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Patterns in the fate of production in seagrass and other ecosystem types, based on a compilation of over 200 literature reports (Cebrián 1999). For each variable, boxes encompass the 25 th and 75 th quartiles of the data, and the central line represents the median. The proportion of seagrass production consumed directly by grazers varies up to about 50% but is low, on average, relative to algal-based aquatic ecosystems.
Contexts in source publication
Context 1
... within sediments, makes experimental ma- nipulation of these plants difficult. Secondly, seagrass ecology has traditionally been dominated by classical investigations of plant physiology and the impacts of eutrophication on seagrass growth, with little attention to the community-oriented approaches employed effectively in other branches of marine ecology. Whatever the reason, we perceive (as have others, e.g. Williams and Heck 2001; Domning 2001) a lack of appreciation for the potentially critical role of grazing, historically and currently, in determining the structure and productivity of seagrass food webs. Indeed, one of the few existing paradigms of marine ecology is that little of the production of seagrasses is grazed by marine herbivores (e.g. Cebrián 1999, 2002; Mateo et al., Chapter 7; Fig. 2). In this chapter, we review existing infor- mation, and offer new hypotheses, for the current and historical role of grazing in the ecology and evolution of seagrasses, with particular reference to its role in mediating the relative dominance of seagrasses versus benthic algae. Our review leads us to suggest that the current bottom-up paradigm of seagrass ecology, which has focused primarily on control of seagrass populations by light, nutrients, and other abiotic variables, is incomplete. We review the substantial evidence that grazing—on either living seagrasses, or their attached epiphytic algae—is of critical importance in mediating population dynamics, community composition, and materials fluxes in seagrass ecosystems. A unifying theme that emerges from our analysis is that trophic interactions generally, and grazing in particular, can have pervasive impacts on many aspects of the population, com- munity, and ecosystem ecology of seagrass beds. Moreover, seagrass morphologies, growth patterns, and life histories are likely to have been molded by historical grazing regimes quite different than those prevailing today. In short, we propose that the structure and functioning of seagrass ecosystems cannot be understood except in the context of the larger food webs they support. Although we find the weight of evidence for these points compelling, there is a clear need for more and better experimental research focused on the role of trophic interactions in the ecology of seagrass systems. Accordingly, we close our chapter by suggesting several hypotheses that we hope will stimulate new approaches to assessing the organization and dynamics of seagrass ecosystems, and their importance in supplying production to marine food webs. The basic qualitative biology of herbivory in seagrass beds is reasonably well known. Despite consid- erable variance in feeding strategies among species, we can generalize that vertebrate herbivores (fishes, turtles, sirenians, and waterfowl) and sea urchins often graze seagrasses directly, whereas smaller mesograzers (crustaceans, gastropods, and other invertebrates) usually feed on algae attached to seagrass leaves. These patterns have been thoroughly documented by previous reviews (Orth and van Montfrans 1984; van Montfrans et al. 1984; Thayer et al. 1984; Klumpp et al. 1989; Jernakoff et al. 1996; Valentine and Heck 1999; Williams and Heck ...
