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Influence of microscale turbulence on the phytoplankton of a temperate coastal embayment, Western Australia
... The horizontal and vertical spatial variability of phytoplankton concentrations and their temporal succession have long been recognized as being of central ecological importance in the management of aquatic systems [e.g., Denman and Gargett, 1983;Machado et al., 2014;Vidal et al., 2014]. It is known that both the scales of the phytoplankton patches and the phytoplankton species succession are influenced by the time scales of the physical mixing processes at both the patch scale and the phytoplankton cell scales [e.g., Kierstead and Slobodkin, 1953;Durham et al., 2013;Machado et al., 2014]. ...
... The horizontal and vertical spatial variability of phytoplankton concentrations and their temporal succession have long been recognized as being of central ecological importance in the management of aquatic systems [e.g., Denman and Gargett, 1983;Machado et al., 2014;Vidal et al., 2014]. It is known that both the scales of the phytoplankton patches and the phytoplankton species succession are influenced by the time scales of the physical mixing processes at both the patch scale and the phytoplankton cell scales [e.g., Kierstead and Slobodkin, 1953;Durham et al., 2013;Machado et al., 2014]. The motion in the water column of a stratified lake experiences a great range of scales, all influencing the physical aspects of the phytoplankton habitat. ...
... All rights reserved. phytoplankton succession [George and Heaney, 1978;Cuypers et al., 2011;Machado et al., 2014]. Machado et al. [2014] recently showed that patch scales and the biomass concentration of a phytoplankton bloom depend on the time scales of the vertical and horizontal mixing processes because within a patch an equilibrium state is reached between the biological growth rate and horizontal dispersion, as first hypothesised by Kierstead and Slobodkin [1953]. ...
A combination of field observations and 3D hydrodynamic simulations were used to identify the phytoplankton species and to estimate the various time scales of the dominant physical and biological processes in Lake Iseo, a deep subalpine lake located in northern Italy, during a stratified period (July 2010). By ordering the rate processes time scales we derive a phytoplankton patch categorization and growth interpretation that provides a general framework for the spatial distribution of phytoplankton concentration in Lake Iseo and illuminates the characteristics of their ecological niches. The results show that the diurnal surface layer was well mixed, received strong diurnal radiation, had low phosphorus concentrations and the phytoplankton biomass was sustained by the green alga Sphaerocystis schroeterii. The vertical mixing time scales were much shorter than horizontal mixing time scales causing a depth-uniform chlorophyll a concentration. The horizontal patch scale was determined by horizontal dispersion balancing the phytoplankton growth time scale, dictating the success of the observed green algae. The strongly stratified nutrient-rich metalimnion had mild light conditions and Diatoma elongatum and Planktothrix rubescens made up the largest proportions of the total phytoplankton biomass at the intermediate and deeper metalimnetic layers. The vertical transport time scales were much shorter than horizontal transport and vertical dispersion leading to growth niche for the observed phytoplankton. The study showed that time-scale hierarchy mandates the essential phytoplankton attributes or traits for success in a particular section of the water column and/or water body. This article is protected by copyright. All rights reserved.
... The impact of turbulence on diatoms species has increasingly become a subject of study, uncovering a multitude of adaptive responses to turbulent stress including nutrients absorption, light utilization, and cell morphology alterations (MaChado et al., 2014;Dell'Aquila et al., 2017). Diatoms, especially the chain forming ones, are considered to optimally thrive in turbulent environments . ...
... They are known for their siliceous shells, which confers a certain degree of resistance to the shear stress induced by turbulence. Investigations into the influence of microscale turbulence on the large phytoplankton of a coastal embayment found that formation of siliceous cell chains provide diatoms an advantage in accessing nutrients in turbulent coastal ecosystems (MaChado et al., 2014). Amato et al. studied the morphological response of Chaetoceros decipiens to turbulence and suggested that this species produced shorter chains in turbulence (Amato et al., 2017). ...
The effects of turbulence on phytoplankton growth have received considerable attentions. However, the complexity of turbulence poses a significant challenge to its systematic characterization in the laboratory, resulting in relatively limited data on the effects of turbulence on several algal species. Here, a laboratory turbulence simulation system was set up to systematically investigate the growth of three common diatom species (Thalassiosira pseudonana, Skeletonema costatum, and Phaeodactylum tricornutum) under stationary and turbulent conditions (at 60, 120, 180 rpm), and measurements were taken for the algal biomass, algal photosynthetic activity, and nutrients consumption. By comparing the growth of three algae species under different turbulence exposure intensities, this study found that different algae exhibit varying sensitivities to turbulence, and therefore have different shear thresholds. Meanwhile, cell morphology is the key factor influencing the different shear threshold values observed in the three diatom species. Additionally, turbulence could impact algal aggregation and light availability, and dramatically improve nutrient uptake by phytoplankton. Our study will provides theoretical support for future endeavors in using turbulence to cultivate phytoplankton or combat algal blooms.
... To date, more than 100 MCs variants have been discovered, among which microcystin-RR (MC-RR) are the most frequently detected one (Corbel et al., 2016;Wu et al., 2010). It is widely reported that phytoplankton communities can be regulated not only by physical and chemical parameters such as temperature, nutrients, PH, transparency and illumination (Çelekli et al., 2014;Zhao et al., 2015), but also by hydrodynamic alternations, for example, flow velocity, inflow and turbulence (Choi et al., 2017;Machado et al., 2014;Rao et al., 2017). ...
... A wide range factors in aquatic environments, including turbulence mixing, nutrient concentrations and SPM content are responsible for controlling the growth and physiological activities of phytoplankton (Machado et al., 2014;Marisol et al., 2018;Marmorino et al., 2017). Phytoplankton density changes are also not controlled by individual factors but by a series of environmental parameters (He et al., 2017;Heisler et al., 2008). ...
The effect of turbulence on phytoplankton growth has been widely studied; however, its effects with respects to suspended particulate matter (SPM) on the development of phytoplankton communities and the behavioral responses of phytoplankton to turbulence and SPM are poorly understood. Here, an approximately homogeneous turbulence simulation system (AHTS, mainly consisting of an oscillating-grid apparatus) was established to gain insight into the mechanisms underlying phytoplankton community responses in turbid, well-mixed waters. The results revealed that maintaining the turbulence dissipation rates (Ɛ) of 2.25 × 10⁻³ and 1.80 × 10⁻² m²/s³ caused significant reductions in algal density, and the effects could be substantially enhanced when 500 mg/L of SPM were added before day 12. In contrast to the constant decrease of algal density for the Ɛ of 2.25 × 10⁻³ m²/s³, a dramatic increase in the phytoplankton density occurred after 16 days of incubation for a Ɛ of 1.80 × 10⁻² m²/s³, irrespective of SPM. Addition of SPM in the Ɛ of 1.80 × 10⁻² m²/s³ treatments did not considerably affect the algal density profile compared to that without SPM, of which unicellular algae decreased and colonial algae dominated the phytoplankton community. On the other hand, the phytoplankton can regulate the SPM properties. During the 18 days’ coincubation, extracellular polymeric substances (EPS) released from algal cells induced larger particle sizes and round surfaces of SPM, which can reduce the damage received to algal cells. Here we demonstrated that the phytoplankton communities could actively counteract the effects of turbulence + SPM and adapt the couple stress, jointly through the release of EPS, the modification of SPM surface properties and the conversion of their assemblage pattern, thereby contributing to rebalance the ecosystem. These findings highlight the strategies employed during the reconstruction of phytoplankton under the dual effects of turbulence and SPM for the first time, consequently enabling the forecasting of the dominant species of phytoplankton in turbulent waters.
... Microturbulence may also favour unicellular autotrophs, bringing them into the upper illuminated layer of the ocean, the euphotic zone 5 . The arguments above, together with cell size and the ability to produce chains [6][7][8][9][10] , are frequently invoked to explain why diatoms should be favoured in turbulent environments 11,12 . ...
... This implies that in nutrient repletion (i.e. when nutrients are never limiting for growth) diatoms should not be affected by turbulence. Previous studies have demonstrated that diatoms are favoured by turbulence in mesocosm experiments 7,9,42,43 and in situ 44 , but how the response is triggered is unclear. ...
