Zooplankton Community Size Structure Research Articles

Zooplankton size-structure dynamics of a lowland tropical floodplain lake

Floodplains are highly complex ecosystems representing high biodiversity and conservation values, but they are also one of the world’s most threatened ecosystems due to extensive development and anthropogenic activities. Lake Tempe is a lowland riverine floodplain lake in Sulawesi Island, Indonesia, that is subject to multiple stressors such as flow alteration, eutrophication and invasive species. In this study, the dynamics of the zooplankton community size structure was investigated in Lake Tempe. Five size-based metrics, including zooplankton mean body size, total abundance, total, biomass, and normalised biomass size spectra (NBSS) slope and intercept were assessed from net-sampled zooplankton collected monthly from March to December 2016, and the role of environmental variables in shaping these size-based metrics were also examined. Zooplankton community size structure is a useful metric as it is less labour intensive than traditional approaches, provides more data accuracy and does not require highly specialised taxonomic expertise. The zooplankton community in Lake Tempe was characterised by high density of small-sized zooplankton with an average mean body size < 500 µm equivalent spherical diameter. While the zooplankton density was characterised by the dominance of the size class 300–700 µm, there was evidence of a significant contribution of the size > 700 µm towards total biomass. Seasonal hydrological regimes, water temperature and eutrophication parameters, as indicated by total phosphorus and chlorophyll-a concentration, had a significant role in driving the variability of zooplankton community size structure in this system. The results of this study are not only a significant step in providing critical baseline information on the zooplankton assemblage of Lake Tempe, but is also a significant contribution to the overall understanding of zooplankton community structure in tropical lakes, and will aid in improving lake management plans in these regions.

Zooplankton community size structure across lakes within a semi-arid landscape: the effect of temperature and salinity

AbstractBody size is a master trait controlling biological communities and ecosystem functioning. Mean population size not only depends on the size of individuals, but also on the size distribution of individuals within the population. Mean community size is additionally influenced by the composition of species (larger- or smaller-sized species). Shallow lakes within semi-arid landscapes are prone to experience large changes in temperature and salinity, which affect the zooplankton size structure. Higher temperatures are expected to result in smaller average body size, while the effect of salinity appears to depend on the range under study. Here we analyze zooplankton body size patterns across shallow lakes from the semi-arid central region of Argentina. All community size descriptors point to decreasing size and a narrow size range at higher temperatures. On the other hand, the maximum average community body size occurred at intermediate (~30 gL−1) salinity levels. The combined effect of both variables resulted in a bell-shaped pattern, with maximum community body size toward lower temperatures and intermediate salinities. Based on future temperature scenarios, one may anticipate an overall decrease in community body size. But such prediction is strongly conditioned by regional and local trends in salinity.

Seasonal variation of zooplankton assemblages and their responses to water chemistry and microcystin content in shallow lakes in Thailand

This study examines zooplankton assemblage structure and density from five hypereutrophic urban shallow lakes between cool and hot periods in 2018-2019. We analyzed the variation of zooplankton and their relationship with environmental factors. Samples of zooplankton were collected from shallow lakes in different regions of Thailand. Four groups of zooplankton were identified, of which Rotifera was the most abundant group, followed by Copepoda, Protozoa, and Cladocera. Zooplankton assemblages were influenced by seasons, as indicated by multidimensional scaling analysis. The number of species and density of zooplankton were lower during the cool period than during the hot period. The increased density of zooplankton in the hot period may have been due to increased phytoplankton density as food sources. Pearson?s correlation coefficient revealed that Rotifera and Copepoda positively correlated with the temperature and pH, and Rotifera was negatively correlated with total phosphorus; a negative correlation was also observed between Protozoa and dissolved oxygen. The microcystin content tended to have a negative impact on specific small species such as Protozoa (Coleps sp.). Information from this research is important for further study involving factors affecting the size structure of zooplankton communities, especially large-bodied species in tropical regions.