Context 2
... throughout their 50 My history. Many dugongine sirenians evolved large blade-like tusks that were probably adaptations to uprooting and feeding on the large, tough rhizomes of seagrass taxa such as Thalassia and Zostera . Indeed, after review- ing the fossil history of both seagrasses and sirenians in the Caribbean region, Domning (2001) concluded that “for most of the past 50 Ma, Caribbean seagrass communities have had to withstand sustained grazing pressure from several sympatric lineages of large mammalian herbivores” (p. 45); “Under these conditions (which prevailed up to about 2-3 Ma), most primary productivity was presumably consumed by herbivores, in contrast to the detritus-based seagrass ecosystems of today, which include few if any large herbivores” (p. 27). These conclusions may even understate the historical dominance of seagrass grazing given the formerly great abundances of seagrass-grazing green turtles and perhaps waterfowl (Jackson et al. 2001). Thus, in past times grazing vertebrates in seagrass beds may have been as important in determining productivity as they were in terrestrial grasslands (cf. McNaughton 1984). Today the situation is quite different. For the most part, large vertebrate grazers of seagrasses are func- tionally extinct, due to human exploitation, throughout most of the global ocean (Jackson et al. 2001). Thus, many modern seagrass ecosystems are of the detritus-based type, with little seagrass production grazed directly (Fig. 2, Cebrián and Duarte 2001; Cebrián 1999, 2002; Mateo et al., Chapter 7). Although waterfowl and sparid fishes have locally intense impacts on seagrass biomass in some areas, the most abundant primary consumers in most temperate seagrass ecosystems are smaller invertebrate mesograzers that feed mainly on algae attached to seagrass leaves (Kikuchi 1974; Orth and van Montfrans 1984; Klumpp et al. 1989; Jernakoff et al. 1996). In many warmer seas, seagrasses are intensely grazed by sea urchins and some fishes (Thayer et al. 1984; Valentine and Heck 1999). Thus, the major components in a general- ized seagrass food web must include at least the following: seagrasses and their attached algae, invertebrate mesograzers, detritivores, vertebrate grazers, small invertebrate-feeding predators, piscivorous predators—and, of course, humans (Fig. 3). Direct and indirect interactions among these functional groups have potentially important influences on the relative dominance of seagrasses, macroalgae, and microalgae, as shown by growing experimental evidence from seagrass beds (see below), and inferences drawn from other ecosystems. Understanding the functioning of seagrass ecosystems, and predicting and mitigating their responses to anthropogenic stresses, requires understanding these food-web processes and how they interact with changing resource ...
Context 3
... occupy a central role in determining the structure and productivity of most marine food webs. Not only can grazers influence the biomass, productivity, and community structure of primary producers (Lubchenco and Gaines 1981; Cyr and Pace 1993; Menge 1995; Shurin et al. 2002), they are also an important conduit for the transfer of energy from primary producers to higher trophic levels. Thus, grazers exert an important bottom-up control over the nutritional status, abundance, and composition of higher-order consumers. This is especially true of the small invertebrate mesograzers, the key link between primary producers and higher trophic levels in shallow marine systems (Edgar and Shaw 1995; Taylor 1998). As such, the influence of grazers cascades both downwards and upwards in food webs. Accordingly, the roles of grazers can be approached from two distinct perspectives, focusing on their influence on community structure, versus on ecosystem processes. Paine (1980) emphasized that these two types of influence, which he referred to as functional webs (also called interaction webs) versus energy-flow webs, respectively, are often not necessarily concordant. That is, a trophic link with negligible influence on energy and materials flows under normal circumstances, can nonetheless have a critical influence on the relative abundance of species in the community (Paine 1980), and on the system’s stability (McCann et al. 1998; Berlow 1999). Similarly, key conduits for the transfer of energy among adjacent trophic levels need not be strong interactors from a community perspective (e.g. Kirsch et al. 2002). Below we consider both the community-level and ecosystem-level influences of grazing. We consider separately the mostly tropical systems in which direct grazing of seagrasses can still be substantial, and the invertebrate-dominated, mostly temperate systems in which grazers feed primarily on attached epiphytic algae, and impacts on seagrasses are primarily indirect. One of the few existing paradigms of marine ecology holds that, in most systems, little seagrass primary production is directly grazed (Fig. 2). Most marine texts report that, on average, < 30% of sea- grass production reaches higher order consumers via the grazing pathway (e.g. Lalli and Parson 1993; Valiela 1995; Levinton 2000; Nybakken 2000; see also Mateo et al., Chapter 7). As a result, the primary conduit for seagrass production to reach higher order consumers is consistently reported to be the detrital pathway (e.g. Fenchel 