Diatoms are a fundamental microalgal phylum that thrives in turbulent environments. Despite several experimental and numerical studies, if and how diatoms may profit from turbulence is still an open question. One of the leading arguments is that turbulence favours nutrient uptake. Morphological features, such as the absence of flagella, the presence of a rigid exoskeleton and the micrometre size would support the possible passive but beneficial role of turbulence on diatoms. We demonstrate that in fact diatoms actively respond to turbulence in non-limiting nutrient conditions. TURBOGEN, a prototypic instrument to generate natural levels of microscale turbulence, was used to expose diatoms to the mechanical stimulus. Differential expression analyses, coupled with microscopy inspections, enabled us to study the morphological and transcriptional response of Chaetoceros decipiens to turbulence. Our target species responds to turbulence by activating energy storage pathways like fatty acid biosynthesis and by modifying its cell chain spectrum. Two other ecologically important species were examined and the occurrence of a morphological response was confirmed. These results challenge the view of phytoplankton as unsophisticated passive organisms.
... Only a limited number of studies have previously used microstructure observations to investigate the relationship between turbulence and phytoplankton activity and composition. Among those, most studies were focused on analyzing the role of mixing and nutrient supply as drivers of phytoplankton community structure (Sharples et al., 2007(Sharples et al., , 2009Machado et al., 2014;Villamaña et al., 2017Villamaña et al., , 2019. As far as we know only two studies have used microturbulence observations to investigate phytoplankton growth under the CTH framework. ...
Previous studies focused on understanding the role of physical drivers on phytoplankton bloom formation mainly used indirect estimates of turbulent mixing. Here we use weekly observations of microstructure turbulence, dissolved inorganic nutrients, chlorophyll a concentration and primary production carried out in the Ría de Vigo (NW Iberian upwelling system) between March 2017 and May 2018 to investigate the relationship between turbulent mixing and phytoplankton growth at different temporal scales. In order to interpret our results, we used the theoretical framework described by the Critical Turbulent Hypothesis (CTH). According to this conceptual model if turbulence is low enough, the depth of the layer where mixing is active can be shallower than the mixed-layer depth, and phytoplankton may receive enough light to bloom. Our results showed that the coupling between turbulent mixing and phytoplankton growth in this system occurs at seasonal, but also at shorter time scales. In agreement with the CTH, higher phytoplankton growth rates were observed when mixing was low during spring-summer transitional and upwelling periods, whereas low values were described during periods of high mixing (fall-winter transitional and downwelling). However, low mixing conditions were not enough to ensure phytoplankton growth, as low phytoplankton growth was also found under these circumstances. Wavelet spectral analysis revealed that turbulent mixing and phytoplankton growth were also related at shorter time scales. The higher coherence between both variables was found in spring-summer at the ~16–30 d period and in fall-winter at the ~16–90 d period. These results suggest that mixing could act as a control factor on phytoplankton growth over the seasonal cycle, and could be also involved in the formation of occasional short-lived phytoplankton blooms.
... Estuaries, as the seaward continuum between lands and oceans, generally have enriched levels of nutrients mainly derived from river discharge, atmosphere, and ocean as well, which regulates phytoplankton production in coastal marine ecosystems [9][10][11][12]. Excess nutrients from farmlands entering into coastal ecosystems as organic and inorganic compounds pose a major threat to the ecological stability of this environment [13]. ...
The Pearl River estuary is an ecologically dynamic region located in southern China that experiences strong gradients in its biogeochemical properties. This study examined the seasonality of nutrient dynamics, identified related environmental responses, and evaluated how river discharge regulated nutrient sink and source. The field investigation showed significant differences of dissolved nutrients with seasons and three zones of the estuary regarding the estuarine characteristics. Spatially, nutrients exhibited a clear decreasing trend along the salinity gradient; temporally, their levels were obviously higher in summer than other seasons. The aquatic environment was overall eutrophic, as a result of increased fluxes of nitrogen and silicate. This estuary was thus highly sensitive to nutrient enrichment and related pollution of eutrophication. River discharge, oceanic current, and atmospheric deposition distinctly influenced the nutrient status. These factors accordingly may influence phytoplankton that are of importance in coastal ecosystems. Phytoplankton (in terms of chlorophyll) was potentially phosphate limited, which then more frequently resulted in nutrient pollution and blooms. Additionally, the nutrient sources were implied according to the cause–effect chains between nutrients, hydrology, and chlorophyll, identified by the PCA-generated quantification. Nitrogen was constrained by marine-riverine waters and their mutual increase-decline trend, and a new source was supplemented along the transport from river to sea, while a different source of terrestrial emission from coastal cities contributed to phosphate greatly.
... Estuaries and adjacent coastal waters, which are known as transitional zones due to their positions at the terrestrial, freshwater, and marine interface, are of primary ecological importance in marine management schemes (Cao et al., 2011;Vidal et al., 2014;Machado et al., 2014). As a result of the complex interactions between seawater and freshwater, estuaries are generally characterized by strong physical and biogeochemical gradients of various factors, including dissolved oxygen (DO), chemical oxygen demand (COD), salinity, water temperature (T), suspended sediment (SS), and dissolved nutrients (e.g., ammonium, NH 4 ; nitrate, NO 3 ; nitrite, NO 2 ; phosphorus, PO 4 ; and silicate, Si) (Yin et al., 2001Li et al., 2014;Lu and Gan, 2015). ...
... Previous studies have reported that phytoplankton variability is influenced by a wide range of abiotic environments. For example, a change in salinity may affect the phytoplankton community (Lionard et al., 2005;Jiang et al., 2014); tidal forcing affects the vertical mixing process, which controls the vertical distribution of phytoplankton (Woernle et al., 2014;Machado et al., 2014); sediment absorbs and scatters light intensity, implying that phytoplankton are limited by light availability in the high turbidity zone (Byun et al., 2007;Shi et al., 2017); nitrogen may limit phytoplankton growth, whereas in some cases, phosphate availability plays a key role in regulating phytoplankton population numbers (Davey et al., 2008;Kim et al., 2014). ...
... Another approach is to use different proxies for quantifying turbulence and nutrient supply in the field (Bowman et al., 1981;Cermeño et al., 2008;Irwin et al., 2012;Pearman et al., 2017). Currently, instruments designed to measure the dissipation rate of turbulent kinetic energy and/or thermal variance (Prandke and Stips, 1998;Stevens et al., 1999;Wolk et al., 2002) allow the study of mixing (Machado et al. 2014) and nutrient supply (Fernández-Castro et al., 2015;Mouriño-Carballido et al., 2016;Sharples et al., 2007;Villamaña et al., 2017) as drivers of phytoplankton community structure in the field. Moreover, the original Margalef's mandala was constrained by the sampling procedures of its era (Wyatt, 2014). ...
We investigate the role of mixing, through its effect on nutrient and light availability, as a driver of phytoplankton community composition in the context of Margalef’s mandala. Data on microstructure turbulence, irradiance, new nitrogen supply and phytoplankton composition were collected at 102 stations in three contrasting marine environments: the Galician coastal upwelling system of the northwest Iberian Peninsula, the northwestern Mediterranean, and the tropical and subtropical Atlantic, Pacific and Indian oceans. Photosynthetic pigments concentration and microscopic analysis allowed us to investigate the contribution of diatoms, dinoflagellates, pico- and nanoeukaryotes, and cyanobacteria to the phytoplankton community. Simple linear regression was used to assess the role of environmental factors on community composition, and environmental overlap among different phytoplankton groups was computed using nonparametric kernel density functions. Mixing and new nitrogen supply played an important role in controlling the phytoplankton community structure. At lower values of mixing and new nitrogen supply cyanobacteria dominated, pico- and nanoeukaryotes were dominant across a wide range of environmental conditions, and finally enhanced new nitrogen supply was favourable for diatoms and dinoflagellates. Dinoflagellates were prevalent at intermediate mixing levels, whereas diatoms spread across a wider range of mixing conditions. Occasional instances of enhanced diatom biomass were found under low mixing, associated with the high abundance of Hemiaulus hauckii co-occurring with high N2 fixation in subtropical regions, and with the formation of thin layers in the Galician coastal upwelling. Our results verify the Margalef’s mandala for the whole phytoplankton community, emphasizing the need to consider nutrient supply, rather than nutrient concentration, as an indicator of nutrient availability.