Experimental assessment of salinization effects on freshwater zooplankton communities and their trophic interactions under eutrophic conditions

Freshwater ecosystems are becoming saltier due to human activities. The effects of increased salinity can lead to cascading trophic interactions, affecting ecosystem functioning and energy transfer, through changes in community and size structure. These effects can be modulated by other environmental factors, such as nutrients. For example, communities developed under eutrophic conditions could be less sensitive to salinization due to cross-tolerance mechanisms. In this study, we used a mesocosm approach to assess the effects of a salinization gradient on the zooplankton community composition and size structure under eutrophic conditions and the cascading effects on algal communities. Our results showed that zooplankton biomass, size diversity and mean body size decreased with increased chloride concentration induced by salt addition. This change in the zooplankton community did not have cascading effects on phytoplankton. The phytoplankton biomass decreased after the chloride concentration threshold of 500 mg L−1 was reached, most likely due to direct toxic effects on the osmotic regulation and nutrient uptake processes of certain algae rather than as a response to community turnover or top-down control. Our study can help to put in place mitigation strategies for salinization and eutrophication, which often co-occur in freshwater ecosystems.

Hydrological Regime and Fish Predation Regulate the Zooplankton Community Size Structure in a Tropical Floodplain Lake

Floodplain ecosystems are characterised by alternating flood and drought periods that can affect the structure of the aquatic community. Dynamic changes in the hydrological regimes from flooding to dry periods influence the migration and dispersal of aquatic fauna and the exchange of particulate matter and nutrients. Riverine floodplains are among the most productive ecosystems; however, increasing pressure from anthropogenic activities has altered the hydrological regimes, threatening aquatic biodiversity. In this study, we examined the temporal patterns of zooplankton community size structure and fish density during three distinct hydrological events in a tropical floodplain lake, Lake Tempe, Indonesia. We included fish density data and three contrasting hydrological conditions, moderate-, high-, and low-water periods, as the environmental factors regulating zooplankton community structure. In high- and low-water conditions, the ecosystem heterogeneity was characterised by high chlorophyll-a, total nitrogen, and total phosphorus concentrations; high fish density; and high zooplankton abundance and biomass. During the early flood period, the ecosystem was characterised by lower concentrations of trophic indicators and significant decreases in zooplankton abundance and biomass, as well as decreased fish density. While there was no clear association between hydrological conditions and zooplankton size structure, our findings indicate that fish predation probably suppressed zooplankton size diversity in Lake Tempe, shown by the dominant contribution of small-sized zooplankton towards total abundance and biomass under all hydrological conditions. Our results indicate that the patterns of environmental variables, zooplankton community, and fish density are affected by hydrological conditions, highlighting the role of water level fluctuation as the driving factor for zooplankton community structure. Our results also indicated that fish predation led to the development of a small-sized population of zooplankton in Lake Tempe.

Macroscale patterns of oceanic zooplankton composition and size structure

Ocean plankton comprise organisms from viruses to fish larvae that are fundamental to ecosystem functioning and the provision of marine services such as fisheries and CO2 sequestration. The latter services are partly governed by variations in plankton community composition and the expression of traits such as body size at community-level. While community assembly has been thoroughly studied for the smaller end of the plankton size spectrum, the larger end comprises ectotherms that are often studied at the species, or group-level, rather than as communities. The body size of marine ectotherms decreases with temperature, but controls on community-level traits remain elusive, hindering the predictability of marine services provision. Here, we leverage Tara Oceans datasets to determine how zooplankton community composition and size structure varies with latitude, temperature and productivity-related covariates in the global surface ocean. Zooplankton abundance and median size decreased towards warmer and less productive environments, as a result of changes in copepod composition. However, some clades displayed the opposite relationships, which may be ascribed to alternative feeding strategies. Given that climate models predict increasingly warmed and stratified oceans, our findings suggest that zooplankton communities will shift towards smaller organisms which might weaken their contribution to the biological carbon pump.