1970, 1977; Kikuchi and Peres 1977; Nienhaus and Van Ierland 1978; Kikuchi 1980; Thayer et al. 1984; Nienhaus and Groenendijk 1986; Zieman and Zieman 1989; Cebrián 2002; Mateo et al., Chapter 7). One contrib- utory factor to this view is the observation that in seagrass systems, as in most systems on land, “the world is green” (Hairston et al. 1960). That is, since seagrasses form lush vegetated habitats along the coasts of every continent except Antarctica, herbivores would seem unlikely to play a significant role in controlling their biomass. Despite this plausible qualitative picture, there have been relatively few critical assessments of the importance of grazing in seagrass beds, and fewer direct estimates of the proportion of seagrass production consumed in the field. Consequently, investigations of the factors controlling seagrass growth and biomass emphasize the primacy of the sediment nutrient supply (e.g. Patriquin 1972; Short 1987; Powell et al. 1989; Fourqurean et al. 1992; Short et al. 1993), light availability, and/or physical factors (e.g. Patriquin 1975; Backman and Barilotti 1976; Dennison and Alberte 1985; Thom and Albright 1990). The history of this “bottom-up” paradigm for seagrass ecosystems is intriguingly similar to the analogous worldview of early terrestrial ecologists, who also perceived grazing on terrestrial plants to be low. Just as early assessments of terrestrial plant-animal interactions are now known to have been overly simplistic (e.g. Karban and Baldwin 1997; Lowman 1984, 1985; McNaughton 1985; McNaughton et al. 1996), it is increasingly clear that seagrass-grazer interactions are more important and dynamic than previously recognized. In this section we provide an updated overview of recent evidence of direct grazing on seagrasses. Perhaps surprisingly, there is ample evidence that grazing on seagrasses is significant in many areas of the world’s oceans. This evidence suggests that it is premature to conclude in general terms how important direct grazing of seagrasses is in determining the structure and productivity of modern coastal food webs. Two distinct, but not mutually exclusive, explanations have been offered for the apparently low levels of herbivory on seagrasses. These can ...
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... They play an extremely important role in maintaining the seagrass beds and increasing seagrass resilience. Their feeding behaviour helps decrease the organic matter in the sediments, which in turn stimulates biodiversity, decreases hypoxia and improves the health of the seagrass beds (Valentine and Duffy 2006). It can also help decrease the risk of harmful algal overgrowth, as well as decrease the number of seagrass leaf diseases (Valentine and Duffy 2006). ...
... Their feeding behaviour helps decrease the organic matter in the sediments, which in turn stimulates biodiversity, decreases hypoxia and improves the health of the seagrass beds (Valentine and Duffy 2006). It can also help decrease the risk of harmful algal overgrowth, as well as decrease the number of seagrass leaf diseases (Valentine and Duffy 2006). This means that areas that support large numbers of dugongs can provide better quality food than those that support few or no dugongs (Aragones and Marsh 2000). ...
Marine mammals have always captured the imagination of the people they share their environment with, but few people know that various species populate the waters of the United Arab Emirates (UAE). The world’s second largest population of dugongs reside in the Abu Dhabi Emirate, while two species of dolphins (Indian Ocean humpback and Indo-Pacific bottlenose dolphin) and the elusive finless porpoises inhabit Abu Dhabi and Dubai waters. Facing both the Sea of Oman and the Arabian Gulf, the UAE hosts a total of 18 species of marine mammals. These include the biggest animal on Earth, the blue whale, Bryde’s, humpback whales and killer whales, to name a few. However, ecological information about these species is still scarce. With the exponential increase of anthropogenic pressure and effects of climate change on the UAE marine environment, they are facing the risk of disappearing unnoticed. It is imperative to gain a better understanding of their ecology and main threats that affect them to support the implementation of effective conservation measures. Here we review the evolutionary history and adaptation to the water environment of these charismatic group of animals and provide the most up-to-date information on their status in UAE.
... When scaled up to the area of the meadow, there would be between 25,465 and 32,873 viable fragments in Pioneer Bay, and 17,545-22,649 viable fragments in Midge Point per day during the growing season, and 4,630-7,408 viable fragments in Pioneer Bay and 3,190-5,104 viable fragments in Midge Point per day during the senescent season. The probability of successful dispersal by individual vegetative fragments is low (Ewanchuk and Williams 1996;Smith et al., 2018) due to deterioration over time (Weatherall et al., 2016), consumption by various herbivores (Valentine and Duffy 2006), and settlement in locations incompatible with growth (Grech et al., 2016). However, the large number of viable fragments available for settlement discovered in our study indicates this mode of dispersal could play an important role in meadow connectivity. ...