... As turbulence became stronger, energy dissipation increased, which caused the Batchelor microscale to decrease. Thus, nutrient diffusion dominated by fluctuating turbulence was more closely related to nutrient uptake by algal cells (Machado et al., 2014). Based on this information, we chose turbulent dissipation, which according to the Batchelor microscale functioned as a hydrodynamic indicator describing the effect of nutrient uptake on growth. ...
Hydrodynamic conditions play a key role in algal blooms, which have become an increasing threat to aquatic environments, especially reservoirs. Microcystis aeruginosa is a dominant species in algal blooms in reservoirs and releases large amounts of algal toxins during algal bloom events. The algal growth characteristics and the corresponding mechanism of the influence of hydrodynamic conditions were explored using custom hydraulic rotating devices. The long-term experimental results were as follows: (1) a moderate flow velocity increased the algal growth rate and prolonged algal lifetime relative to static water; (2) moderate water turbulence promoted energy metabolism and nutrient absorbance in algal cells; (3) moderate shear stress reduced oxidation levels in algal cells and improved algal cell morphology; (4) under hydrodynamic treatment, algal cell deformation was confirmed by scanning electron microscopy (SEM), and a high shear stress of 0.0104 Pa induced by a flow of 0.5 m/s may have destroyed cell morphology and disturbed reactive oxygen species (ROS) metabolism; (5) algal cell morphology evaluation (including circle ratio, eccentricity, diameter increasing rate, and deformation rate) was established; (6) based on algal growth status and specific effects, five independent intervals (including 'positive-promotion', 'middle-promotion', 'negative-promotion', 'transition', and 'inhibition') and the hydrodynamic threshold system (including flow velocity, turbulent dissipation, and shear stress) were established; and (7) for M. aeruginosa, the optimum flow velocity was 0.24 m/s, and the static-equivalent flow velocity was 0.47 m/s. These results provide a basic summary of the hydrodynamic effects on algal growth and a useful reference for the control of M. aeruginosa blooms in reservoirs.
... Additionally, an intensive field study was conducted over 9 d, from 24 May to 01 June 2012 where profiling was conducted using the Portable Flux Profiler (PFP, Machado et al. 2014). The PFP included a fine scale profiler that measured pressure, temperature, conductivity, dissolved oxygen, pH, photosynthetic active radiation (PAR), and turbidity (Imberger and Head 1994). ...
Many factors including depth, vegetation density, wind, and gyres may act to influence the littoral exchange in a water body but very few studies have investigated the interaction between more than two of these factors at any time. To investigate these controls on horizontal exchange in a large tropical reservoir, we conducted a 9-d intensive field experiment in Lake Argyle, Western Australia. The experiment began with a 7-d cooling period that generated water in the shallows of the reservoir embayments that was persistently cooler than the interior. This led to an underflow of dense water that moved from the lake boundary toward the center of the reservoir. A three-dimensional hydrodynamic model (ELCOM) was able to adequately reproduce this thermal structure and was used to demonstrate its sensitivity to wind-sheltering effects and submerged macrophyte presence. Further analysis of the predictions indicated that when the ratio of the shear and buoyancy force, averaged over 6 h (B6h) and 6-h averaged wind speed in the direction of the embayment (U6h) was (1) greater than 0.5 ms−1 and 4.5 ms−1 respectively, the exchange was dominated by a topographic gyre formation, (2) when 0.1 < B6h < 0.5 and 2.4 ms−1 < U6h < 4.5 ms−1, the resulting circulation was a combination of differential cooling flows and a topographic gyre circulation, and (3) when U6h fell below 2.4 ms−1, purely buoyancy driven flow occurred but only if the buoyancy forces across the embayments were an order of magnitude greater than wind-induced velocity shear.
... The ability of such a probe, the Fluoro-Probe (FP-BBE FluoroProbe, Moldaenke, Germany), to estimate phytoplankton biomass and taxonomic composition has been demonstrated in different water bodies, mainly in temperate climate regions (Catherine et al., 2012;Gregor et al., 2005;Gregor and Maršálek, 2004;Rolland et al., 2010). This probe has been used to study the temporal variability in marine phytoplankton photosynthetic activity (Houliez et al., 2015), the impact of the resuspension of contaminated sediment on phytoplankton (Lafabrie et al., 2013), the influence of microscale turbulence on coastal phytoplankton (Machado et al., 2014), the relationships between toxin and cyanobacteria concentrations in lakes and reservoirs (Salmaso et al., 2014). FP also provided high-resolution data for three-dimensional hydrodynamic and ecological lake modelling (Carraro et al., 2012;Marti et al., 2011). ...
... Biswas et al. (2010) reported from long-term observations of a phytoplankton data set from a mangrove dominated estuarine system of India (NW Bay of Bengal) that S. costatum blooms were noticed during turbid monsoonal months when silicate concentrations and TSM were significantly high. Machado et al. (2014) investigated the impact of small scale turbulence under low nutrient condition (nitrate) in a temperate coastal embayment, Western Australia and observed that mainly the chain forming diatoms including the Skeletonema sp. were able to benefit by siliceous chain formation under turbulent and low nutrient conditions. Consistent lower values of N:P relative to the classical Redfield values of 16:1 (Redfield et al., 1963) indicate that in the study area the supply of DIN is lesser relative to phosphate and hence phosphate limitation was never noticed. ...
In recent decades, material fluxes to coastal waters from various land based anthropogenic activities have significantly been enhanced around the globe which can considerably impact the coastal water quality and ecosystem health. Hence, there is a critical need to understand the links between anthropogenic activities in watersheds and its health. Kakinada Bay is situated at the SW part of the Bay of Bengal, near to the second largest mangrove cover in India with several fertilizer industries along its bank and could be highly vulnerable to different types of pollutants. However, virtually, no data is available so far reporting its physicochemical status and microalgal diversity at this bay. In order to fill this gap, we conducted three time series observations at a fixed station during January, December and June 2012, at this bay measuring more than 15 physical, chemical and biological parameters in every 3 h over a period of 36 h in both surface (0 m) and subsurface (4.5 m) waters. Our results clearly depict a strong seasonality between three sampling months; however, any abnormal values of nutrients, biological oxygen demand or dissolved oxygen level was not observed. A Skeletonema costatum bloom was observed in December which was probably influenced by low saline, high turbid and high Si input through the river discharge. Otherwise, smaller diatoms like Thalassiosira decipiens, Thalassiothrix frauenfeldii, and Thalassionema nitzschioides dominated the bay. It is likely that the material loading can be high at the point sources due to intense anthropogenic activities, however, gets diluted with biological, chemical and physical processes in the offshore waters.
This study investigates the nutrient content, water quality, and its relationship with chlorophyll-a in the Socah Estuary, Bangkalan Regency. The research was conducted in four stations. Determination of nitrate and nitrite in samples was performed by the colorimetric method. Meanwhile, phenate and Nessler reagent spectrophotometric methods were used to determine the ammonia and ammonium content in samples, respectively. The phosphate and chlorophyll-a were determined using ascorbic acid and spectrophotometry, respectively. The Spearman rank correlation was used to measure the degree of relationship between nutrients and chlorophyll- a content. Based on the results of Spearman rank correlation, the nitrate showed the strongest correlation (rho = 0.76) compared to other nutrients. Moreover, the increased chlorophyll-a level in the water will be closely related to the increased nitrate level. According to the value of Secchi depth, nutrients, and chlorophyll-a concentration, eutrophic is a trophic state in Socah Estuary. This condition is caused by the input of shrimp pond wastewater which triggers nutrient enrichment, decreasing the water quality and increasing the phytoplankton biomass in Socah Estuary.