A first approach to build and test the Copepod Mean Size and Total Abundance (CMSTA) ecological indicator using in-situ size measurements from the Plankton Imager (PI)

Being located between primary producers and fish, zooplankton are a key element of marine food webs, the trophic structure of which is dependent upon the size distribution of species. Changes in zooplankton community size structure have the potential to alter the food web structure and ultimately the quality of food for planktivorous fish. Zooplankton therefore play a key role in overall ecosystem health and are ideal indicators of environmental variability due to their short life cycle and sensitivity to environmental changes. In this study, we used a first approach to build and test the Copepod Mean Size and Total Abundance (CMSTA) ecological indicator in the Celtic Sea, using a similar methodology to that of the HELCOM Mean Size and Total Stock (MSTS) indicator. We explored relationships between zooplankton mean size, total abundance and biomass with hydrographic and biological variables representative of both lower and higher trophic levels (i.e. Chlorophyll-a levels and the biomass distribution of herring, sardine, anchovy, sprat and horse mackerel). Herring, the species with the strongest preference for larger prey, was the only fish that displayed a statistically significant positive correlation with copepod mean size. Displaying the stations spatially within the 2-dimentional (copepod mean size versus abundance) indicator plots, resulted in a strong pattern of herring distribution related to areas where copepods were the largest rather than the most abundant. Our preliminary results are in line with those obtained from previous studies, confirming that zooplankton mean size is able to reflect the state of the food web, and thus reinforce the importance as zooplankton size as a key trait to be routinely monitored. Our in-situ zooplankton size measurements were collected with the Plankton Imager (PI). The ability of this automated system to take measurements for every single organism in a sample meant that the mean size was calculated based on the actual size distribution of the sampled population. Additionally, because of the non-normal distribution of copepod size, the geometric mean was found to be a much more representative description of the community size than the arithmetic mean. We have demonstrated (1) the potential of the CMSTA as a potential indicator of ecosystem health status and climate effects, and (2) the value of the PI in collecting routine in-situ zooplankton size information for improving the power of the mean size indicator approach to describe the community structure.

Zooplankton community size-structure change and mesh size selection under the thermal stress caused by a power plant in a semi-enclosed bay

Zooplankton samples were collected using 505, 160 and 77 µm mesh nets around a power plant during four seasons in 2011. We measured total length of zooplankton and divided zooplankton into seven size classes in order to explore how zooplankton community size-structure might be altered by thermal discharge from power plant. The total length of zooplankton varied from 93.7 to 40 074.7 µm. The spatial distribution of mesozooplankton (200 −2 000 µm) populations were rarely affected by thermal discharge, while macro- (2 000 −10 000 µm) and megalo-zooplankton (>10 000 µm) had an obvious tendency to migrate away from the outfall of power plant. Thus, zooplankton community tended to become smaller and biodiversity reduced close to power plant. Moreover, we compared the zooplankton communities in three different mesh size nets. Species richness, abundance, evenness index and Shannon-Wiener diversity index of the 505 µm mesh size were significantly lower than those recorded from the 160 and 77 µm mesh size. Average zooplankton abundance was highest in the 77 µm mesh net ((27 690.0±1 633.7) ind./m3), followed by 160 µm mesh net ((9 531.1±1 079.5) ind./m3), and lowest in 505 µm mesh net ((494.4±104.7) ind./m3). The ANOSIM and SIMPER tests confirmed that these differences were mainly due to small zooplankton and early developmental stages of zooplankton. It is the first time to use the 77 µm mesh net to sample zooplankton in such an environment. The 77 µm mesh net had the overwhelming abundance of the copepod genus Oithona, as an order of magnitude greater than recorded for 160 µm mesh net and 100% loss through the 505 µm mesh net. These results indicate that the use of a small or even multiple sampling net is necessary to accurately quantify entire zooplankton community around coastal power plant.

Oligotrophication affects the size structure and potential ecological interactions of planktonic microcrustaceans

The oligotrophication of freshwater is the reduction of the nutrient concentration in the water column, which causes the depletion of organic matter and diminishes the biological production of the ecosystem. To elucidate the zooplankton response to nutrient depletion (oligotrophication), an experiment was performed in mesocosms simulating real scenarios. Phytoplankton and zooplankton inocula were collected in the Upper Parana River Floodplain and subjected to combinated nutrient and transparency treatment. We evaluated whether resource limitation and increased transparency (both associated with oligotrophication) affect the size structure of planktonic crustaceans. The effects of predation were corroborated in the treatments with low nutrient concentrations. Thus, the oligotrophic scenario (high transparency and low nutrient concentration) indicated a decreased size structure of zooplankton, probably because of the predation pressure, supporting the predictions of the size-efficiency hypothesis (SEH). When resources were abundant (in treatments with high nutrient concentrations), the decrease in the size structure indicated that the enrichment of nutrients favoured small individuals. Our results showed that nutrient and transparency manipulation affected species richness, the abundance of individuals, and the zooplankton community size structure. Therefore, we suggest that oligotrophication affects predation and competition dynamics in zooplankton.