Background and aims
Long distance dispersal (LDD) contributes to the replenishment and recovery of tropical seagrass habitats exposed to disturbance, such as cyclones and infrastructure development. However, our current knowledge regarding the physical attributes of seagrass fragments that influence LDD predominantly stems from temperate species and regions. The goal of this paper is to measure seagrass fragment density and viability in two tropical species, assessing various factors influencing their distribution.
Methods
We measured the density and viability of floating seagrass fragments for two tropical seagrass species (Zostera muelleri and Halodule uninervis) in two coastal seagrass meadows in the central Great Barrier Reef World Heritage Area, Australia. We assessed the effect of wind speed, wind direction, seagrass growing/senescent season, seagrass meadow density, meadow location and dugong foraging intensity on fragment density. We also measured seagrass fragment structure and fragment viability; i.e., potential to establish into a new plant.
Key results
We found that seagrass meadow density, season, wind direction and wind speed influenced total fragment density, while season and wind speed influenced the density of viable fragments. Dugong foraging intensity did not influence fragment density. Our results indicate that wave action from winds combined with high seagrass meadow density increases seagrass fragment creation, and that more fragments are produced during the growing than the senescent season. Seagrass fragments classified as viable for Z. muelleri and H. uninervis had significantly more shoots and leaves than non-viable fragments. We collected 0.63 (±0.08 SE) floating viable fragments 100 m−2 in the growing season, and 0.13 (±0.03 SE) viable fragments 100 m−2 in the senescent season. Over a third (38%) of all fragments collected were viable.
Conclusion
There is likely to be a large number of viable seagrass fragments available for long distance dispersal. This study's outputs can inform dispersal and connectivity models that are used to direct seagrass ecosystem management and conservation strategies.
... Herbivorous marine amphipods have long been recognized as important grazers on filamentous and ephemeral benthic algae (Duffy, 1990). The ecological roles of these herbivores diverse from trophic grazing, assisting the spore dispersal, to controlling the epiphytic microalgae and detritus, and hence regulating the species composition and fitness of the host benthic seaweeds across different ecosystems (Duffy, 1990;Sano et al., 2003;Valentine et al., 2006). With the increasing occurrence of floating macroalgal blooms in China's coastal waters (Qi et al., 2017;Liu et al., 2018;Song et al., 2019;Xiao et al., 2021), epiphytic amphipods have been noted and speculated to be important biological predators on the drifting seaweeds Wang G. C. et al., 2020). ...
An epiphytic gammarid species, Apohyale sp., was abundant in the floating Ulva prolifera (U. prolifera), which forms large-scale green tides in the Yellow Sea (YSGT). Field observation and laboratory experiments were subsequently conducted to study the species identity, abundance, and grazing effects on the floating algal biomass. The abundance of Apohyale sp. showed great spatial variation and varied from 0.03 to 1.47 inds g⁻¹ in the YSGT. In average, each gram of Apohyale sp. body mass can consume 0.43 and 0.60 g algal mass of U. prolifera per day, and the grazing rates varied among the algae cultured with different nutritional seawaters. It was estimated that grazing of Apohyale sp. could efficiently reduce ~0.4 and 16.6% of the algal growth rates in Rudong and Qingdao, respectively. The U. prolifera fragments resulting from gnawing of Apohyale sp. had a higher growth rate and similar photosynthetic activities compared to the floating algae, indicating probably positive feedback on the floating algal biomass. This research corroborated the significant impact of Apohyale sp. on the floating algal mass of YSGT through the top-down control. However, further research is needed to understand the population dynamics of these primary predators and hence their correlation with the expansion or decline of YSGT, especially under the complex food webs in the southern Yellow Sea.