In this paper, a solid-gas-liquid phase double slip model for flat plate was established by correcting the boundary conditions with the boundary layer slip velocity. According to Karman boundary layer momentum theorem, the velocity distributions both in the solid-gas boundary layer and in the gas-liquid boundary layer were deduced. The boundary layer thickness, displacement thickness and momentum loss thickness, as well as friction coefficient were also obtained, which offer an understanding for the laminar boundary layer characteristic. The theoretical results indicate that the flow field velocity near the solid surface can be increased by the micro-bubbles on the solid surface. In the case of low Reynolds number, the boundary slip phenomenon occurs in the bubble surface boundary layer, which can lead to the resistance reduction effect. The bubble with higher height offers a longer boundary ship length, and also a more significant resistance reduction. Experimental research was also made by establishing a holographic optical experiment system and the results show that there is higher flow velocity distribution in the bubble surface boundary layer.
This contribution reviews relationships between turbulence and marine phytoplankton ecology, ranging from the mesoscale to the smallest scales. Phytoplankton life-forms, considered to be survival strategies in a turbulent environment, are briefly presented. The importance of mesoscale hydrodynamics on phytoplankton distributions and organisms physiology and behaviour, are examined. Finally, direct effects of small scale turbulence on phytoplankton are considered.
This paper is written in honour of the influential career of Professor T.J. Smayda. It celebrates the brief, but productive, period in which we collaborated on the formulation of the principles and mechanisms of community assembly in plankton. The topics considered then provide the structure of the present paper - the origin of communities (nascence), the common and differential requirements of phytoplankton species that might affect their selection, the general and specialist adaptive traits that differentiate their performances and their relative successes, and the sensitivity of performances to sub-ideal or hostile environmental conditions. In all these instances, it is the reactivity of the net dynamic responses of the species present to imposed environmental variations that shapes the structure of the assemblage and the composition of the community. These are the high-level properties that emerge from the behaviour of low-level components. The patterns that emerge at the community level - selection by performance or competition, dominance and temporal succession - are qualitatively predictable and, at the assemblage level, are amenable to quantitative modelling. In turn, it is theoretically possible to make deductions about the likelihood of biological events that involve species occurring in abundances sufficient to affect water quality and/or constitute risks to human health (harmful algal blooms).
Diatoms developed a variety of mechanisms to form chain-like colonies, resulting in diverse morphologies and bulk mechanical properties. These properties affect translation, rotation, and deformation of colonies in ambient flows as well as their susceptibility to breakage by flow- and grazer-induced forces. Morphological characteristics of diatom chains have been extensively studied, yet no studies have examined their mechanical properties. We studied the flexibility of four morphologically distinct species (Lithodesmium undulatum, Stephanopyxis turris, Lauderia annulata, and Guinardia delicatula) by measuring their deflections when held across a capillary tip in developing pipe flow and applying simple beam theory and a finite-difference analysis of curvature to calculate flexural stiffness. Flexural stiffness varies greatly, with at least four orders of magnitude difference among the examined species (from 1.7 × 10−13 N m2, the most rigid, to 1.3 × 10−17 N m2, the most flexible), but two other species (Melosira nummuloides and a Thalassiosira sp.) were too flexible to measure with our apparatus. Vulnerability to breakage by flow also varied between species and, for species with heavily silicified joints between cells, was enhanced under nutrient depletion. These results highlight yet another attribute underlying the biodiversity of diatoms and their potential for utilizing highly differentiated ecological niches. Quantitative information from this study can now be used in the design of more mechanically realistic models that capture the dynamic coupling between elastic particles and flow to study diatom-flow interactions and their effects on nutrient acquisition, encounter with grazers, aggregate formation, and settling.
The influence of different nutrient sources on the seasonal variation of nutrients and phytoplankton was assessed in the northern area of the Perth coastal margin, south–western Australia. This nearshore area is shallow, semi-enclosed by submerged reefs, oligotrophic, nitrogen-limited and receives sewage effluent via submerged outfalls. Analysis of 14 year of field observations showed seasonal variability in the concentration of dissolved inorganic nitrogen and phytoplankton biomass, measured as chlorophyll-a. For 2007–2008, we quantified dissolved inorganic nitrogen inputs from the main nutrient sources: superficial runoff, groundwater, wastewater treatment plant effluent, atmospheric deposition and exchange with surrounding coastal waters. We validated a three-dimensional hydrodynamic-ecological model and then used it to assess nutrient-phytoplankton dynamics. The model reproduced the temporal and spatial variations of nitrate and chlorophyll-a satisfactorily. Such variations were highly influenced by exchange through the open boundaries driven by the wind field. An alongshore (south–north) flow dominated the flux through the domain, with dissolved inorganic nitrogen annual mean net-exportation. Further, when compared with the input of runoff, the contributions from atmospheric-deposition, groundwater and wastewater effluent to the domain’s inorganic nitrogen annual balance were one, two and three orders of magnitude higher, respectively. Inputs through exchange with offshore waters were considerably larger than previous estimates. When the offshore boundary was forced with remote-sensed derived data, the simulated chlorophyll-a results were closer to the field measurements. Our comprehensive analysis demonstrates the strong influence that the atmosphere–water surface interactions and the offshore dynamics have on the nearshore ecosystem. The results suggest that any additional nutrient removal at the local wastewater treatment plant is not likely to extensively affect the seasonal variations of nutrients and chlorophyll-a. The approach used proved useful for improving the understanding of the coastal ecosystem.
Active turbulence in lakes is confined to the surface mixed layer, to boundary layers on the lake sides and bottom, and to turbulent patches in the interior. The density stratification present in most lakes fundamentally alters the pathways connecting external mechanical energy inputs, for example by wind, with its ultimate fate as dissipation to heat; the density stratification supports internal waves and intrusions that distribute the input energy throughout the lake. Intrusions may be viewed as internal waves with zero horizontal wavenumber and are formed each time localised mixing occurs in a stratified fluid. Intrusions are also formed in the epilimnion by differential heating or cooling and by differential deepening. The fraction of lake volume below the diurnal mixed layer that is subject to active turbulence is very small, probably of the order of 1% or less in small to medium‐sized lakes. By contrast, in the surface mixed layer, turbulence is less intermittent and maintains phytoplankton in suspension and controls their exposure to the underwater solar flux. Nutrient transport to individual cells depends not only on the cell Reynolds number but also on the Peclet number, which, if large, implies enhanced mass transfer above purely diffusive values.
A method to match the response of the SBE-3 temperature sensor and the SBE-4 conductivity cell is described. The technique uses a recursive filter in the time domain, which allows direct calculation of salinity and density, and thus offers a significant computational advantage over other methods. The response of any sensor may be matched or sharpened using this method provided that the sensor can be modeled appropriately. Using this method the useful bandwidth of the SBE-3 temperature sensor may be improved by a factor of between 3 and 7, depending on the permissible signal-to-noise ratio degradation. It is also possible to match the SBE-3 and SBE-4 responses closely and thus remove spikes in the profiles of calculated salinity and density.
From the literature we obtained experimental data on the effects of small-scale turbulence on plankton at the organism scale. Normalized rates in response to turbulence were calculated in the present study for parameters related to growth,ingestion and energy expenditure. Growth rates are, in general, negatively affected by turbulence. Nevertheless, the data are highly biased towards a specific group of organisms, dinoflagellates, which could have peculiar physiological impediments under turbulence. Ingestion rates seem to be increased by turbulence, especially at low and intermediate levels. The few data available on energy expenditure indicate increases under turbulent conditions; Although many experiments were conducted at very high levels of turbulence with respect to oceanic conditions, in most studies the Kolmogorov size microscale remained larger than the size of the planktonic organisms. Thus, organisms responded to turbulence or turbulence-derived stimuli below the Kolmogorov lengthscale. The ecological. relevance of interactions between turbulence and other size-related parameters, such as reactive distances, mean free paths and Batchelor microscales, are mentioned mainly in terms of predation, nutrient uptake and competition. Specifically, mean free paths seem relevant to determine encounter rates for organisms that show concentrations similar to those typical for large protozoa and algae (both within these organisms and between them and their potential predators). Characteristic time scales for different planktonic organisms are also explored in relation to the corresponding Kolmogorov time microscale. All planktonic organisms will experience fluctuations in the nutrient or food-particle fields over their lifetime. If other characteristic times are considered, such as the time between particle capture or nutrient uptake events, fluctuations may or may not be experienced over those times depending on the feeding mode and the turbulence intensity.