Size Spectra Modeling of Mesozooplankton over a Tropical Continental Shelf

de Souza, C.S.; da Conceicao, L.R.; Freitas, T.S.S.; Aboim, I.L.; Schwamborn, R.; Neumann-Leitao, S., and Mafalda, P.d.O., Jr., 2020. Size spectra modeling of mesozooplankton over a tropical continental shelf. Journal of Coastal Research, 36(4), 795–804. Coconut Creek (Florida), ISSN 0749-0208.Size spectra analysis is a useful way of studying biomass and energy flow in ecosystems across trophic levels. The size spectrum reflects a balance of energy and matter between gains through growth and recruitment and losses though mortality in a size-structured community. Therefore, changes in biomass and normalized biomass size spectrum (NBSS) slope and intercept between time or environment can be used to assess the energy transfer efficiency within ecosystems. Linear models were developed to analyze zooplankton biomass and community size structure for the narrow continental shelf and shelf break off Salvador, Brazil, using samples that were collected during 10 oceanographic cruises, from April 2013 to October 2014, and analyzed with a ZooScan. Three descriptors of zooplankton size spectra were examined: size diversity, NBSS slope, and intercept. Copepods were always dominant in abundance. Size spectra varied considerably along a short coast-to-offshore transect of 17 km. The pattern of spatial change presented a predominance of larger size fractions nearshore (flat NBSS slope, –0.56) and the predominance of smaller sized organisms in oceanic waters (steep NBSS slope, –0.88). This gradient was mostly due to the higher abundance of large (>5 mm equivalent spherical diameter) gelatinous organisms (mainly hydromedusae) nearshore. The results suggest that the tropical zooplankton community at the continental shelf off Salvador was characterized by low productivity and low trophic efficiency, especially offshore. Although copepods were dominant in numbers, large-sized gelatinous predators were the key structuring zooplankton taxon in this area.

Taxonomic and size structures of zooplankton communities in the fjords along the Atlantic water passage to the Arctic

Abstract Current climate change is significantly modifying Arctic ecosystems and their components, but the reference and direction of these changes are still undefined. The aim of this study was to provide well-grounded data on the present composition and structure of zooplankton from boreal to Arctic latitudes. This comparative study was performed with a unified methodology in six fjords located along the west coasts of Norway and Svalbard. Comparisons between the fjords' environmental characteristics (temperature, salinity, chlorophyll, and particles) and the zooplankton community, described with several metrics (abundance and biovolume) and traits (taxonomy and size), were performed to recognise the potential of various ecological indicators for future monitoring. While hydrography and the taxonomic composition of zooplankton were specific to each fjord and changed along the latitudinal environmental gradient, the clustering of particle characteristics (size, concentration, and transparency) and zooplankton size structures alludes to their susceptibility to additional, local processes. The differences in the biotic and abiotic characteristics distinguished the semi-closed fjords from the fjords prone to Atlantic water advection. The fjords with a limited water mass exchange incorporated local or Arctic-type waters and were host to specific zooplankton communities and particles, with the marked presence of species adapted to high particle concentrations (Microsetella norvegica) or species of Arctic affiliation (Calanus glacialis). In the open fjords, zooplankton communities were characterised by the predomination of Oithona similis and Calanus finmarchicus, confirming influence of Atlantic water. The zooplankton community size structure, expressed as abundance, was mainly shaped by changes in the contributions of small copepods, whereas the size structure expressed as biovolume reflected the differences in the co-occurring sibling species of Calanus spp. and/or differences in their dominating life stages. The results of this integrative across-system study constitute a necessary baseline for the future monitoring of changes in the zooplankton communities, which are likely to occur in response to climate change at high latitudes.

