel, you will have been among vast areas of eucalypt savannas and native grasslands, broken only by an occasional cleared paddock, a scattering of small towns, and the rivers and wetlands that give life to the country. This landscape of savanna and rainforest, rivers and wetlands, is of great significance. On a global scale, such large natural areas are now very rare. Northern Australia stands out as one of the few very large natural areas remaining on Earth: alongside such global treasures as the Amazon rainforests, the boreal conifer forests of Alaska, and the polar wilderness of Antarctica. Unlike much of southern and eastern Australia, nature remains in abundance in the North. Great flocks of birds still move over the land searching for nectar, seeds and fruit. Rivers still flow naturally. Floods come and go. In fertile billabongs, thousands of Magpie Geese, brolgas, egrets and other water birds still congregate. The intact nature of the North provides a basis for much of the economic activity and the general quality of life for residents of the area. Most of the major industries – tourism, pastoralism, Indigenous economies – rely on productive, functioning and healthy natural ecosystems. Across the North, recreational activities such as fishing, four-wheel driving and visiting beautiful country depend on the opportunities provided by a largely intact and natural landscape. Being in and among nature remains a normal part of life for people in the North, in contrast to the situation for those living in the now highly transformed, cleared and urbanised areas of southern Australia. For the high proportion of Northern Australian residents who are Indigenous, country is part of the essence of life. Knowledge of and links to the land remain strong, and there remains an enduring responsibility to look after the land, and its plants and animals.
The complex yet poorly understood interactions between riparian vegetation, large woody debris and fluvial geomorphology in an anthropogenically undisturbed reach of an alluvial, sand-bed forest stream in SE Australia have been determined. Riparian vegetation exhibits lateral and vertical zonation of understorey and overstorey species. The dominant riparian tree species, Tristaniopsis laurina (water gum), grows within the channel and on the floodplain within one channel width of the stream. Larger Eucalyptus species only grow on the highest parts of the flood plain and on a low Pleistocene river terrace. A complete LWD census conducted in the 715-m long study reach revealed that water gum comprises 17.6% of the total LWD loading, which, at 576 m3ha-1, is high for a stream with a catchment area of 187 km2. Although most LWD has a small diameter (0.1-0.3 m), the greatest contribution to the total volume of LWD is by pieces with a diameter between 0.3 and 0.7 m. A high proportion of LWD (10.4%) has a blockage ratio greater than 10%. The spatial distribution of LWD is random both longitudinally and within individual meander bends. Dominant recruitment processes of LWD vary by species. Tristaniopsis laurina trees are recruited to the channel by minor bank erosion and senescence while the Eucalyptus species are predominantly recruited from the highest parts of the flood plain/low river terrace by episodic windthrow during large storms. Multiple radiocarbon dates of outer wood of immobile LWD indicate a maximum residence time of 240 + 40 yBP for T. laurina timber. The high loading of LWD combined with the extensive root systems of riparian vegetation stabilize Tonghi Creek. Log-steps form natural wooden drop-structures with a mean height of 29 mm that were responsible for 20.5% of the total head loss under base flow conditions (Q = 0.08 m3s-1). Large woody debris is buried in the bed at depths of up to 2.3 m and is responsible for an estimated 49% of the 11, 600 m3 of sand stored in the study reach. Pools are spaced at 0.8 channel widths and 82% of pools are formed by scour over, under, around, or beside LWD or by the impoundment of water upstream of debris dams. Due to the high density of hardwood timber species, debris dams however, do not readily form in Tonghi Creek as the timber is difficult to transport and LWD usually sinks to the bed of the stream. Despite the high degree of channel stability provided by LWD, high blockage ratios in the channel result in relatively frequent overbank flows. These flows are often concentrated in chutes across the neck of meanders or multiple loops, which can develop into cutoffs and channel avulsions, respectively.
This report was originally produced for the Northern Territory Department of Infrastructure, Planning and Environment (DIPE) by the Supervising Scientist Division of Environment Australia in fulfilment of a project initiated and funded by the Natural Heritage Trust (Environment Australia) and DIPE as part of the National River Health Program. The views and opinions expressed in this publication are those of the authors and do not necessarily reflect those of the Northern Territory and Commonwealth Governments. While reasonable efforts have been made to ensure that the contents of this publication are factually correct, the Northern Territory and Commonwealth Governments do not accept responsibility for the accuracy or completeness of the contents, and shall not be liable for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the contents of this publication. The information contained in this work has been published by Supervising Scientist Division of Environment Australia and the Northern Territory's Department of Infrastructure, Planning and Environment to help develop community, industry and management expertise in sustainable water resources management and raise awareness of river health issues and the needs of our rivers. The Northern Territory and Commonwealth Governments recommend that readers exercise their own skill and care with respect to their use of the material published in this report and that users carefully evaluate the accuracy, currency, completeness and relevance of the material for their purposes.
1. Northern Australia is characterised by a tropical wet–dry climate that regulates the distinctive character of river flow regimes across the region. There is marked hydrological seasonality, with most flow occurring over only a few months of the year during the wet season. Flow is also characterised by high variability between years, and in the degree of flow cessation, or intermittency, over the dry season.
2. At present, the relatively low human population density and demand for water in the region means that most rivers have largely unmodified flow regimes. These rivers therefore provide a good opportunity to understand the role of natural flow variability in river ecosystem structure and processes.
3. This review describes the major flow regime classes characterising northern Australian rivers, from perennial to seasonally intermittent to extremely intermittent, and how these regimes give rise to marked differences in the ecological character of these tropical rivers, particularly their floodplains.
4. We describe the key features of these flow regimes, namely the wet and dry seasons and the transitions between these seasons, and how they regulate the biophysical heterogeneity, primary productivity and movement of biota in Australia’s wet–dry tropical rivers.
5. We develop a conceptual model that predicts the likely hydrological and ecological consequences of future increases in water abstraction (e.g. for agriculture), and suggest how such impacts can be managed so that the distinctive ecological character of these rivers is maintained.
1. Globally, tropical floodplains are highly productive ecosystems. This is largely because of predictable seasonal rains providing replenishing floodwaters that stimulate nutrient turnover which, in turn, substantially boosts both primary and secondary productivity. This is associated with concomitant shifts in the types of primary producers and associated food webs.
2. The Magela Creek floodplain on Kakadu National Park in northern Australia is one of the most studied tropical freshwater ecosystems in Australia and provides an opportunity to collate and examine information on organic carbon sources and pathways through food webs to gain a fundamental understanding of how these systems may function.
