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Shellfish reefs have been lost from bays and estuaries globally, including in the Swan-Canning Estuary in Western Australia. As part of a national program to restore the ecosystem services that such reefs once provided and return this habitat from near extinction, the mussel Mytilus galloprovincialis was selected for a large-scale shellfish reef construction project in this estuary. To assess the potential filtration capacity of the reef, estuary seston quality, mussel feeding behavior, and valve gape activity were quantified in the laboratory and field during winter and summer. In general, estuary water contained high total particulate concentrations (7.9–8.7 mg L−1). Standard clearance rates were greater in winter (1.9 L h−1; 17 °C) than in summer (1.3 L h−1; 25 °C), the latter producing extremely low absorption efficiencies (37%). Mussel valves remained open ~97% and ~50% of the time in winter and summer, respectively. They often displayed erratic behavior in summer, possibly due to elevated temperatures and the toxic microalgae Alexandrium spp. Despite numerous stressors, the reef, at capacity, was estimated to filter 35% of the total volume of the estuary over winter, incorporating 42.7 t of organic matter into mussel tissue. The reefs would thus make a substantial contribution to improving estuary water quality.

We analyzed the spatial and temporal variability of seston parameters at 4 locations in the Ría de Ares-Betanzos (NW Spain) over 5 yr. Seston content was higher in the inner part of the ría and during winter, while seston quality was higher in the outer part of the ría with maximum values during summer, and exhibited a marked relationship with water circulation. Interannual differences were detected only in the organic content of seston (which was not always well-correlated with chlorophyll a), and only at some locations. Seston quality had the strongest relationship with meteorological factors, and was the only variable that was consistent at the 4 sites within the embayment. This led us to develop an empirical model that explains the spatial–temporal variability of seston quality in terms of wind stress and river discharge.

We hypothesized that C. fluminea could remove anthropogenic N at a rate sufficient to affect particulate N in the water column of North Buffalo Creek, North Carolina, USA, a 4th-order stream that receives treated urban wastewater. We used stable-isotope analysis to evaluate trophic relationships between seston and C. fluminea and conducted field sampling and laboratory experiments to evaluate the potential qualitative and quantitative effects of C. fluminea on seston. Corbicula fluminea δ15N was 3 to 5‰ enriched compared to seston along a longitudinal transect downstream of the wastewater treatment plant (WWTP), a result consistent with use of seston N. However, seston δ13C declined and C. fluminea δ13C showed no pattern with distance downstream, a result that was inconsistent with a trophic relationship between seston and C. fluminea. In 2 laboratory experiments designed to measure filter-feeding rate and qualitative effects on seston, seston ash-free dry mass and chlorophyll a data indicated that C. fluminea either was not filtering or was filtering at a rate insufficient to affect seston concentration over the course of the experiments. δ15N data showed that the sediment was an N source for C. fluminea, but δ13C and C∶N data from the same experiments indicated that C. fluminea probably affected seston quality by suspending benthic algae and returning settled algae to the water column. These results illustrate that the food sources for C. fluminea and implications of C. fluminea activity in stream ecosystems should be evaluated more fully.

Changes in the near-bottom abundance of zooplankton on scales of centimeters to meters and hours to seasons are of great importance to corals and other benthic zooplanktivores. Our objective was to characterize such spatio-temporal changes over several coral reefs in the Gulf of Aqaba, Red Sea. Using arrays of underwater pumps, we found a substantial depletion of zooplankton near the bottom. Vertical gradients in zooplankton abundance were steeper during the night than day, mostly due to a greater nocturnal increase in zooplankton biomass higher in the water column. On average, the layer <1 m above bottom (mab) was depleted by 2.6±2.2 mg m^sup -3^ (46±35%) and 1.4±1.4 mg m^sup -3^ (37±43%) during night and day, respectively. A long time series of bi-weekly samples at 0.5 mab, lasting 1.5 years, indicated a doubling of the biomass during the night with no apparent seasonality. The diel change was due to an increase in the abundance of only large (>200 μm) zooplankters around dusk and their disappearance in the morning. Diurnal predation by zooplanktivorous fish, sediment resuspension by benthivorous fish and zooplankton behavior appear to control the dynamics of suspended particles over the reef, creating sharp vertical gradients and a remarkable diel cycle in the ratio between nutritious plankton and inorganic particles.[PUBLICATION ABSTRACT]