The mass balance and activity ratios of naturally occurring radium (Ra) isotopes (223Ra, 224Ra, 226Ra, 228Ra) were investigated in Cockburn Sound (Western Australia) to further understand submarine groundwater discharge (SGD) into these coastal waters. Water samples from 11 marine stations and 20 groundwater sites (encompassing three aquifer layers) were analysed for Ra at four times, at the end of winter (September 2003), early summer (December 2003), late summer (March 2004) and mid-winter (July 2004). Variable isotopic signatures of groundwater suggested that vertical mixing may occur between different aquifer layers and two isotopically different water sources were identified in marine waters both inside and outside of Cockburn Sound. A mass balance of the long lived radium isotopes (228Ra and 226Ra) produced a range of discharge estimates from 0.8 × 107 L day−1 in late summer to 2.7 × 107 L day−1 at the end of winter. Signatures of radium isotopes in the coastal waters suggested that groundwater discharge was not confined to the shoreline and may have occurred from a number of aquifer sources at a temporally variable scale.
The role of fluid motion in delivery of nutrients to phytoplankton cells is a fundamental question in biological and chemical oceanography. In the study of mass transfer to phytoplankton, diatoms are of particular interest. They are non-motile, are often the most abundant components in aggregates and often form chains, so they are the ones expected to benefit most from enhancement of nutrient flux due to dissipating turbulence. Experimental data to test the contribution of advection to nutrient acquisition by phytoplankton are scarce, mainly because of the inability to visualize, record and thus imitate fluid motions in the vicinities of cells in natural flows. Laboratory experiments have most often used steady Couette flows to simulate the effects of turbulence on plankton. However, steady flow, producing spatially uniform shear, fails to capture the diffusion of momentum and vorticity, the essence of turbulence. Thus, numerical modelling plays an important role in the study of effects of fluid motion on diffusive and advective nutrient fluxes. In this paper we use the immersed boundary method to model the interaction of rigid and flexible diatom chains with the surrounding fluid and nutrients. We examine this interaction in two nutrient regimes, a uniform background concentration of nutrients, such as might be typical of an early spring bloom, and a contrasting scenario in which nutrients are supplied as small, randomly distributed pulses, as is more likely for oligotrophic seas and summer conditions in coastal and boreal seas. We also vary the length and flexibility of chains, as whether chains are straight or bent, rigid or flexible will affect their behaviour in the flow and hence their nutrient fluxes. The results of numerical experiments suggest that stiff chains consume more nutrients than solitary cells. Stiff chains also experience larger nutrient fluxes compared to flexible chains, and the nutrient uptake per cell increases with increasing stiffness of the chain, suggesting a major advantage of silica frustules in diatoms.
To explore new mechanisms for planktonic thin layer formation, particle and continuum models of gyrotactically swimming phy-toplankton are embedded in simulations of a dynamically unstable stratified shear layer. Two trapping mechanisms are observed in the developing Kelvin-Helmholtz (K-H) billow train. Within the K-H billows, a particle can remain preferentially in downwelling regions, cancelling its upward swimming motion. In the braids that separate the billows, intense shear defeats the gyrotactic stabi-lization mechanism and causes cells to tumble. Particle and continuum models are compared statistically to reveal both consistencies and weaknesses in each. A scaling based on Reynolds number and swimming speed is used to predict the maximum concentration generated by an instability event. Although K-H billows are short lived in comparison with planktonic thin layers observed in the coastal oceans, the resulting trapping causes aggregation at rates which could account for the observed cell concentrations.
This article summarizes the results presented in a series of invited contributions which were submitted to celebrate the fiftieth
anniversary of publication of the seminal article “Homage to Santa Rosalia or why are there so many kinds of animals” by G.E.
Hutchinson. The authors were asked to explore old and new paradigms of biodiversity in aquatic ecosystems. The contributions
by Hutchinson in this field are truly landmarks in the history of modern ecological sciences. The authors of the contributed
articles, stimulated by one of the most fruitful concept articles in ecology that has appeared over the last half century,
have shown that scientific investigation, although still seeking the causes underlying diversity maintenance, is moving toward
(i) the understanding of the functions and mechanisms of diversity in ecosystems and (ii) the evaluation of the role of a
diversified assemblage in ensuring the integrity of ecosystem services.
KeywordsCoexistence-Predation-Dispersal-Competition-Spatial heterogeneity-Rare species
In addition to answering Hutchinson’s question “Why are there so many species?”, we need to understand why certain species are found only under certain environmental conditions and not others. Trait-based approaches are being increasingly used in ecology to do just that: explain and predict species distributions along environmental gradients. These approaches can be successful in understanding the diversity and community structure of phytoplankton. Among major traits shaping phytoplankton distributions are resource utilization traits, morphological traits (with size being probably the most influential), grazer resistance traits, and temperature responses. We review these trait-based approaches and give examples of how trait data can explain species distributions in both freshwater and marine systems. We also outline new directions in trait-based approaches applied to phytoplankton such as looking simultaneously at trait and phylogenetic structure of phytoplankton communities and using adaptive dynamics models to predict trait evolution.
Keywords: Phytoplankton-Community structure-Functional diversity-Traits-Growth-Temperature-Harmful algal blooms-Adaptive dynamics
Here, we communicate a point of departure in the development of aquatic ecosystem models, namely a new community-based framework, which supports an enhanced and transparent union between the collective expertise that exists in the communities of traditional ecologists and model developers. Through a literature survey, we document the growing importance of numerical aquatic ecosystem models while also noting the difficulties, up until now, of the aquatic scientific community to make significant advances in these models during the past two decades. Through a common forum for aquatic ecosystem modellers we aim to (i) advance collaboration within the aquatic ecosystem modelling community, (ii) enable increased use of models for research, policy and ecosystem-based management, (iii) facilitate a collective framework using common (standardised) code to ensure that model development is incremental, (iv) increase the transparency of model structure, assumptions and techniques, (v) achieve a greater understanding of aquatic ecosystem functioning, (vi) increase the reliability of predictions by aquatic ecosystem models, (vii) stimulate model inter-comparisons including differing model approaches, and (viii) avoid 're-inventing the wheel', thus accelerating improvements to aquatic ecosystem models. We intend to achieve this as a community that fosters interactions amongst ecologists and model developers. Further, we outline scientific topics recently articulated by the scientific community, which lend themselves well to being addressed by integrative modelling approaches and serve to motivate the progress and implementation of an open source model framework.
Using a classical physiological model based on nutrient uptake kinetics, we explored the effect of turbulence on resource competition and succession between 2 phytoplankton functional groups on ecological and geological time scales. The 2 groups we considered are silica-precipitating diatoms and carbonate-precipitating coccolithophorids. Using published experimental laboratory data for parameterization, our model results suggest that diatoms dominate under highly turbulent regimes, while coccolithophorids tend to dominate under stable, nutrient-depleted conditions. We attribute the success of diatoms in highly dynamic systems to luxury uptake of nutrients afforded by the evolution of storage vacuoles. In contrast, coccolithophorids are more successful in resource-depleted waters, due to their lower minimum limiting-nutrient requirement (R*). We examine how these differences in nutrient acquisition strategy potentially explain the long-term trends in the fortunes of these 2 taxa on geological time scales. The fossil record indicates that coccolithophorids rose to ecological prominence in the mid-Jurassic and reached an apex in the mid-Cretaceous, but have declined throughout the Cenozoic. In contrast, diatoms have risen rapidly in the late Cenozoic, especially from early-Miocene time to the present. Based on paleoclimate reconstructions, from Mesozoic times, we hypothesize that the relative success of the 2 functional groups reflects, in part, long-term changes in upper ocean turbulence and its influence on the temporal distribution of nutrients.
Phytoplankton-produced dimethylsulfoniopropionate (DMSP) provides underwater and atmospheric foraging cues for several species
of marine invertebrates, fish, birds, and mammals. However, its role in the chemical ecology of marine planktonic microbes
is largely unknown, and there is evidence for contradictory functions. By using microfluidics and image analysis of swimming
behavior, we observed attraction toward microscale pulses of DMSP and related compounds among several motile strains of phytoplankton,
heterotrophic bacteria, and bacterivore and herbivore microzooplankton. Because microbial DMSP cycling is the main natural
source of cloud-forming sulfur aerosols, our results highlight how adaptations to microscale chemical seascapes shape planktonic
food webs, while potentially influencing climate at the global scale.