3. We reviewed biophysical information published since the early 1980s to construct an assessment of the carbon resources for the channel and floodplain.
4. We conclude that macrophytes, largely in the form of grasses and aquatic plants, produce the greatest above-ground biomass on the Magela Creek floodplain. Although macrophytes provide suitable substrata for the attachment of epiphytes, they do not appear to be an important carbon source for aquatic consumers themselves. Nevertheless, macrophytes do provide critical seasonal food and habitat structure for other producers and consumers on the floodplain, such as the abundant magpie geese.
5. We developed a generalised conceptual food web and carbon budget contrasting the ‘wet’ and ‘dry’ seasons for the Magela Creek system, as a representative of tropical seasonal floodplain systems.
6. Our conceptual model of tropical floodplains indicates that knowledge of the seasonal and spatial links and exchanges between the floodplain and the river is critical in understanding ecosystem function.
Woody debris has several important roles in running water. Less is known about the ecology of wood in great rivers than in smaller rivers and streams. We used a probability survey to estimate the abundance of littoral and shoreline wood along the following mid-continent great rivers of the United States in summer 2004–2006: the Missouri River, Upper Mississippi River, and the Ohio River. We counted wood pieces >0.3 m in diameter from a zone between the bank full level out into the river 10 m. We categorized wood according to its origin and function as “beached” (transported from upriver but not providing aquatic habitat), “wet” (origin unknown and providing aquatic habitat; includes snags), or “anchored” (attached to the bank at its current location and providing aquatic habitat). We counted 5900 pieces of wood at 447 sites across rivers. Approximately 56 percent of pieces were beached, 30 percent were wet, and 14 percent were anchored. Overall, mean abundance of wood was 2.6 pieces of wood 100 m−1 of shoreline (approximately 3.0 m3 100 m−1). Abundance of wood (pieces per unit distance of river) was much lower than has been reported for many smaller streams and rivers. There was more wood along the Upper Mississippi River (3.3 pieces 100 m−1) than elsewhere (≤2.4 pieces 100 m−1). The mean abundance of wood on the Ohio River decreased significantly between the 2004 and 2005 survey periods due to high flows. Longitudinal patterns in wood abundance were weak. There was less anchored and wet wood along shorelines protected by revetment (e.g., rip rap). There was generally more wood along shorelines where the riparian land use was characterized as forest rather than agriculture or developed. Mean abundance of wood along forested, un-revetted shorelines was approximately four pieces 100 m−1 of shoreline (= 80 pieces km−1 of river). This estimate of mean wood abundance for what amounts to least disturbed riparian and shoreline conditions is relevant for great river bioassessment and management. Published in 2009 John Wiley & Sons, Ltd.
A field experiment was conducted to examine the influence of variable density (complexity) of small patches of woody debris
on the abundance and taxonomic richness of macroinvertebrates and fishes in the Brazos River, a meandering lowland river in
east-central Texas. Woody debris patches contained bundles of either 8 or 16 sticks of two sizes, and reference plots contained
no woody debris. The experiment was conducted in the littoral zone in the river channel and a nearby oxbow lake. Organisms
were collected from each patch after 14 days. Abundance and taxonomic assemblage structure of macroinvertebrates in both the
river channel and oxbow were significantly and positively influenced by complexity of woody debris. For fish in the oxbow,
abundance and species richness were greater in woody debris than sites lacking structure, but the opposite trend was observed
for fish in the river channel. This difference could be associated with isolation from source habitats and low colonization
of the constructed woody debris patches in the river by fishes with affinities for complex habitats. Small lotic-adapted minnows
were captured from reference habitats in the channel, but these species were rare in woody debris patches. This was in contrast
to aquatic insects in the river channel, such as caddisfly and midge larvae, that efficiently colonized the small isolated
patches of woody debris. In a lotic environment, woody debris provides vertical surfaces that intercept drifting insect larvae
and provides protection from the water current. We speculate that greater abundance of macroinvertebrates in woody debris
patches in both habitats results from the combined influence of high food resource availability and refuge from predation
provided by structurally complex habitats.
Multiple-scale assessments of fish-habitat associations are limited despite the fact that riverine fish assemblages are influenced
by factors operating over a range of spatial scales. A method for assessing fish-habitat assemblages at multiple scales is
proposed and tested in a large Australian dryland river, the Barwon–Darling River. Six discrete mesohabitat types (large wood,
smooth bank, irregular bank, matted bank, mid-channel and deep pool) nested within 10km long river reaches were sampled.
Individual reaches were, in turn, nested within four larger geomorphological zones, previously identified along the river.
Fish assemblages varied significantly between mesohabitat types and at different spatial scales. Golden perch (Macquaria ambigua), Murray cod (Maccullochella peelii peelii) and common carp (Cyprinus carpio) were strongly associated with large wood, but golden perch and Murray cod exhibited higher habitat specificity than carp.
Bony herring (Nematalosa erebi) were more common in shallow edgewater habitats. At the river-scale, regional differences in the fish assemblage occurred
at scales closely corresponding to geomorphological zones and these differences were associated with changes in the relative
abundance of species rather than the addition or replacement of species. The proposed hierarchical framework improves the
efficiency of fish surveys in large rivers by viewing meso-scale fish-habitat associations in the context of larger-scale
geomorphological processes.
The line-intersect technique was used to measure the loading of large woody debris in a 1 .8 km reach of the Thomson River, Victoria (catchment area of 3540 km 2). A debris census (measuring every item present) was done over 0 .775 km of this reach. The transect technique overestimated the actual loading revealed by the census. The loading of debris >0 .01 m in diameter for the total 1 .8 km reach was 0 .0172 m 3 m-2 , which is higher than that measured in many headwater streams in other parts of the world. The volume loading of debris measured from low level aerial photographs was only 4 .8% of the value estimated by the line-intersect technique. The line-intersect estimates were biased due to non-random orientation of debris in the stream (causing estimated errors of +8% for volume loading and +16% for surface area loading). It is recommended that to avoid this problem, when using the line-intersect transect technique in lowland rivers, each line should comprise at least two obliquely-angled transects across the channel. The mean item of debris (>0 .1 m in diameter) had a trunk basal diameter of 0 .45 m, a length of 7 .4 m, and volume of 0 .7 m3. The riparian trees and the in-channel debris were of similar dimensions. The debris tended to be close to the bed and banks and was oriented downstream by the flow at a median angle of 27°. Because of this orientation, most debris had a small projected cross-sectional area, with the median value being only 1 m 2. Thus, the blockage ratio (proportion of projected area of debris to channel cross-sectional area) was also low, ranging from 0 .0002 to 0.