We examined whether seston food quality was a significant factor to predict zooplankton, specifically Daphnia pulex, biomass dynamics in Lake Berryessa, California, U.S.A. The seston composition changed dramatically during the study period. Total macrozooplankton to seston biomass ratios were high during the springtime, as was D. pulex biomass. In monthly growth experiments using natural seston, eicosapentaenoic acid (EPA) concentration was best in explaining D. pulex growth, although α-linolenic acid (ALA) also showed considerable relationship with D. pulex growth. We built D. pulex growth models using combinations of EPA concentration, C:P ratio, and water temperature. The model using EPA concentration, C:P ratio and water temperature was best in explaining D. pulex biomass in springtime in Lake Berryessa. However, this model did not work for other seasons, suggesting that resource control via seston food quality was prevalent in springtime, but other factors may also become important in other seasons in this reservoir.[PUBLICATION ABSTRACT]

Extreme precipitation is occurring with greater frequency and intensity as a result of climate change. Such events boost the transport of allochthonous organic matter (allo-OM) to freshwater ecosystems, yet little is known about the impacts on dissolved organic matter (DOM) quality and seston elemental stoichiometry, especially for lakes in warm climates. A mesocosm experiment located in a Turkish freshwater lake was designed to simulate a pulse event leading to increased inputs of allo-OM by examining the individual effects of increasing water colour (HuminFeed®, HF), the direct effects of the extra energetic inputs (alder tree leaf leachate, L), and the interactions of the single treatment effects (combination of both sources, HFL), along with a comparison with unmanipulated controls. Changes in the DOM quality and nutrient stoichiometry of the allo-OM treatment additions was examined over the course of the experiments. Results indicated that there was an increase of high recalcitrant DOM components in the HF treatment, in contrast to an increase in less aromatic microbially derived molecules for the L treatment. Unexpectedly, seston C:P ratios remained below a severe P-limiting threshold for plankton growth and showed the same temporal pattern in all mesocosms. In contrast, seston N:P ratios differed significantly between treatments, with the L treatment reducing P-limiting conditions, whilst the HF treatment increased them. The effects of the combined HFL treatment indicated an additive type of interaction and chlorophyll-a was highest in the HFL treatment. Our results demonstrate that accounting for the optical and stoichiometric properties of experimental allo-OM treatments is crucial to improve the capacity to explain extrapolated conclusions regarding the effects of climate driven flooding on freshwater ecosystems in response to global climate change.

Concerns are being raised that microplastic pollution can have detrimental effects on the feeding of aquatic invertebrates, including zooplankton. Both small plastic fragments (microplastics, MPs) produced by degradation of larger plastic waste (secondary MPs; SMPs) and microscopic plastic spheres used in cosmetic products and industry (primary MPs; PMPs) are ubiquitously present in the environment. However, despite the fact that most environmental MPs consist of weathered plastic debris with irregular shape and broad size distribution, experimental studies of organism responses to MP exposure have largely used uniformly sized spherical PMPs. Therefore, effects observed for PMPs in such experiments may not be representative for MP-effects in situ. Moreover, invertebrate filter-feeders are generally well adapted to the presence of refractory material in seston, which questions the potential of MPs at environmentally relevant concentrations to measurably affect digestion in these organisms. Here, we compared responses to MPs (PMPs and SMPs) and naturally occurring particles (kaolin clay) using the cladoceran Daphnia magna as a model organism. We manipulated food levels (0.4 and 9 μg C mL-1) and MP or kaolin contribution to the feeding suspension (<1 to 74%) and evaluated effects of MPs and kaolin on food uptake, growth, reproductive capacity of the daphnids, and maternal effects on offspring survival and feeding. Exposure to SMPs caused elevated mortality, increased inter-brood period and decreased reproduction albeit only at high MP levels in the feeding suspension (74% by particle count). No such effects were observed in either PMP or kaolin treatments. In daphnids exposed to any particle type at the low algal concentration, individual growth decreased by ~15%. By contrast, positive growth response to all particle types was observed at the high algal concentration with 17%, 54% and 40% increase for kaolin, PMP and SMP, respectively. When test particles comprised 22% in the feeding suspension, both MP types decreased food intake by 30%, while kaolin had no effect. Moreover, SMPs were found to homoaggregate in a concentration-dependent manner, which resulted in a 77% decrease of the ingested SMPs compared to PMPs. To better understand MP-processing in the gut, gut passage time (GPT) and evacuation rate of MPs were also assayed. SMPs and PMPs differed in their effects on daphnids; moreover, the particle effects were dependent on the MP: algae ratio in the suspension. When the MP contribution to the particle abundance in the medium changed from 1 to 4%, GPT for daphnids exposed to SMPs increased 2-fold. Our results suggest that MPs and, in particular, SMPs, have a greater capacity to negatively affect feeding in D. magna compared to naturally occurring mineral particles of similar size. Moreover, grazer responses observed in experiments with PMPs cannot be extrapolated to the field where SMPs dominate, because of the greater effects caused by the latter.