... Seagrass meadows provide a multitude of ecosystem services, such as supporting a complex food web and serving as a nursery habitat for commercially important species (Valentine and Duffy 2006), while also providing coastal protection and mitigation of climate change by carbon sequestration (Nordlund et al. 2018). Anthropogenic impacts threaten seagrasses worldwide and have led to their decline (Orth et al. 2006;Waycott et al. 2009). ...
Seagrass conservation and management requires scientific understanding of spatial and temporal variability, information that is currently limited for the Eastern Tropical Pacific (ETP). Here, we analysed seagrass presence based on previous reports, herbarium collections and stakeholder knowledge, combined with field characterization in Golfo Dulce, southern Pacific coast of Costa Rica. Seagrasses were found at multiple locations along a narrow border close to shore and in up to 6 m depth within Golfo Dulce, dating back to 1969. Two seagrass species were found, Halophila baillonii and Halodule beaudettei. Seagrass biomass values for Golfo Dulce (12.0 ± 8.5 g DW m ⁻² ) were lower and water nutrient concentrations were higher than previously reported in the gulf. Shoot density (1513 ± 767 shoots m ⁻² ) was similar to previous reports. Stable isotope values in seagrass were −11.3 ± 1.0‰ δ ¹³ C and 1.2 ± 0.9‰ δ ¹⁵ N; while those in sediments were −26.1 ± 1.3 and 2.5 ± 0.9‰. In Golfo Dulce, isotopic values of both seagrass species do not overlap with other known primary producers. Management strategies should aim to minimize known seagrass stressors, protect potential seagrass habitat, and take into account the dynamic life strategies of the two seagrass species found.
... Sea Urchins are found in temperate, tropical, and subtropical seagrass beds [3] . They are considered as an integral part of seagrass ecosystem dynamics [43] . Despite a considerable number of sea urchins species (nearly about 15 species) are known to consume living seagrass tissue [3] ; their contribution in seagrass faunal composition is relatively low. ...
Aspects of the distribution, ecology and taxonomy of fibulariid species inhabiting seagrass beds at Hurghada and About Monkar Island, Northern Red Sea, Egypt were characterized. The fibulariid fauna comprised four species belonging to two genera, among them two endemic species to the Red Sea, Fibularia ovulum and Fibularia volva. On the other hand, the other two species, Fibulariella acuta and Fibularia cribellum were recorded for the first time from the Red Sea; they are considered new record to the Red Sea echinoderm fauna. The distribution of the recorded species from different regions of the Red Sea were compared with other adjacent areas as well as the ecological geographical distribution were discussed.
... Changes in the species composition of seagrass communities have been primarily attributed to bottom-up control mechanisms, such as resource availability (Touchette & Burkholder, 2000;Ralph et al., 2007), with top-down mechanisms, such as herbivory, only playing a minor role. The drastic declines of large herbivores, like green turtles, manatees, and dugongs, caused by human overexploitation (Jackson, 1997(Jackson, , 2001Hughes et al., 2004;Valentine & Duffy, 2006) has contributed to the undervaluation of top-down controls in seagrass communities. In recent decades, conservation strategies have led to local increases in abundance of green turtles (Chelonia mydas), with noticeable impacts of turtle grazing on seagrass communities (Zieman, Iverson & Ogden, 1984;Kaladharan et al., 2013;Kelkar et al., 2013;Molina-Hernández & Van Tussenbroek, 2014). ...
In the Caribbean, green turtles graze seagrass meadows dominated by Thalassia testudinum through rotational grazing, resulting in the creation of grazed and recovering (abandoned) patches surrounded by ungrazed seagrasses. We evaluated the seagrass community and its environment along a turtle grazing gradient; with the duration of (simulated) grazing as a proxy for the level of grazing pressure. The grazing levels consisted of Short-term (4 months clipping), Medium-term (8 months clipping), Long-term grazing (8 months of clipping in previously grazed areas), 8-months recovery of previously grazed patches, and ungrazed or unclipped patches as controls. We measured biomass and density of the seagrasses and rhizophytic algae, and changes in sediment parameters. Medium- and Long-term grazing promoted a shift in community species composition. At increasing grazing pressure, the total biomass of T. testudinum declined, whereas that of early-successional increased. Ammonium concentrations were highest in the patches of Medium-term (9.2 + 0.8 μM) and Long-term grazing levels (11.0 + 2.2 μM) and were lowest in the control areas (4.6 + 1.5 μM). T. testudinum is a late-successional species that maintains sediment nutrient concentrations at levels below the requirements of early-successional species when dominant. When the abundance of this species declines due to grazing, these resources become available, likely driving a shift in community composition toward a higher abundance of early-successional species.