There is growing awareness that patterns of habitat use by animals cannot be isolated from issues of scale. Recently, techniques have been devised which allow empirical testing of hypotheses related to the effects of spatial scale on habitat use. We used spatially explicit statistical procedures to examine the roles of scale and habitat arrangement in determining fish distribution patterns in a reach of an Australian lowland river. Native golden perch and introduced common carp were tracked day and night using radiotelemetry over a 10 day period and their distributions compared with mapped habitat variables. Golden perch were significantly associated with depth, current velocity, substratum and cover at larger scales of analysis both day and night, and at smaller scales during the day. At night, however, associations between golden perch and habitat variables were generally much weaker at small scales. Common carp were generally not significantly associated with habitat variables at larger scales of analysis, whereas at smaller scales they were associated with depth, current velocity, substrate and cover. Associations were generally stronger during the day than at night. Our study emphasises the need to consider scale-dependence in studies of fish-habitat associations. Management and restoration of fish habitat in lowland rivers should be based on an understanding of habitat use at scales relevant for the fish rather than at scales arbitrarily defined by humans.
A landscape perspective of wood in world rivers accounts for spatial and temporal patterns of sources of wood from streamside forests, processes of wood delivery to channels, transport of wood through river networks, and trapping sites of wood. Amounts of wood in a river system also depend on productivity of forests in source areas and decomposition rates. Collectively, these factors determine the amount and arrangement of individual pieces and accumulations of wood through a river network, which, in turn, affect ecological, geomorphic, social, and other features of rivers. Research to date deals with subsets of these components of wood in rivers, but there has been limited development of a general framework for wood in river networks. This chapter considers a framework for examining the arrangement of wood in river landscapes and how it may reflect the history of spatial patterns and timing of wood input and redistribution. Field studies provide examples of different spatial patterns and architectures of wood accumulations. Wood accumulations are shaped by input processes, trapping sites, and transport processes. Reaches in river networks may switch from wood patterns dominated by one set of controls to another because of gradual or abrupt input and redistribution. A framework for future studies and management includes interpretation of these different controls through time and over river networks.
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This chapter describes the various geomorphic and hydrologic processes such as catchments that influence riparian system development and maintenance. Catchments are areas of the land surface in which all the runoff drains to a single point on a stream or river channel, and are bounded by drainage divides; catchments have been known to range from hundreds of square meters in size to millions of square kilometers. Catchment drainage networks may have dendritic, palmate dendritic, or trellised forms, depending on the nature of underlying geology. These networks vary in drainage density and gradient, which affect riparia by impacting flood intensity and stream power, respectively. The most basic geomorphic processes in catchments are erosion, transport, and deposition. These processes operate across all time and space scales but vary in relative importance along drainage networks. Erosive processes dominate headwater regions, whereas deposition processes dominate the bottom of catchments draining to the ocean or into enclosed basins. Transport dominates in the mid-reaches of river systems. Erosion scours and eliminates riparian habitats and occurs when the shear stress imposed by flowing water exceeds the shear strength of the material over which it flows. The dominant forms of erosion include down-cutting and lateral movement of channels and scouring of channels and floodplains. Hydrologic processes strongly influence riparian habitats as the transport medium for sediments, but the presence or absence of water by itself is also an important control on riparian form and function. Flooding is a key process that distributes surface water to riparian environments and sets up gradients that drive surface water-groundwater exchanges. Four characteristics of floods, which are especially important to riparian and floodplain ecosystems are magnitude, frequency, timing, and duration.
Freshwater Fishes of North-Eastern Australia provides details of the ecology, systematics, biogeography and management of 79 species of native fish present in the region. It includes detailed information on their identification, evolutionary history, breeding biology, feeding ecology, movement patterns, macro-, meso- and micro-habitat use, water quality tolerances, conservation status and current threats, as well as environmental flow and management needs. Based on the results of extensive field surveys and a comprehensive review of existing literature, it is designed to assist environmental practitioners and managers to make informed decisions about future management strategies. It will also encourage a greater research effort into the region’s aquatic fauna by providing a comprehensive resource that enables other researchers to adopt a more quantitative and strategic framework for their research. Joint winner of the 2005 Whitley Medal.
Wild juvenile coho salmon (Oncorhynchus kisutch) were individually marked in October 1990 and 199l to evaluate the effects of habitat complexity and fish size on over-winter survival in Big Beef Creek, Washington. Habitat complexity was quantified for the habitat unit where the fish were collected and, in 1991, also for the 500-m reach downstream from the collection site. Survival, estimated from recovery of marked smolts at the stream's mouth, differed between years (25.4 and 46.2%) and also varied among habitat units and reaches within years. Survival was at most weakly correlated with complexity of the habitat units but was strongly correlated with the quantity of woody debris and density of habitat units in the 500-m reach, and distance from the estuary. Because distance covaried with habitat complexity, we could not ascertain which factor had the primary influence on survival. In addition, larger fish generally survived at a higher rate than smaller individuals. However, fish tagged above William Symington Lake were smaller in the fall but larger as smolts and had higher survival rates than those tagged below the lake. Taken together, these results reveal complex relationships between size, habitat, and growth that may affect over-winter survival and subsequent life-history events.
Removal of all organic debris dams from a 175-m stretch of second-order stream at the Hubbard Brook Experimental Forest in New Hampshire led to a dramatic increase in the export of organic carbon from this ecosystem. Output of dissolved organic carbon (<0.50 @mm) increased 18%. Fine particulate organic carbon (0.50 @mm-1 mm) export increased 632% and coarse particulate organic matter (>1 mm) export increased 138%. Measurement of the standing stock of coarse particulate organic matter on streambeds of the Hubbard Brook Valley revealed that organic debris dams were very important in accumulating this material. In first-order streams, debris dams contain nearly 75% of the standing stock of organic matter. The proportion of organic matter held by dams drops to 58% in second-order streams and to 20% in third-order streams. Organic debris dams, therefore, are extremely important components of the small stream ecosystem. They retain organic matter within the system, thereby allowing it to be processed into finer size fractions in headwater tributaries rather than transported downstream in a coarse particulate form.