Potential shifts in zooplankton community structure in response to changing ice regimes and hydrologic connectivity

ABSTRACTChanging Arctic climate may alter freshwater ecosystems as a result of warmer surface waters, longer open-water periods, reduced wintertime lake ice growth, and altered hydrologic connectivity. This study aims to characterize zooplankton community composition and size structure in the context of hydrologic connectivity and ice regimes in Arctic lakes. Between 2011 and 2016, we sampled the phytoplankton, zooplankton, and fish communities from a set of representative lakes on the Arctic Coastal Plain (ACP) of northern Alaska to determine potential food web responses to changing Arctic ecosystems. Multivariate analyses showed that time from ice-out had a strong influence on zooplankton community structure and that seasonal succession of zooplankton differed between lakes with varying hydrologic connectivity. Trends were observed suggesting that large-bodied zooplankton (Daphnia, calanoid copepods) may be more prevalent in poorly connected lakes with low fish diversity. Large-bodied zooplankton displayed higher biomass in lakes with high occurrences of bedfast ice, while small-bodied zooplankton (Bosmina, rotifers) displayed highest biomass in deeper lakes with low occurrences of bedfast ice. Our results contribute to limited knowledge of zooplankton in remote lakes of the ACP and suggest that the anticipated changes to aquatic ecosystems in the Arctic may include energetically less efficient plankton food webs.

8. The Effect of Bythotrephes Longimanus on the Size Structure of Zooplankton Communities in the Muskoka Region, Ontario

Invasive species are considered the greatest threat to aquatic ecosystem biodiversity. Bythotrephes longimanus, an exotic zooplankton species introduced to North America in the 1980s, is threatening the structure of indigenous aquatic ecosystems as it continues to invade inland Ontario lakes. As a predacious zooplankton species, B. longimanus has been shown to decrease zooplankton abundance, species richness and shift zooplankton community size structure in invaded lakes. However, much of the previous research concerning the predatory effects of B. longimanus has been on surveys of a small number of lakes or has been in controlled mesocosm or lab-based experiments. This study examines the effects of B. longimanus on the zooplankton community using size-structure characterizations (grouping individuals from the community based on size) as community measures for 311 lakes in the Muskoka Region, a highly invaded watershed in Southern Ontario. More specifically, the study explores the size-spectra of invaded versus uninvaded lakes, with reference to an array of environmental lake characteristics (water chemistry, lake morphometry,etc.), and the relevance of B. longimanus activity on the regional scale. By using such a large-scale survey we will be able to appreciate regional-scale effects, as well as encompass the multiple and more indirect trophic interactions that B. longimanus is likely having with the entire aquatic community. (Funding: NSERC &amp; CAISN.)

Scaling copepod grazing in a coastal upwelling system: the importance of community size structure for phytoplankton C flux

Crustacean zooplankton, often dominated by copepods and euphausiids, are the major phytoplankton grazers in coastal upwelling systems. It has been argued that grazing by zooplankton is a size-dependent process, such that models incorporating the size structure of zooplankton are appropriate for describing herbivore C-transfer. Here, b ased on the size-spectrum theory and on gut-fluorescence experiments, conducted with numerically dominant copepods from two upwelling sites off the Chilean coast , we show that C-specific ingestion rates of copepods are size-dependent. We further show that the size structure of the copepod community, synthesized by the slope of the normalized size spectrum, determines the impact of grazing on phytoplankton. C-specific ingestion rates, depending on species size, were in the range of 0.02-8.40 (µg C x µg C -1 d -1 ). A modelled normalized biomass-spectra of a copepod community in the size range of 0.5 to 74.0 µg C showed that C-specific grazing impact can increase by a factor of 4 when small-sized species (0.1-10 µg C indiv -1 ), such as Paracalanus cf. indicus , Acartia tonsa , Oncaea spp. and Corycaeus spp., dominate the community in terms of biomass. By contrast, when larger-sized copepods dominate (10-100 µg C indiv -1 ), such as Calanus chilensis , Calanoides patagoniensis and Rhyncalanus nasutus , total zooplankton biomass may increase, but with a sharp decrease in the efficiency of C transfer via herbivores. Our findings indicate that processes affecting the size structure of zooplankton communities can substantially impact on phytoplankton C flux through the pelagic food web, thus controlling production of higher trophic levels.