In this exciting and innovative textbook, two leading oceanographers bring together the fundamental physics and biology of the coastal ocean in a quantitative but accessible way for undergraduate and graduate students. Shelf sea processes are comprehensively explained from first principles using an integrated approach to oceanography that helps build a clear understanding of how shelf sea physics underpins key biological processes in these environmentally sensitive regions. Using many observational and model examples, worked problems and software tools, the authors explain the range of physical controls on primary biological production and shelf sea ecosystems. Boxes throughout the book present extra detail for each topic and non-mathematical summary points are provided for physics sections, allowing students to develop an intuitive understanding. The book is fully supported by extensive online materials, including worked solutions to end-of-chapter exercises, additional homework/exam problems with solutions and simple MATLAB and FORTRAN models for running simulations.
Rapid in situ measurements of some components of fluorescent spectra are now possible with submersible multi-wavelength fluorometers, which implies that phytoplankton composition can be measured, at least implicitly, at a spatial resolution that allows many scales of patches to be resolved. We present a method for identifying the location of patches of distinct fluorescent groupings of phytoplankton by using principal component analysis (PCA) to process in situ spectral data. The processing method potentially allows retention of more information from the raw data than existing methods because it depends on fewer assumptions. Furthermore, it can be applied without the need for site-specific calibration of the fluorometer. A series of idealized spectral data sets were used to explain the conceptual basis of the approach; the method was then applied to field spectral data sampled in Lake Victoria, Kenya. The results demonstrate that the main features of large sample sets of multi-component spectral data can be summarized in a single graph that reveals the number of spectrally distinct groups of phytoplankton at the site, and allows information about the spatial structure of those different phytoplankton groups to be derived from subsequent analysis. In this way, fluorescent spectral data collected at high spatial resolution can be used to identify the locations of patches and facilitate targeted water sample collection from those locations to investigate the species diversity and distribution at a study site.
The two main wastewater discharges on the west coast of Australia, near the city of Perth, were monitored for their potential impact on phytoplankton communities between 1996 and 2000. The wastewater outlets varied in their location, length and depth (at point of discharge), volume of discharge and nutrient load. Phytoplankton responses were markedly different in the vicinity of the two outlets. In the vicinity of the southern outlet with the greatest effluent volume and nutrient load, phytoplankton responses were at or near the level of detection, except during summer. In the vicinity of the northern outlet significant environmental impacts were noted: light attenuation was 32% greater, phosphate and nitrate concentrations were 301% and 548% greater, respectively, and phytoplankton biomass was 221% greater than the relevant control site. The magnitude of the environmental responses at these two outlets is hypothesized to be associated with differences in current speed and stratification. At the northern outlet, where a greater phytoplankton response was detected, biomass was negatively correlated with wind speed, which suggests that measurable biological impacts were associated with the retention of the buoyant effluent in the vicinity of the outlet during periods of slower along-shore transport.
Seasonal cycles and horizontal distribution of temperature, salinity and dissolved nutrients over a 3-year period are described for coastal waters off Perth, Western Australia. Adjacent oceanic nutrient concentrations are low and there is little terrestrial runoff. The coastal waters contain nutrients in low concentrations, yet support highly productive benthic macrophyte communities. The waters are well mixed and characterized by summer maxima of phosphate and salinity and winter maxima of silicate and nitrate. Horizontal differences in nitrate concentrations within a semi-enclosed lagoon during autumn and winter appear to be due in part to a decrease with depth in the ability of benthic communities to reduce nitrate concentrations in the overlying water.
Turbulent mixing within the metalimnion of a stratified lake was investigated using a portable flux profiler (PFP) capable of resolving all three components of the velocities, the conductivity, and the temperature microstructure. Presented is a detailed description of the techniques used in the data processing, particularly in the separation of the turbulence from the internal wave signal. The sampling, carried out in Lake Kinneret (Israel) during the summer for 3 consecutive years, showed that most of the time the vertical flux through the metalimnion was negligible, but, at times, the eddy diffusivity did reach values as high as 10 22 m 2 s 21 . A comparison between direct measurement of the vertical fluxes obtained from the PFP data with that from indirect estimates of the fluxes shows good agreement for the 6 # Frg , 100 range. Scaling of the turbulence based on Frg and Ri reveal two classes of turbulent regimes: (1) due to traumata characteristic of internal wave-wave interaction and, another, (2) more energetic and due to shear-driven turbulence. The PFP penetrated the water relatively slowly (0.1 m s 21 ) allowing the measurement of temperature fluctuations down to 1 mm and, at the same time, also providing information of the velocity fluctuations. This is different from previous oceanographic measurements, which are always gathered with instrument traversing the water column at velocities closer t o1ms 21 , preventing regime (1) from being detected.
Increased biological and chemical reaction rates within permeable continental-shelf sediment can result from the action of passing surface waves, especially when the seabed is rippled. The effect of this on the exchange of nitrogen between the sediment and water column is the focus of the present paper. The continental shelf of Western Australia is used as an example. A time series of chlorophyll a is compared with surface-wave height revealing seasonal and sub-seasonal correlation between the two variables. At the same time, results from a coupled pelagicbenthic biogeochemical model show that temperature-controlled changes in sedimentary nitrogen efflux cannot account for the observed seasonal changes in chlorophyll a in the overlying water column. It is proposed that enhanced pore-water circulation within the sediment, caused by the action of passing surface waves, results in an increase in the efflux of nitrogen from the sediment during winter, supporting higher pelagic phytoplankton production. The parameterisation of sedimentary nitrogen mineralisation as a function of the square of wave height is suggested for the inclusion of this effect in regional-scale continental shelf models.
Field experiments were conducted to investigate the near-field dilution characteristics of a hypersaline brine discharge into coastal waters via an offshore diffuser from a desalination plant. The aim was to determine the dilution of the negatively buoyant plume as it exited the diffuser under three different discharge Froude number regimes (one-third, two-thirds, and full-flow capacity) and to compare these measurements to scaling arguments derived from laboratory measurements. Equations based on the densimetric jet Froude number F, obtained from laboratory experiments, were found to adequately describe the dilution of the brine for cases when F > 20. For F < 20, no laboratory results exist and the dilution was found to be greater than that anticipated from an extrapolation of the laboratory results.
Field measurements were used to investigate circulation, mixing processes, and variations in temperature and salinity in the coastal boundary layer off Perth, Western Australia. This region is characterized by a low amplitude diurnal tidal regime, a strong southwesterly summer sea breeze, and a topography dominated by submerged barrier reefs. Subinertial current dynamics were analyzed using a simplified depth-averaged alongshore momentum equation. Offshore of the reefs, the results showed a balance between wind stress, alongshore pressure gradient, and bottom friction and acceleration forces. The alongshore pressure gradient contributed to accelerating the water body, but wind was the dominant driving force for a majority of the time. Onshore of the reefs (lagoonal waters), the alongshore dynamic balance was mainly between wind stress and bottom friction. The temperature variability in Perth coastal waters was in agreement with the seasonal variations in the net air-sea heat flux. The exchange rates between lagoonal waters in the area and the adjacent shelf waters were estimated, and the results showed an estimated flushing time of 4 to 13 days for the lagoonal waters. The seasonal variations of salinity in the area were dominated by changes in the salinity of the offshore waters and local discharge of groundwater. The vertical structure of the nearshore water was found to be dominated by wind mixing. In summer, solar heating in the morning stratified the water column, the sea breeze mixed the water column in the afternoon, and convective cooling at night sustained this mixing until the early morning; in winter the water column generally remained mixed throughout the day.
The diversity of the morphologies, propulsion mechanisms, flow environments, and behaviors of planktonic microorganisms has long provided inspiration for fluid physicists, with further intrigue provided by the counterintuitive hydrodynamics of their viscous world. Motivation for studying the fluid dynamics of microplankton abounds, as microorganisms support the food web and control the biogeochemistry of most aquatic environments, particularly the oceans. In this review, we discuss the fluid physics governing the locomotion and feeding of individual planktonic microorganisms (⩽1mm). In the past few years, the field has witnessed an increasing number of exciting discoveries, from the visualization of the flow field around individual swimmers to linkages between microhydrodynamic processes and ecosystem dynamics. In other areas, chiefly the ability of microorganisms to take up nutrients and sense hydromechanical signals, our understanding will benefit from reinvigorated interest, and ample opportunities for...