Dam abundance varied from 8-13 dams/100m of stream length and covered only 1-3% of channel surface. Annual mean storage of organic matter (AFDM; particles >0.15mm) in debris dams was 922g/m2 and 3356g/m2 in Colliers Creek and Buzzards Branch (both Virginia), respectively. Wood > 16mm constituted 73-80% and fine particulate organic matter (FPOM; particles 0.15-1mm) <1% of total organic matter in the streams. Dams stored 21% and 85% of the coarse particulate organic matter (CPOM) on the channel surface in Colliers Creek and Buzzards Branch, respectively. Storage was highly seasonal, with highest concentrations following autumnal leaf fall and only 5-17% of December storage present in February. Dams were net exporters of large amounts of small particles of CPOM and FPOM. Macroinvertebrate density and biomass in dams was correlated with changes in organic matter storage. Annual mean densities were 8915 individuals/m2 and 22 302 individuals/m2, and biomass was 0.3g/m2 and 3.2g/m2, in Colliers Creek and Buzzards Branch, respectively. Densities were at least 10 times, and biomass 5 times, as great in dams as on sediment. Increasing the abundance of dams increased organic matter storage, macroinvertebrate abundance, and the relative contribution of shredders to biomass, both in dams and on sediment. Increasing dam abundance also increased retention of leaves during base and especially storm flow. Widespread removal of riparian vegetation throughout the SE Coastal Plain, resulting in a decrease in the number of dams in streams, no doubt has altered the ecological characteristics of these streams. -from Authors
This chapter discusses the physical and chemical features of tropical flowing waters. Tropical flowing waters typically show well. -defined seasonality in depth and velocity of flow, water chemistry, and metabolic rates, but seasonality is based primarily on hydrology alone rather than hydrology in conjunction with temperature, as would be more typical of temperate latitudes. Because of high mean water temperature, the oxygen reserve at saturation is substantially lower for tropical waters at low elevations than for temperate waters averaged over the growing season. Dissolved forms of phosphorus and nitrogen in tropical streams and rivers are present in quantities sufficient to support moderate to high biomass of autotrophs even under pristine conditions. The main significance of temperature for flowing waters is through its effect on metabolism and the capacity of water to hold dissolved oxygen. Tropical flowing waters as a whole have a lower oxygen reserve and a higher potential oxygen demand for a given amount of organic loading than temperate waters, except when temperate waters briefly reach the height of summer warmth. Factors affecting both suspended and dissolved load and concentration include slope, annual precipitation, lithology, disturbance, and natural or anthropogenic impoundment. Tropical flowing waters in some ways differ as a group from their counterparts at higher latitudes, but in other respects latitudinal differences are weak or nil. Tropical streams and rivers are stable thermally but should show seasonality driven by hydrology.
The flood pulse concept of Junk, Bayley and Sparks is a major contribution to our understanding of river–floodplain interactions and has become an important paradigm in lotic ecology. The concept is based mainly on large tropical lowland rivers. Floodplains may, however, develop in all geographical areas and at different locations along a river corridor. We extend this concept to temperate areas by including information derived from near-natural proglacial, headwater and lowland floodplains. Specific attention is directed to the role of temperature as a major determinant of floodplain ecology. Further attention is directed to the importance of expansion–contraction cycles occurring well below bankfull (‘flow pulse’ versus ‘flood pulse’). Selected examples are presented that highlight the complexity of expansion–contraction events and their consequences on habitat heterogeneity and functional processes. Habitat heterogeneity is mainly a product of shifting water sources, different flow paths and the relative importance of autogenic processes. In different floodplain systems, expansion may enhance habitat heterogeneity (e.g. glacial floodplain) or create homogeneity (e.g. Danubian floodplain). Further, the ecological consequences of episodic flow and flood pulses are discussed. Finally, a landscape approach is suggested in order to document expansion and contraction processes and to elucidate how these processes influence landscape heterogeneity and biodiversity patterns. Such a landscape-based ecosystem model can be applied to rigorously assess the ecological integrity of river–floodplain systems. Copyright © 2000 John Wiley & Sons, Ltd.
We illustrate the fundamental importance of fluctuations in natural water flows to the long-term sustainability and productivity of riverine ecosystems and their riparian areas. Natural flows are characterized by temporal and spatial heterogeneity in the magnitude, frequency, duration, timing, rate of change, and predictability of discharge. These characteristics, for a specific river or a collection of rivers within a defined region, shape species life histories over evolutionary (millennial) time scales as well as structure the ecological processes and productivity of aquatic and riparian communities. Extreme events – uncommon floods or droughts – are especially important in that they either reset or alter physical and chemical conditions underpinning the long-term development of biotic communities. We present the theoretical rationale for maintaining flow variability to sustain ecological communities and processes, and illustrate the importance of flow variability in two case studies – one from a semi-arid savanna river in South Africa and the other from a temperate rainforest river in North America. We then discuss the scientific challenges of determining the discharge patterns needed for environmental sustainability in a world where rivers, increasingly harnessed for human uses, are experiencing substantially altered flow characteristics relative to their natural states.
The importance of large woody debris (LWD) in forested stream ecosystems is well documented. However, little is known about LWD in northern boreal forest streams. We investigated the abundance, characteristics, and function of LWD in 13 small tributary streams of the upper Yukon River basin, Yukon Territory, Canada. LWD abundance was similar to values reported from temperate regions, whereas LWD size and total volume were well below values for the Pacific Northwest. LWD formed 28% of the pools, which provide important habitat for juvenile chinook salmon (Oncorhynchus tshawytscha Walbaum). The median diameter of pool-forming pieces was 17 cm, and ring counts on fallen riparian trees indicated that pool-forming pieces were likely 70200 years old when downed. Juvenile chinook salmon density was correlated with LWD abundance in our study reaches. We conclude that despite differences in climate and forest type, LWD in Yukon streams and LWD in temperate regions appear to perform a similar function in creating fish habitat. Resource managers should consider the relatively slow tree growth and thus potentially long recovery times following human disturbances in these watersheds.