Effects of nutrient and water level changes on the composition and size structure of zooplankton communities in shallow lakes under different climatic conditions: a pan-European mesocosm experiment

Lentic ecosystems act as sentinels of climate change, and evidence exists that their sensitivity to warming varies along a latitudinal gradient. We assessed the effects of nutrient and water level variability on zooplankton community composition, taxonomic diversity and size structure in different climate zones by running a standardised controlled 6-months (May to November) experiment in six countries along a European north–south latitudinal temperature gradient. The mesocosms were established with two different depths and nutrient levels. We took monthly zooplankton samples during the study period and pooled a subsample from each sampling to obtain one composite sample per mesocosm. We found a significant effect of temperature on the community composition and size structure of the zooplankton, whereas no effects of water depth or nutrient availability could be traced. The normalised size spectrum became flatter with increasing temperature reflecting higher zooplankton size diversity due to higher abundance of calanoid copepods, but did not differ among depths or nutrient levels. Large-bodied cladocerans such as Daphnia decreased with temperature. Taxonomic diversity was positively related to size diversity, but neither of the two diversity measures demonstrated a clear pattern along the temperature gradient nor with nutrient and water levels. However, genus richness decreased at the warm side of the temperature gradient. Our experiment generally supports recent empirically based findings that a continuing temperature increase may result in lower genus richness and lower abundance of large-sized zooplankton grazers, the latter likely resulting in reduced control of phytoplankton.

Chronic and pulse exposure effects of silver nanoparticles on natural lake phytoplankton and zooplankton.

The increasing use of silver nanoparticles (AgNPs) in consumer products raises concerns regarding the environmental exposure and impact of AgNPs on natural aquatic environments. Here, we investigated the effects of environmentally relevant AgNP concentrations on the natural plankton communities using in situ enclosures. Using twelve lake enclosures, we tested the hypotheses that AgNP concentration, dosing regimen, and capping agent (poly-vinyl pyrrolidone (PVP) vs. citrate) exhibit differential effects on plankton communities. Each of the following six treatments was replicated twice: control (no AgNPs added), low, medium, and high chronic PVP treatments (PVP-capped AgNPs added continuously, with target nominal concentrations of 4, 16, and 64 μg/L, respectively), citrate treatment (citrate-capped AgNPs added continuously, target nominal concentrations of 64 μg/L), and pulse treatment (64 μg/L PVP-AgNPs added as a single dose). Although Ag accumulated in the phytoplankton, no statistically significant treatment effect was found on phytoplankton community structure or biomass. In contrast, as AgNP exposure rate increased, zooplankton abundance generally increased while biomass and species richness declined. We also observed a shift in the size structure of zooplankton communities in the chronic AgNP treatments. In the pulse treatments, zooplankton abundance and biomass were reduced suggesting short periods of high AgNP concentrations affect zooplankton communities differently than chronic exposures. We found no evidence that capping agent affected AgNP toxicity on either community. Overall, our study demonstrates variable AgNP toxicity between trophic levels with stronger AgNP effects on zooplankton. Such effects on zooplankton are troubling and indicate that AgNP contamination could affect aquatic food webs.

High frequency mesozooplankton monitoring: Can imaging systems and automated sample analysis help us describe and interpret changes in zooplankton community composition and size structure — An example from a coastal site

The present work aims to show that high throughput imaging systems can be useful to estimate mesozooplankton community size and taxonomic descriptors that can be the base for consistent large scale monitoring of plankton communities. Such monitoring is required by the European Marine Strategy Framework Directive (MSFD) in order to ensure the Good Environmental Status (GES) of European coastal and offshore marine ecosystems. Time and cost-effective, automatic, techniques are of high interest in this context. An imaging-based protocol has been applied to a high frequency time series (every second day between April 2003 to April 2004 on average) of zooplankton obtained in a coastal site of the NW Mediterranean Sea, Villefranche Bay. One hundred eighty four mesozooplankton net collected samples were analysed with a Zooscan and an associated semi-automatic classification technique. The constitution of a learning set designed to maximize copepod identification with more than 10,000 objects enabled the automatic sorting of copepods with an accuracy of 91% (true positives) and a contamination of 14% (false positives). Twenty seven samples were then chosen from the total copepod time series for detailed visual sorting of copepods after automatic identification. This method enabled the description of the dynamics of two well-known copepod species, Centropages typicus and Temora stylifera, and 7 other taxonomically broader copepod groups, in terms of size, biovolume and abundance–size distributions (size spectra). Also, total copepod size spectra underwent significant changes during the sampling period. These changes could be partially related to changes in the copepod assemblage taxonomic composition and size distributions. This study shows that the use of high throughput imaging systems is of great interest to extract relevant coarse (i.e. total abundance, size structure) and detailed (i.e. selected species dynamics) descriptors of zooplankton dynamics. Innovative zooplankton analyses are therefore proposed and open the way for further development of zooplankton community indicators of changes.