Bacteria play an indispensable role in marine biogeochemistry by recycling dissolved organic matter. Motile species can exploit
small, ephemeral solute patches through chemotaxis and thereby gain a fitness advantage over nonmotile competitors. This competition
occurs in a turbulent environment, yet turbulence is generally considered inconsequential for bacterial uptake. In contrast,
we show that turbulence affects uptake by stirring nutrient patches into networks of thin filaments that motile bacteria can
readily exploit. We find that chemotactic motility is subject to a trade-off between the uptake benefit due to chemotaxis
and the cost of locomotion, resulting in an optimal swimming speed. A second trade-off results from the competing effects
of stirring and mixing and leads to the prediction that chemotaxis is optimally favored at intermediate turbulence intensities.
Marine bacteria influence Earth’s environmental dynamics in fundamental ways by controlling the biogeochemistry and productivity
of the oceans. These large-scale consequences result from the combined effect of countless interactions occurring at the level
of the individual cells. At these small scales, the ocean is surprisingly heterogeneous, and microbes experience an environment
of pervasive and dynamic chemical and physical gradients. Many species actively exploit this heterogeneity, while others rely
on gradient-independent adaptations. This is an exciting time to explore this frontier of oceanography, but understanding
microbial behavior and competition in the context of the water column’s microarchitecture calls for new ecological frameworks,
such as a microbial optimal foraging theory, to determine the relevant trade-offs and global consequences of microbial life
in a sea of gradients.
A fundamental yet elusive goal of ecology is to predict the structure of communities from the environmental conditions they experience. Trait-based approaches to terrestrial plant communities have shown that functional traits can help reveal the mechanisms underlying community assembly, but such approaches have not been tested on the microbes that dominate ecosystem processes in the ocean. Here, we test whether functional traits can explain community responses to seasonal environmental fluctuation, using a time series of the phytoplankton of the English Channel. We show that interspecific variation in response to major limiting resources, light and nitrate, can be well-predicted by lab-measured traits characterising light utilisation, nitrate utilisation and maximum growth rate. As these relationships were predicted a priori, using independently measured traits, our results show that functional traits provide a strong mechanistic foundation for understanding the structure and dynamics of ecological communities.
Diffusion coefficients of aqueous ammonium nitrate, lithium nitrate and ammonium sulfate solutions, and conductances of ammonium nitrate and ammonium chloride solutions are reported for high concentrations (0.1-8.0 moles/liter at 25°). Theoretical expressions for the diffusion coefficient and conductance of the 1:1 electrolytes are found to be remarkably successful. In the case of ammonium nitrate there is evidence of marked ion-pair formation, the extent of which is estimated from the conductance data and then used in the interpretation of the diffusion results.
Fine‐scale measurements and temperature gradient microstructure data are used to describe the diurnal energetics of the mixed layer in the Wellington Reservoir in Western Australia. The data covered a morning period of solar heating, a period of severe wind mixing induced by an afternoon sea breeze, and a period of pure penetrative convection extending through most of the night. The Wedderburn number (W) ranged from a value of 0.02 at the start of the heating phase to a value of about 10 at the end of the convective period. During the intermediate strong wind period the value of W remained about 0.1; the surface wind stress induced a strong tilt of the isotherms at the base of the mixed layer. The other main parameter, the ratio of the Monin‐Obukhov length to the depth of the mixed layer, ranged from about 5.0 during the morning down to 0.005 at the end of the convective period. The full range of mixed‐layer deepening processes could thus be analyzed.
A simple one‐dimensional integral mixed‐layer model was applied to assess the importance of the temporal terms in diurnal simulations and to verify the values of the energy conversion efficiencies used in such models. Overall, the model performed extremely well; however, the results showed the need to develop parameterizations for the energetics of billowing induced by large shears across the base of the mixed layer and for the mixing accompanying upwelling.
Although the main interest in this book is biological, the physical environment of the lake cannot be ignored. The organisms
of the lake, and the ecosystem as a whole, are strongly influenced by physical processes such as vertical mixing, horizontal
circulation, and sediment-water interactions. We are fortunate to have a detailed understanding of the physical limnology
of Lake Mendota, deriving initially from the classical work of Birge but being elaborated to a great extent by the work of
Professor Reid Bryson and his students in the 1950’s and early 1960’s, by Kenton M. Stewart in the 1960’s, and by Robert A.
Stauffer in the 1970’s. In addition to these kinds of physical studies, many biologically- or chemically-oriented workers
have carried out routine physical measurements as part of their special research studies.
Here, we communicate a point of departure in the development of aquatic ecosystem models, namely a new community-based framework, which supports an enhanced and transparent union between the collective expertise that exists in the communities of traditional ecologists and model developers. Through a literature survey, we document the growing importance of numerical aquatic ecosystem models while also noting the difficulties, up until now, of the aquatic scientific community to make significant advances in these models during the past two decades. Through a common forum for aquatic ecosystem modellers we aim to (i) advance collaboration within the aquatic ecosystem modelling community, (ii) enable increased use of models for research, policy and ecosystem-based management, (iii) facilitate a collective framework using common (standardised) code to ensure that model development is incremental, (iv) increase the transparency of model structure, assumptions and techniques, (v) a
Dinoflagellate ecology is based on multiple adaptive strategies and species having diverse habitat preferences. Nine types of mixing-irradiance-nutrient habitats selecting for specific marine dinoflagellate life-form types are recognised, with five rules of assembly proposed to govern bloom-species selection and community organisation within these habitats. Assembly is moulded around an abiotic template of light energy, nutrient supply and physical mixing in permutative combinations. Species selected will have one of three basic (C-, S-, R-) strategies: colonist species (C-) which predominate in chemically disturbed habitats; nutrient stress tolerant species (S-), and species (R-) tolerant of shear/stress forces in physically disturbed water masses. This organisational plan of three major habitat variables and three major adaptive strategies is termed the 3-3 plan. The bloom behaviour and habitat specialisation of dinoflagellates and diatoms are compared. Dinoflagellates behave as annual species, bloom soloists, are ecophysiologically diverse, and habitat specialists whose blooms tend to be monospecific. Diatoms behave as perennial species, guild members, are habitat cosmopolites, have a relatively uniform bloom strategy based on species-rich pools and exhibit limited habitat specialisation. Dinoflagellate bloom-species selection follows a taxonomic hierarchical pathway which progresses from phylogenetic to generic to species selection, and in that sequence. Each hierarchical taxonomic level has its own adaptive requirements subject to rules of assembly. Dinoflagellates would appear to be well suited to exploit marine habitats and to be competitive with other phylogenetic groups, yet fail to do so.
An interdisciplinary study of the waters across the continental shelf off Perth, Western Australia, has provided the first detailed climatology of the physical, chemical, optical, and biological processes across the shelf. In support of this work, remote-sensing data were utilised to provide a broad view of the spatial and temporal chlorophyll concentration dynamics, to support in situ observations, and to help “fill the gaps” inherent in in situ point sampling.In situ validation of remote-sensing products was carried out monthly off Perth over a 27-month period. Biological and physical measurements were made along a 40 km east–west transect, 20 km north of Perth. Results of the study have shown that in water deeper than 30–35 m, and where viewing conditions were suitable, the in situ measurements of chlorophyll concentration were within the 35% uncertainty of the SeaWiFS product.An increase in the in situ measurements of near-coastal chlorophyll concentration was evident during the 1998 winter period (May–July), but the apparent seasonal increase was not evident in the in situ data for 1997. The SeaWiFS chlorophyll concentration estimates have been used to show the seasonal fluctuation of chlorophyll concentration in Perth coastal waters from November 1997 to the end of 2004. The remotely sensed data show a clear seasonal cycle, with maximum chlorophyll concentration occurring during May, June and July for waters off Perth. Also evident from the remotely sensed data was an increase in water column light attenuation during winter, coincident with the increase in chlorophyll concentration. The potential now exists to further develop remote-sensing techniques and to integrate remotely sensed products into routine water-quality monitoring programmes for WA waters.