Since European settlement, vast amounts of wood have been removed from Australian rivers. In recognition of its ecological value, including as habitat for aquatic invertebrates, wood is being reintroduced but with little understanding of optimum placement (pools v. riffles) or structural complexity to enhance invertebrate diversity. We hypothesised that complex woody debris would support higher numbers and more macroinvertebrate taxa, especially in riffles. Wood substrates of two complexities but similar surface areas were introduced into pools and riffles at three sites along the Hunter River, Australia. After 30 days, more taxa and individuals occurred on the complex substrates in pools and riffles at all sites. Substrates in riffles usually supported more taxa and individuals but responses were site-specific. Community composition varied among sites, substrates and habitats. Complex substrates, especially in riffles, trapped drifting organic matter that increased abundance and taxa richness but did not alter overall trends among substrates or habitats within sites. However, densities of rheophilic (flow-loving) taxa were reduced by entrained organic matter. Our results indicated that complex woody debris introduced into riffles could enhance diversity and abundance of macroinvertebrates in the Hunter River. However, these conclusions from a short-term, small-scale experiment need validation from longer-term, large-scale river rehabilitation projects.
any natural rivers are relatively wide and shallow, with an extensive tract of bare sediments during the low flow season - the active tract. Many (called braided rivers) have more than one mobile channel, bounded by bars of bare sediment, while others (island-braided rivers) are also characterized by vegetated islands. Most are lined by riparian woodlands. Along these natural rivers, floods erode, transport, and deposit not only sediment but also trees. Deposited trees, and islands that evolve from them, sustain a suite of aquatic and terrestrial habitats that would otherwise not be present on river bar surfaces. Previous research from a wide range of bioclimatic regions has focused on forested headwater catchments and on the
We present a new multivariate technique for testing the significance of individual terms in a multifactorial analysis-of-variance model for multispecies response variables. The technique will allow researchers to base analyses on measures of association (distance measures) that are ecologically relevant. In addition, unlike other distance-based hypothesis-testing techniques, this method allows tests of significance of interaction terms in a linear model. The technique uses the existing method of redundancy analysis (RDA) but allows the analysis to be based on Bray-Curtis or other ecologically meaningful measures through the use of principal coordinate analysis (PCoA). Steps in the procedure include: (1) calculating a matrix of distances among replicates using a distance measure of choice (e.g., Bray-Curtis); (2) determining the principal coordinates (including a correction for negative eigenvalues, if necessary), which preserve these distances; (3) creating a matrix of dummy variables corresponding to the design of the experiment (i.e., individual terms in a linear model); (4) analyzing the relationship between the principal coordinates (species data) and the dummy variables (model) using RDA; and (5) implementing a test by permutation for particular statistics corresponding to the particular terms in the model. This method has certain advantages not shared by other multivariate testing procedures. We demonstrate the use of this technique with experimental ecological data from intertidal assemblages and show how the presence of significant multivariate interactions can be interpreted. It is our view that distance-based RDA will be extremely useful to ecologists measuring multispecies responses to structured multifactorial experimental designs.
Riparian clearing and the removal of wood from channels have affected many streams in agricultural land-scapes. As a result, these streams often have depauperate in-stream wood loads, and therefore decreased habitat complexity and lower levels of in-stream biodiversity. The introduction of wood was investigated as a possible rehabilitation tech-nique for agricultural streams. Wood was re-introduced to eight streams in two separate high-rainfall, intensively grazed regions of Victoria, Australia and the effect on aquatic macroinvertebrate communities was measured. The addition of wood increased overall family richness and the richness of most functional feeding groups occupying edge and benthic habitats within the stream. Wood addition led to less overlap between benthic and edge macroinvertebrate communities, suggesting increased habitat heterogeneity within the stream ecosystem. Of all sampled habitats, wood supported the greatest density of families and was colonised by all functional feeding groups. Wood habitats also had the highest overall richness and supported the most taxa that were sensitive to disturbance. These findings suggest that re-introducing wood to agricultural streams is an appropriate rehabilitation technique where those streams are affected by reduced habitat complexity. Additional work is needed to confirm these findings over larger spatial and temporal scales.
In second- to fifth-order streams that drain old-growth timber in western Washington, characteristics and function of woody debris changed in relation to stream size. Average diameter, length, and volume of pieces of wood increased as stream size increased, whereas the frequency of occurrence of woody debris decreased. In streams with channel widths less than 7 m, 40% of the pieces of debris were oriented perpendicularly to the axis of flow; in streams with channel widths over 7 m, more than 40% of the pieces were oriented downstream. The types of pools most commonly associated with pieces of wood changed from plunge pools in small streams (42%) to debris scour pools in larger systems (62%). Pool area was correlated with the volume of the piece of wood forming the pool in streams of all sizes. However, this relationship was most evident in larger channels. Nearly 40% of the pieces of wood in channels less than 7 m wide were associated with sediment accumulations. Less than 30% of the pieces retained sediment in channels from 7 to 10 m wide, and less than 20% retained sediment in channels greater than 10 m wide. Surface area of sediment accumulations and the volume of the piece of wood forming the accumulation were related in all streams, but the relationship was clearest in the larger channels. Accumulations of particulate organic matter associated with woody debris were more frequent in small streams but were larger in large streams. No relationship was observed between the volume of fine particulate organic matter accumulated by a piece of wood and the piece of wood's volume.
The importance of structural woody habitat (SWH) as instream fish habitat in eastern Australia has been well documented. In response, many native fish restoration programmes are returning SWH to rivers where it had previously been removed. However, there is little information to direct the placement of this SWH within a river landscape.
Low‐level, high‐resolution aerial photographs were used to investigate the spatial pattern of SWH in the Murray River between Lake Mulwala and Tocumwal. It was found that SWH occurred in aggregations that were closely associated with eroding banks on meanders.
The physical characteristics of the SWH in these aggregations varied (basal diameter range 0.44–2.45 m, length range 1–44 m); however, small‐ to medium‐sized trees (basal diameter range 0.7–1.4 m, length range 5–20 m) were most common.
The association between eroding banks and SWH suggests that bank erosion may be an important determinant in the formation of SWH aggregations.
Copyright © 2004 John Wiley & Sons, Ltd.
This paper reports the development and application of two Bayesian Network models to assist decision making on the environmental flows required to maintain the ecological health of the Daly River (Northern Territory, Australia). Currently, the Daly River is unregulated, with only a small volume of water extracted annually for agriculture. However, there is considerable pressure for further agricultural development in the catchment, particularly with demand for extra water extraction during the dry season (May–November).
The abundances of two fish species—barramundi ( Lates calcarifer ) and sooty grunter ( Hephaestus fuliginosus )—were chosen as the ecological endpoints for the models, which linked dry season flows to key aspects of the biology of each species. Where available, data were used to define flow–fish habitat relationships, but most of the relationships were defined by expert opinion because of a lack of quantified ecological knowledge.
Recent field data on fish abundances were used to validate the models and gave prediction errors of 20–30%. The barramundi model indicated that the adult sub‐population was key to overall fish abundance, with this sub‐population particularly impacted by the timing of abstraction (early vs. late dry season). The sooty grunter model indicated that the juvenile sub‐population dominated the overall abundance and that this was primarily due to the amount of hydraulically suitable riffle habitat. If current extraction entitlements were fully utilized, the models showed there would be significant impacts on the populations of these two fish species, with the probability of unacceptable abundances increasing to 43% from 25% for sooty grunter and from 36% for barramundi under natural conditions. Copyright © 2010 John Wiley & Sons, Ltd.