... There can also be a complex mixture of autotrophic and heterotrophic microorganisms embedded in a matrix of organic detritus, the periphyton, that covers the harder surfaces of ponds (Azim et al., 2005). Periphyton is easily grazed upon by small invertebrates, fishes, and other aquatic animals (Jana et al., 2004;Valentine and Duffy, 2006) and hence may contribute considerably to the productivity of the pond. ...
In order to demonstrate that IMTA can be profitable and a good alternative to regular semi-intensive fish mariculture production in earthen ponds three different production treatments with distinct combinations of trophic levels were designed: (i) a combination of fish, filter feeders, phytoplankton and macroalgae, (ii) fish, filter feeders and phytoplankton and (iii) fish, phytoplankton and macroalgae, to evaluate the role of each trophic level within an Integrated Multi-Trophic Aquaculture system (IMTA). Each treatment was carried out under semi-intensive conditions with two replicates, in a total of 6 earthen ponds of 500 m2 surface and depth of 1.5 m. The results showed that fish, oyster, phytoplankton and macroalgae integrated aquaculture is a healthy sustainable production system for mariculture in earthen ponds, providing much more fish supply compared with the other two treatments. Ponds with filter feeders had significantly lower turbidity (Nephelometric Formazin unit (FNU) of 13 in the morning and 17 in the afternoon) when compared to ponds without filter feeders (16 FNU in the morning and 20 FNU in the afternoon) with increased light penetration throughout water column (61 and 55 cm transparency in ponds with filter feeders compared to 51 cm in ponds without filter feeders) and consequently higher photosynthetic activity with significantly higher dissolved oxygen (5.4 mg L−1 in the morning and 6.7 mg L−1 in the afternoon in ponds with filter feeders compared to 5.3 mg L−1 in the morning and 6.4 mg L−1 in the afternoon in ponds without filter feeders) and carbon sequestration (0.50 and 0.53 mg L−1 8 h−1 in ponds with filter feeders and 0.43 mg L−1 8 h−1 in ponds without filter feeders). In the fish, filter feeder, phytoplanton and macroalgae IMTA treatment, phytoplankton played a crucial role because they increased DO levels, removed the excess of nutrients from animal excretion, and was used as food by the filter feeders. Almost as important is the presence of filter feeders since they control the density of the microalgae and particulate matter in the ponds contributing to a more constant level of DO and higher transparency of the water column. The increased transparency and pond fertilization by oyster excretion, resulted in higher proliferation of phytoplankton (chlorophyll a concentrations of 16.5 μg L−1 and 20.2 μg L−1 in ponds with filter feeder and 13.3 μg L−1 in ponds without filter feeder) with benefits not only for filter feeders themselves but also for the macroalgae. At the end there was higher water quality and higher savings (14% day−1) in the energy costs for pond aeration. Meagre, white seabream and flathead grey mullet enhance their performance in IMTA systems with the presence of filter feeders with food conversion rates (FCR) of 1.52 when compared with 2.46 in the regular semi-intensive system composed by fish, phytoplankton and macroalgae. Meagre grew significantly more in IMTA systems with controlled macroalgae while white seabream and flathead grey mullet enhance their performance in the presence of macroalgae. The results show that the fish, oyster, phytoplankton and macroalgae integrated production in earthen ponds is an improved system compared to the regular semi-intensive fish production. The enhanced water quality in these systems leads to improved fish performance and higher biomass production, and to reduction in the energy power used, contributing to greater profitability.