The Eastern Oyster (Crassostrea virginica) is a commercially important aquaculture species and food resource along the Atlantic and Gulf coasts of the USA. In addition to its economic value, oyster aquaculture provides ecological value such as water quality improvement. Oyster filtration is highly variable as filtration behavior is influenced by environmental conditions, oyster size, and oyster energetic demands. However, average rates generated in laboratory experiments are often used to estimate the ecological impact of oyster filtration, and there is a need for field-based, farm-specific estimates of filtration that account for this variation. In this study, field experiments were conducted between September 2020 and September 2021 to estimate seasonal oyster filtration physiology at oyster farms in three different bays in the Mid-Atlantic (Barnegat Bay and Delaware Bay in New Jersey and Rehoboth Bay in Delaware). The physiological activity of oysters at each farm varied such that oysters at Barnegat Bay were the most active and oysters at Rehoboth Bay were the least active. Seasonal physiological trends were observed such that filtration behavior generally increased in warmer months. An increase in physiological activity across all farms was associated with an increase in salinity and temperature, but physiological activity at each farm was associated with a different suite of environmental variables including total particulate matter and the organic content of seston. This study provides a robust dataset which can be incorporated into models estimating ecological filtration rates in the Mid-Atlantic and adds to the growing body of evidence supporting bivalve aquaculture as a nutrient reduction strategy.

We used data from whole-lake studies to assess how changes in food quantity (phytoplankton biomass) and quality (phytoplankton community composition, seston C:P and N:P) with N fertilization affect zooplankton biomass, community composition and C:N:P stoichiometry, and their N:P recycling ratio along a gradient in lake DOC concentrations. We found that despite major differences in phytoplankton biomass with DOC (unimodal distributions, especially with N fertilization), no major differences in zooplankton biomass were detectable. Instead, phytoplankton to zooplankton biomass ratios were high, especially at intermediate DOC and after N fertilization, implying low trophic transfer efficiencies. An explanation for the observed low phytoplankton resource use, and biomass responses in zooplankton, was dominance of colony forming chlorophytes of reduced edibility at intermediate lake DOC, combined with reduced phytoplankton mineral quality (enhanced seston N:P) with N fertilization. N fertilization, however, increased zooplankton N:P recycling ratios, with largest impact at low DOC where phytoplankton benefitted from light sufficiently to cause enhanced seston N:P. Our results suggest that although N enrichment and increased phytoplankton biomass do not necessarily increase zooplankton biomass, bottom-up effects may still impact zooplankton and their N:P recycling ratio through promotion of phytoplankton species of low edibility and altered mineral quality.

A mesocosm approach was used to investigate the effects of ocean acidification (OA) on a natural plankton community in coastal waters off Norway by manipulating CO₂ partial pressure (pCO₂). Eight enclosures were deployed in the Raunefjord near Bergen. Treatment levels were ambient (~320 μatm) and elevated pCO₂ (~2000 μatm), each in 4 replicate enclosures. The experiment lasted for 53 d in May−June 2015. To assess impacts of OA on the plankton community, phytoplankton and protozooplankton biomass and total seston fatty acid content were analyzed. In both treatments, the plankton community was dominated by the dinoflagellate Ceratium longipes. In the elevated pCO₂ treatment, however, biomass of this species as well as that of other dinoflagellates was strongly negatively affected. At the end of the experiment, total dinoflagellate biomass was 4-fold higher in the control group than under elevated pCO₂ conditions. In a size comparison of C. longipes, cell size in the high pCO₂ treatment was significantly larger. The ratio of polyunsaturated fatty acids to saturated fatty acids of seston decreased at high pCO₂. In particular, the concentration of docosahexaenoic acid (C 22:6n3c), essential for development and reproduction of metazoans, was less than half at high pCO₂ compared to ambient pCO₂. Thus, elevated pCO₂ led to a deterioration in the quality and quantity of food in a natural plankton community, with potential consequences for the transfer of matter and energy to higher trophic levels