Stream channel development in forested areas is profoundly influenced by large organic debris (logs, limbs and rootwads greater than 10 cm in diameter) in the channels.
In low gradient meandering streams large organic debris enters the channel through bank erosion, mass wasting, blowdown, and collapse of trees due to ice loading. In small streams large organic debris may locally influence channel morphology and sediment transport processes because the stream may not have the competency to redistribute the debris. In larger streams flowing water may move large organic debris, concentrating it into distinct accumulations (debris jams). Organic debris may greatly affect channel form and process by: increasing or decreasing stability of stream banks; influencing development of midchannel bars and short braided reaches; and facilitating, with other favourable circumstances, development of meander cutoffs.
In steep gradient mountain streams organic debris may enter the channel by all the processes mentioned for low gradient streams. In addition, considerable debris may also enter the channel by way of debris avalanches or debris torrents. In small to intermediate size mountain streams with steep valley walls and little or no floodplain or flat valley floor, the effects of large organic debris on the fluvial processes and channel form may be very significant. Debris jams may locally accelerate or retard channel bed and bank erosion and/or deposition; create sites for significant sediment storage; and produce a stepped channel profile, herein referred to as ‘organic stepping’, which provides for variable channel morphology and flow conditions.
The effect of live or dead trees anchored by rootwads into the stream bank may not only greatly retard bank erosion but also influence channel width and the development of small scour holes along the channel beneath tree roots. Once trees fall into the stream, their influence on the channel form and process may be quite different than when they were defending the banks, and, depending on the size of the debris, size of the stream, and many other factors, their effects range from insignificant to very important.
Wood storage within the active zone of the dynamic, gravel-bed, Fiume Tagliamento, Italy, was investigated at eight sites along the river's main stem. The quantity, nature, and mode of wood storage revealed a number of trends related to active zone morphology, cover type, and distance from the river's source. Relatively small quantities of wood were stored on open-gravel surfaces (estimates ranged from 1 to 21 t ha−1), intermediate quantities were associated with established islands (24–186 t ha−1), and large quantities were associated with pioneer islands (293–1664 t ha−1). Thus, variations in the geomorphological style of the river, which are associated with changes in these three cover types, are reflected in variations in the amount of wood that is stored in different reaches. In addition, although wood was found in many locations within the active zone, it was preferentially stored in three specific locations: (i) bar crests (the main open-gravel location for wood accumulations and pioneer islands); (ii) the margins and (iii) surfaces of established islands. The proportion of the stored wood that was living (sprouting) increased downstream and was higher on the open gravel than in association with established islands. There was a downstream gradient in the dominant type of wood accumulation. Individual logs predominated at the most upstream site. Thereafter, on the open gravel, whole shrubs and trees dominated the more confined sites in the headwaters and middle reaches, whereas, jams were the most frequent form of accumulation in the downstream reaches. Jams were the most frequent type of accumulation associated with established islands throughout the river. In contrast to small streams, where debris dams constitute the major type of wood accumulation, complex patterns and trends of wood storage were revealed along the Tagliamento. Although further studies are needed, it is clear that erosion of woody vegetation, its subsequent transport and deposition, play a major role in structuring the geomorphological and ecological character of this relatively natural, large European river-system. Insight into the mechanisms underlying the observed spatial patterns will contribute to a better understanding of the dynamic processes involved, and is essential for more effective management of river ecosystems.
Performance of two Centaurea species in response to dif-ferent root-associated microbial communities and to alterations in nutrient availability. — Ann. Bot. Fennici 41: 263–271. The perennial forbs Centaurea jacea and C. scabiosa have relatively similar morpho-logical traits, but different distributions and local abundance. Both occur in ruderal sites and in calcareous grasslands, but C. jacea may dominate in calcareous grass-lands, while C. scabiosa is a subordinate species. We hypothesised that: (1) C. jacea is dependent on the presence of the root-associated microbial community of a calcareous grassland (CG inoculum), as opposed to the microbial community from a ruderal site (old field inoculum, OF), while C. scabiosa is not; and (2) if there is a positive response of C. jacea to CG inoculum, it will be more evident in oligotrophic soil conditions. In our experiment the two Centaurea species were grown with two different inocula (CG, OF) at three different nutrient levels. CG inoculation considerably increased the leaf number of C. jacea as compared with the OF inoculation. This response was inter-preted as an investment into competitive ability in the future. CG inoculum resulted in a significantly higher root biomass in C. scabiosa, while the biomass of C. jacea did not vary among differentially inoculated plants. This was interpreted as an increased allocation to storage in C. scabiosa, to improve performance in stress conditions. Thus, compared with ruderal inoculum (OF), grassland (CG) inoculum enhanced 'competitive behaviour' in C. jacea and 'stress-tolerating behaviour' in C. scabiosa. Both responses were more evident at high nutrient levels. Fertilisation decreased the shoot to root ratio in both species, which may represent an 'element of stress-tolerating behaviour', much more common among rhizomatous perennial plants than is usually recognized.
The interplay of biological and physical patterns and processes within river ecosystems generates a complex matrix of interactions. A challenge in interdisciplinary river science is to dissect patterns and processes in multi-causal river ecosystems into hierarchical levels of organization. Hierarchy theory, and the associated concept of scale, provides a sound framework for achieving this. We present two interdisciplinary case studies that demonstrate how a multi-scale approach can dissect hierarchies of organization in river ecosystems. The first case study examined patterns of large wood character and distribution at three scales of a hierarchy of morphological river system organization in the large, lowland River Murray. The character and distribution of large wood was uniform at the largest reach scale (95 km length of river) because stream energy conditions are relatively uniform within the reach. However, there was an association between lower-level functional sets (straight or bend sections of river) and functional units (12 quadrats within each functional set) and the character and distribution of large wood, because stream energy differs between straight and bend morphologies, and the inner-and outer-channel functional units. Thus, functional sets and functional units are important levels of organization for large wood in the River Murray. The second case study examined the associations between macroinvertebrate assemblage distribution and environmental influences across a hierarchy of river system organization in the upland Murrumbidgee River catchment. We previously demonstrated that macroinvertebrate assemblages were arranged hierarchically at the region, cluster within region, reach within cluster and riffle within reach scales, with region and reach being the strongest signatures. In this study we related different scaled environmental factors, collected across a hierarchy of catchment, zone (valley confinement), reach (similar stream orders) and riffle scales to the region and cluster levels of macroinvertebrate distribution. The hierarchical pattern of large, region-level and local, reach-level macroinvertebrate distribution was matched by a large catchment-scale and local reach-scale of environmental influence. Intermediate zone-scale environmental factors and smaller riffle-scale factors were not important influences. Thus, large regions and catchments and local reaches are important levels of organization for macroinvertebrate-environment associations in rivers of the upper Murrumbidgee catchment. Both case studies support the applicability of hierarchy theory to describe the organization of physical–biological associations in river ecosystems. The multi-scaled approach allowed the detection of levels of hierarchical organization, and showed other hierarchical characteristics such as emergent properties and top–down constraint/bottom– up influence. Hierarchical understanding of river ecosystem organization will enhance river conservation and management because it facilitates a holistic, ecosystem perspective rather than a partial, single-scale, single-component or single-discipline perspective.