... range in size from small invertebrate mesograzers to megaherbivores such as waterfowl, turtles, manatees, and dugongs (Thayer et al. 1984;Valentine and Duffy 2006). Many of the herbivores in seagrass beds are also consumers of the epiphytes that grow on the seagrasses, and thus some grazers may provide a net benefit to the plants (Orth and Montfrans 1984). ...
... This taxon directly influences the seagrass bed by exerting both negative and positive top-down controls (Hemminga & Duarte, 2000;Holzer et al., 2011a), which promote changes in the sediment and nutrient availability for plants (Peterson & Heck, 2001;Reynolds et al., 2007). Molluscs can also act indirectly as important trophic connections within the food chain, and due to the considerable number of individuals present they can also be very important components of the high biological productivity of seagrass meadows (Bologna & Heck, 2002;Valentine & Duffy, 2006). ...
... Grazers tend to be more common than herbivores in marine meadows due to the greater difficulty of digestion and energetic response associated with consuming plant (i.e. seagrass) vs algal tissue (Bologna & Heck, 1999;Arroyo et al., 2006;Valentine & Duffy, 2006;Holzer et al., 2011aHolzer et al., , 2011b. ...
... The grazers E. affine, A. incerta and N. virginea, and the herbivorous S. viridis, were the gastropod species found at the four highest densities in the meadow of Barra Grande in this study. These two trophic guilds (grazers and herbivores) are closely linked to the dynamics within the H. wrightii meadow at Barra Grande because they are an important means of mobilizing carbon from the plants and the periphyton for transfer to higher trophic guilds (Valentine & Duffy, 2006). Grazers associated with marine angiosperms have been shown to exert direct control over epiphytic biomass (Valentine & Duffy, 2006), which could otherwise compromise the growth, distribution and productivity of marine angiosperms (Howard & Short, 1986). ...
This study aimed to qualitatively and quantitatively analyse the molluscan assemblages associated with a Halodule wrightii seagrass bed in a rarely studied area within a conservation unit in north-eastern Brazil. Seasonal and spatial changes in several seagrass meadow characteristics, including sediment, were evaluated to explain temporal and spatial variations in the molluscs found there. The molluscan community differed in its structure among periods and meadows, as well as in the composition of its infaunal and epifaunal assemblages. The results of this study indicated that molluscs are affected by the particular characteristics of a seagrass meadow, especially by its location in the intertidal zone, more than by the area of the meadow. Molluscs were also affected by other characteristics of the seagrass meadow, such as above-ground biomass and shoot density. Changes in all molluscan assemblages were also mediated by differences among months and seasons in this region of the western equatorial Atlantic, but not by seasonal changes of the meadow. The studied meadow was found to be one of the densest in Brazil, which has considerable importance to its associated fauna.
... From such studies it is clear that feedbacks in grazed ecosystems can influence seagrass species composition [15]. Therefore, quantifying how environmental factors regulate herbivore grazing rates is central to understanding system dynamics of seagrass ecosystems [16]. ...
The role of environmental-stress gradients in driving trophic processes like grazing, has potential to shape ecosystem responses to environmental change. In subtidal seagrass systems, however, the variation in top-down processes along stress gradients are poorly understood. We deployed herbivory assays using the five most common seagrass species of Shark Bay, to determine whether herbivory pressure changed across a salinity-stress gradient from oceanic (38 PSU) to hyper-saline (51 PSU) conditions. Seagrass tissue removed from herbivory assays by fishes decreased as environmental stress increased, and herbivores consumed greater amounts of tropical seagrass species compared to the temperate species that dominate seagrass cover in Shark Bay. This heightened consumption was correlated with enriched seagrass nutrient concentrations. Our work suggests there’s a fundamental relationship between trophic interactions and environmental conditions within complex marine settings. Abiotic stressors like salinity directly impact seagrass communities physiologically; however we show that salinity stressors also shift biotic interactions, indirectly influencing grazing rates and thus having a greater effect on seagrasses than physiological impacts alone. In Shark Bay where restoration efforts are being employed to address large scale loss of seagrasses, the relationship between herbivory pressure and salinity-stress could therefore prove crucial to restoration success.