We present a seasonal climatology of the nutrient environment for waters off southwestern Australia with the intention of identifying spatial and seasonal characteristics of the nutrient environment and identify situations where the shelf may be susceptible to anthropogenic nutrient stress. The seasonal climatologies were generated from historical hydrographic data contained within the CSIRO Atlas of Regional Seas. The data presented here suggest the surface waters of the southwestern Australian shelf, the Leeuwin Current and offshore are all low in nitrogen (less than 0.5 μM) year round and that primary productivity is nitrogen limited. The shelf waters contain some dissolved phosphate, at relatively low levels (up to 0.25 μM) but diatom production may be limited by low levels of silicic acid (silicate) which are less than 2 μM year round. The Leeuwin Current is largely devoid of phosphate but contains reasonably high levels of silicate (up to 4 μM) and may be a silicate source to surrounding waters. A cross-shelf gradient in chlorophyll a biomass suggests that terrestrial nutrient sources make an important contribution to primary productivity. Offshore, a seasonal (wintertime) increase in chlorophyll a biomass coincides with a deepening of the mixed layer and is presumably supported by the mixing of deep water nutrients or chlorophyll from the deepwater maximum into the euphotic zone associated with this deepening. Further observations, particularly cross-shelf profiles from winter and profiles along the core of the Leeuwin Current, are required to fully separate the influence of the Leeuwin Current from other potential seasonal nutrient sources.
An integrated mass balance and modelling approach for analysis of estuarine nutrient fluxes is demonstrated in the Swan River Estuary, a microtidal system with strong hydrological dependence on seasonal river inflows. Mass balance components included estimation of gauged and ungauged inputs to the estuary and losses to the ocean (outflow and tidal exchange). Modelling components included estimation of atmospheric (N fixation, denitrification) and sediment–water column nutrient exchanges. Gross and net denitrification derived using two independent methods were significantly correlated (r2 = 0.49, p < 0.01) with net rates averaging 40% of gross. Annual nitrogen (N) and phosphorus (P) loads from major tributaries were linearly correlated with annual freshwater discharge and were 3-fold higher in wet years than in dry years. Urban drains and groundwater contributed, on average, 26% of N inputs and 19% of P inputs, with higher relative contributions in years of low river discharge. Overall, ungauged inputs accounted for almost 35% of total nitrogen loads. For N, elevated loading in wet years was accompanied by large increases in outflow (7x) and tidal flushing (2x) losses and resulted in overall lower retention efficiency (31%) relative to dry years (70%). For P, tidal flushing losses were similar in wet and dry years, while outflow losses (4-fold higher) were comparable in magnitude to increases in loading. As a result, P retention within the estuary was not substantially affected by inter-annual variation in water and P loading (ca. 50% in all years). Sediment nutrient stores increased in most years (remineralisation efficiency ca. 50%), but sediment nutrient releases were significant and in some circumstances were a net source of nutrients to the water column.
The motility and migrational behaviour of upwelling dinoflagellates as adaptations for growth in upwelling systems is evaluated. Traits considered include hydrodynamic streamlining; chain formation; motility rates of single cells and chains; adaptations to turbulence; turbulence sensing; and migrational scattering to avoid turbulence, including its role in the maintenance of indigenous populations. Motility rates are compared to vertical mixing and upwelling rates. Diverse combinations of cell shape, size and motility rates characterize the dinoflagellate species selected for growth in physically energetic upwelling systems. Specific or unique combinations of cell shape, size, propulsion system and swimming rate are not evident. The traits are shared with dinoflagellates generally, and probably reflect their swim-based ecology. Experimental evidence – primarily from Alexandrium catenella – suggests upwelling dinoflagellates can sense turbulence leading to three distinct, but coherent, adaptive responses: chain formation (in such species); increased swimming speed (including non-chain-forming species); and the capacity to re-orient swimming trajectory in response to changes in turbulence, and at time-scales appropriate to survival and growth in the turbulence field being experienced. The added swimming power that dinoflagellates gain through chain formation does not appear to be a major requirement for their selection or success in upwelling systems. Only three of the 42 most prominent dinoflagellates that bloom in eastern boundary upwelling systems form chains, a representation far below expectations. Most chain-forming dinoflagellates are excluded from those upwelling systems. The role of temperature in this exclusion is evaluated. Field and experimental evidence suggests that strong turbulence would be required to overwhelm the swimming-based ecology of the upwelling dinoflagellates and deter their blooms. The Yamazaki–Kamykowski model demonstrating that the cells within a migrating population exposed to wind-induced vertical mixing do not migrate uniformly, but scatter differentially within the mixing layer, is applicable to upwelling dinoflagellates. The adaptive value of migrational scattering is that it protects some of the cells from turbulence-induced mortality or impairment, with the survivors available as potential seed stock for subsequent blooms. It also allows an indigenous population to develop and be maintained. The physically robust features of upwelling systems do not appear to be major impediments to the survival and growth of dinoflagellates.
The influence of diatom bloom behaviour, dinoflagellate life cycles, propagule type and upwelling bloom cycles on the seeding of dinoflagellate blooms in eastern boundary current upwelling systems is evaluated. Winter–spring diatom bloom behaviour is contrasted with upwelling bloom behaviour because their phenology impacts dinoflagellate blooms. The winter–spring diatom bloom is usually sustained, whereas the classical upwelling diatom bloom occurs as a series of separate, recurrent mini-blooms intercalated by upwelling-relaxation periods, during which dinoflagellates often bloom. Four sequential wind-regulated phases characterize upwelling cycles, with each phase having different effects on diatom and dinoflagellate bloom behaviour: bloom “spin up”, bloom maximum, bloom “spin down”, and upwelling relaxation. The spin up – bloom maximum is the period of heightened diatom growth; the spin down – upwelling-relaxation phases are the periods when dinoflagellates often bloom. The duration, intensity and ratio of the upwelling and relaxation periods making up upwelling cycles determine the potential for dinoflagellate blooms to develop within a given upwelling cycle and prior to the subsequent “spin up” of upwelling that favours diatom blooms. Upwelling diatoms and meroplanktonic dinoflagellates have three types of propagules available to seed blooms: vegetative cells, resting cells and resting cysts. However, most upwelling dinoflagellates are holoplanktonic, which indicates that the capacity to form resting cysts is not an absolute requirement for growth and survival in upwelling systems. The long-term (decadal) gaps in bloom behaviour of Gymnodinium catenatum and Lingulodinium polyedrum, and the unpredictable bloom behaviour of dinoflagellates generally, are examined from the perspective of seeding strategies. Mismatches between observed and expected in situ bloom behaviour and resting cyst dynamics are common among upwelling dinoflagellates. This disassociation suggests unrecognized upwelling system factors that fall within the physical–chemical–biological domain are more important than life cycle in selecting dinoflagellates species having the survival-seeding strategies and ecophysiological adaptations required for growth in physically robust upwelling systems. It is conjectured that diatom life cycles, as a group, are geared towards exploiting seeding opportunities, whereas dinoflagellates have evolved life-cycle behaviour more attuned to survival. The role of ecological dormancy and ecological release from bloom inhibition underlying dinoflagellate bloom irregularity is considered. The expectation that the dinoflagellate species selected to bloom from among the common upwelling flora would be the same in all eastern boundary upwelling systems is not realized.
A nonlinear electric circuit which produces signals similar in power spectrum to the velocity fluctuations of the turbulent boundary layer on a flat plate is described and demonstrated. The input signal is rectified by a diode, and the output signal is fed to an FFT analyzer and plotted. The outputs resulting from inputs of two or three square waves of equal amplitude and similar frequency are shown to be similar to turbulent-boundary-layer spectra, suggesting that a small number of instability waves of similar frequency may give rise to the spectrum of turbulence.
A 27-month study of the water properties across the continental shelf off Perth, Western Australia (the "Hillarys Transect") has provided the first systematic inter-disciplinary climatology of the physical, chemical, optical and biological cycles across the shelf. This paper describes the main features of the seasonal and cross-shelf variability of the physical oceanography and chemistry, while companion papers discuss some of the links between the biology and physics of the region