Abstract Habitat availability is often regarded as the primary factor that limits population and community recovery in degraded ecosystems, and physical habitat is thus often targeted in restoration. The identification of which habitat(s) to attempt to restore is a critical step in the restoration process, but one for which there is often a paucity of useful information. Here we examine the distribution of fish in three lowland streams in Victoria, Australia, that have been degraded by severe sedimentation. We aim to identify habitats that are associated with high abundances of native fish, and that thus might be appropriate to target in habitat restoration. Associations between native fish abundances and physical habitat characteristics were examined at three spatial scales (among streams, among sites and within sites) to determine the types of habitat to which fish respond, and the scales over which these responses occur. Of the four species of native fish found in the streams, three (Galaxias olidus Günther, Gadopsis marmoratus (Richardson) and Nanoperca australis Günther) showed significant habitat associations at small spatial scales (i.e. within sites). In particular, these species were generally found in deeper water, and in close proximity to cover (typically either coarse or fine woody debris or vegetation). Differences in habitat availability among sites and streams were less influential, except in the case of G. marmoratus, which was completely absent from both the ephemeral streams. Although our results suggest that these species collectively respond to habitat at several spatial scales, fish distributions were allied to the presence of habitat structure at the scale of metres, the smallest spatial scale examined. We hypothesize that fish abundances are currently limited by the low availability of habitat at these small spatial scales. It may therefore be possible to increase fish abundances in these creeks by augmenting the amount of available habitat via stream restoration.
The natural tendency of woody debris to accumulate into complex debris jams has been adapted by the restoration industry because of the morphological and ecological benefits of these structures. While much work has been done on woody debris, there is a lack of understanding of the dynamics of debris jams including the controls on their formation and the associated changes in hydraulics. Treatment of jams as static structures, whose hydraulics may be described by that of a single-solid object, prevents optimal success of wood-based restoration projects. This paper reviews the state of the science on the initiation and accumulation of wood forming a debris jam. This review is used to develop a conceptual model of the evolution of a single debris jam focussing on the relationship between the structure and hydraulics and the feedback that exists between them. The proposed mechanisms behind debris jam evolution are supported by a case-study of three natural jams. Incorporation of this model into restoration and management plans will result in more successful and cost-efficient projects. Copyright © 2008 John Wiley & Sons, Ltd.
1. We measured responses in macroinvertebrate secondary production after large wood additions to three forested headwater streams in the Upper Peninsula of Michigan. These streams had fine-grained sediments and low retention capacity due to low amounts of in-channel wood from a legacy of past logging. We predicted that wood addition would increase macroinvertebrate secondary production by increasing exposed coarse substrate and retention of organic matter.
2. Large wood (25 logs) was added haphazardly to a 100-m reach in each stream, and a 100-m upstream reach served as control; each reach was sampled monthly, 1 year before and 2 years after wood addition (i.e. BACI design). Macroinvertebrate secondary production was measured 1 year after wood addition in two habitat types: inorganic sediments of the main channel and debris accumulations of leaf litter and small wood.
3. Overall macroinvertebrate production did not change significantly because each stream responded differently to wood addition. Production increased by 22% in the main-channel of one stream, and showed insignificant changes in the other two streams compared to values before wood addition. Changes in main-channel macroinvertebrate production were related to small changes in substrate composition, which probably affected habitat and periphyton abundance. Macroinvertebrate production was much greater in debris accumulations than in the main-channel, indicating the potential for increased retention of leaf litter to increase overall macroinvertebrate production, especially in autumn.
4. Surrounding land use, substrate composition, temperature and method of log placement are variables that interact to influence the response of stream biota to wood additions. In most studies, wood additions occur in altered catchments, are rarely monitored, and secondary production is not a common metric. Our results suggest that the time required for measurable changes in geomorphology, organic matter retention, or invertebrate production is likely to take years to achieve, so monitoring should span more than 5 years, and ecosystem metrics, such as macroinvertebrate secondary production, should be incorporated into restoration monitoring programs.
The Sabie River, located within the semi-arid savanna of South Africa, experienced a ∼100-year return interval flood in 2000, providing a rare opportunity to quantify the resultant pattern and consequence of woody debris piles for riparian areas. Biophysical attributes of rivers in arid and semi-arid regions differ strongly from those in mesic regions, but differences in the role and dynamics of woody debris remain largely unknown. Our primary goal was to determine the basic ecology and distribution of wood piles formed by a large infrequent flood in a bedrock-confined river. In particular, we sought to determine the mechanism of pile formation, their position, size, and structural attributes of piles as well as regeneration from resprouting and seedling recruitment within piles in different channel types. We observed relatively distinct patterns of distribution and mechanisms of formation in 421 piles and provide a description of the distribution, abundance and short-term fate of each pile type. Patterns of wood accumulation were largely controlled by characteristics of the trapping sites, particularly toppled trees that remain rooted in their growth location. In general, wood was primarily strewn against toppled, transported and standing trees, though several large piles formed on bedrock outcrops. Buried piles were also discovered upon excavation of depositional features (alluvial mounds). Resprouting of toppled trees with intact roots and living tissues was the dominant mode of vegetation reestablishment in the piles. We observed resprouting in 36% of piles surveyed. Resprouting piles were most common (48%) in the seasonally inundated portion of the macro-channel. Tree seedlings were recorded in 28% of the piles. The flood appeared to create heterogeneity in the establishment of new woody vegetation by forming distinct wood piles from the ruins of the previous riparian forest. Even though the input of wood to semi-arid rivers appears to be a fairly rare occurrence and wood piles are rapidly depleted by decay and fire, we expect their influence will be long-lived, preserved in the pattern of vegetation establishment and alluvial mounds within the macro-channel. Copyright © 2006 John Wiley & Sons, Ltd.
1. Large wood forms an important component of woodland river ecosystems. The relationship between large wood and the physical characteristics of river systems varies greatly with changes in the tree species of the marginal woodland, the climatic and hydrological regime, the fluvial geomorphological setting and the river and woodland management context.
2. Research on large wood and fluvial processes over the last 25 years has focussed on three main themes: the effects of wood on flow hydraulics; on the transfer of mineral and organic sediment; and on the geomorphology of river channels.
3. Analogies between wood and mineral sediment transfer processes (supply, mobility and river characteristics that affect retention) are found useful as a framework for synthesising current knowledge on large wood in rivers.
4. An important property of wood is its size when scaled to the size of the river channel. ′Small′ channels are defined as those whose width is less than the majority of wood pieces (e.g. width < median wood piece length). `Medium' channels have widths greater than the size of most wood pieces (e.g. width < upper quartile wood piece length), and `Large' channels are wider than the length of all of the wood pieces delivered to them.
5. A conceptual framework defined here for evaluating the storage and dynamics of wood in rivers ranks the relative importance of hydrological characteristics (flow regime, sediment transport regime), wood characteristics (piece size, buoyancy, morphological complexity) and geomorphological characteristics (channel width, geomorphological style) in `Small', `Medium' and `Large' rivers.
6. Wood pieces are large in comparison with river size in `small' rivers, therefore they tend to remain close to where they are delivered to the river and provide important structures in the stream, controlling rather than responding to the hydrological and sediment transfer characteristics of the river.
7. For `Medium' rivers, the combination of wood length and form becomes critical to the stability of wood within the channel. Wood accumulations form as a result of smaller or more mobile wood pieces accumulating behind key pieces. Wood transport is governed mainly by the flow regime and the buoyancy of the wood. Even quite large wood pieces may require partial burial to give them stability, so enhancing the importance of the sediment transport regime.
8. Wood dynamics in `Large' rivers vary with the geometry of the channel (slope and channel pattern), which controls the delivery, mobility and breakage of wood, and also the characteristics of the riparian zone, from where the greatest volume of wood is introduced. Wood retention depends on the channel pattern and the distribution of flow velocity. A large amount is stored at the channel margins. The greater the contact between the active channel and the forested floodplain and islands, the greater the quantity of wood that is stored.
In channel and floodplain habitats of the Cinaruco River, Venezuela, Cichla temensis was more abundant and larger than C. intermedia and C. orinocensis. Seasonal variation in hydrology influenced habitat use, spawning, and predator-prey interactions. The three piscivores partitioned habitat, with C. intermedia showing a strong affinity for structured habitats in the main channel during all water level fluctuations. C. orinocensis was most abundant in shallow areas with submerged structure in lagoons and, to a lesser extent, in low velocity regions of the channel, and C. temensis occupied a wide range of lotic and lentic habitats. During the low-water period, the feeding frequency and body condition of all three species declined, and this was related, in part, to preparation for spawning near the end of the low-water season. The diet of C. intermedia was least similar to its two congeners during falling and rising water. C. orinocensis and C. temensis had lowest diet overlap during the low-water conditions, the period when many individuals of these two species move into lagoons for nesting. Prey in stomachs were significantly larger during the falling-water than the rising-water period, and predation by Cichla and other large piscivores during the falling-water period may have reduced the abundance of large prey, particularly Semaprochilodus kneri. These migratory detritivorous fish were important prey for C. temensis during the falling-water period and probably contributed a substantial fraction of the annual energy intake for this species. Together, the three Cichla species consume a wide spectrum of prey from a diverse fish assemblage, but prey are subdivided based on habitat, prey type, and season.
1. River valleys resemble dynamic mosaics, composed of patches which are natural, transient features of the land surface produced by the joint action of a river and successional processes over years to centuries. They simultaneously regulate and reflect the distribution of stream energy and exchanges of sediment, wood and particulate organic matter between riparian and aquatic environments.
2. We determined the structure, composition, dynamics and origin of seven patch types at the reach scale in the Queets River valley in the temperate coastal forests of the Olympic Mountains, Washington (U.S.A.). Patch types included: (1) primary and (2) secondary channels; (3) pioneer bars; (4) developing and (5) established floodplains; and (6) transitional and (7) mature fluvial terraces.
3. Lateral channel movements strongly shape patch distribution, structure and dynamics. The primary channel moved laterally 13 m year−1, on average from 1939 to 2002, but was highly variable among locations and over time. Mean lateral movement rates ranged from 1 to 59 m year−1 and moving averages (2 km) ranged from 3 to 28 m year−1 throughout the valley.
4. Each patch type exhibited characteristic vegetation, soil and accumulations of large wood. Pioneer bars contained peak stem density (69 778 stems ha−1) and volume of large wood (289 m3 ha−1). Mature fluvial terraces contained the highest mean stem (1739 m3 ha−1) and canopy volume (158 587 m3 ha−1). These patches also contained the most soil nitrogen (537 kg ha−1) and carbon (5972 kg ha−1).
5. Patch half-life (the time required for half of the existing patches to be eroded) ranged from 21 to 401 years among forested patch types. Erosion rates were highest in pioneer bars (2.3% year−1) and developing floodplains (3.3% year−1), compared with only 0.17% year−1 in mature fluvial terraces. New forests formed continually, as pioneering vegetation colonised 50% of the channel system within 18 years, often unsuccessfully.
6. In the Queets River, the structure, composition, and dynamics of the patchy riparian forest depends on the interplay between channel movements and biophysical feedbacks between large wood, living vegetation and geomorphic processes. The cycle of patch development perpetuates a shifting-mosaic of habitats within the river valley capable of supporting diverse biotic assemblages.
Surveys of wood along 30 forested headwater stream reaches in La Selva Biological Station in north-eastern Costa Rica represent the first systematic data reported on wood loads in neotropical streams. For streams with drainage areas of 0·1–8·5 km2 and gradients of 0·2–8%, wood load ranged from 3 to 34·7 m3 wood/100 m channel and 41–612 m3 wood/ha channel. These values are within the range reported for temperate streams. The variables wood diameter/flow depth, stream power, the presence of backflooding, and channel width/depth are consistently selected as significant predictors by statistical models for wood load. These variables explain half to two-thirds of the variability in wood load. These results, along with the spatial distribution of wood with respect to the thalweg, suggest that transport processes exert a greater influence on wood loads than recruitment processes. Wood appears to be more geomorphically effective in altering bed elevations in gravel-bed reaches than in reaches with coarser or finer substrate. Copyright © 2009 John Wiley & Sons, Ltd.