Defining zooplankton habitats in the Gulf of Lion (NW Mediterranean Sea) using size structure and environmental conditions

The size structure of zooplankton communities in the Gulf of Lion, NW Mediterranean Sea, was studied in May 2010 and January 2011. The integrated physical and biological measurements provided a 3D view with high spatial resolution of the physical and biological variables and their correlations over the whole gulf. The effects of physical processes such as freshwater input, coastal upwelling, and water column mixing by winds on phytoplankton and zooplankton distributions were analyzed using these data. During the winter, strong northerly winds mixed the water column, and the vertical distributions of biological variables were uniform over most of the gulf while there were local hot spots with high chlorophyll a (chl a) concentrations in front of the Rhone mouths and in coastal areas. During the spring, light winds and water column stratification resulted in less vertical mixing, and the Rhone River freshwater plume spread over a large part of the gulf. The nutrients delivered by the freshwater input encouraged high primary production in the surface layer. In the pycnocline, a thin layer of high particle concentration was associated with these high phytoplankton biomasses. Three habitats were distinguished based on statistical analysis performed on biological and physical variables: (1) the coastal area characterized by shallow waters, high chl a concentrations, and a steep slope of the normalized biomass size spectrum (NBSS); (2) the area affected by the Rhone with high stratification and flat NBSS slope; and (3) the continental shelf with a deep mixed layer, relatively low particle concentrations, and moderate NBSS slope. Defining habitat is a relevant approach to designing new zooplankton sampling strategies, validating distribution models and including the zooplankton compartment in trophodynamic studies.

Intraspecific variation in a predator drives cascading variation in primary producer community composition

Predation has important cascading impacts on primary producer biomass and community composition in many ecosystems. While most studies have focused on the consequences of interspecific or density differences in predators, it is recognized that phenotypic variation within species can have strong and cascading community and ecosystem consequences at lower trophic levels. In coastal New England lakes, both the presence and life history form of the zooplanktivorous fish alewife, Alosa pseudoharengus, have strong influence on the biomass, size structure and community composition of crustacean zooplankton communities. Here we test the hypothesis that alewife presence and life history will have cascading impacts on phytoplankton biomass and community composition in a mesocosm experiment that previously reported strong biomass and compositional differences of crustacean zooplankton communities among alewife treatments. We show that alewife life history led to small but statistically significant differences in phytoplankton community composition among treatments. This compositional difference was driven primarily by an increase in the density of two edible phytoplankton genera associated with lower zooplankton biomass in the anadromous alewife treatment. Our results show that intraspecific variation in a predator can have cascading effects on primary producer communities. However we did not observe significant differences in total algal biomass.

Effects of Bythotrephes longimanus (Crustacea, Cladocera) on the abundance, morphology, and prey community of Leptodora kindtii (Crustacea, Cladocera)

We hypothesized that native Leptodora kindtii would be shorter and have smaller feeding baskets in central Ontario lakes with greater abundances of small-bodied zooplankton prey, and that differences in zooplankton size among lakes could be attributed to the invasive cladoceran Bythotrephes longimanus. We evaluated these conjectures by comparing size metrics of Leptodora and the size of their preferred cladoceran prey in lakes invaded or not by Bythotrephes. Leptodora was less abundant in invaded lakes, but were smaller bodied with smaller feeding baskets only in lakes with long invasion histories. Small cladoceran abundance was greater in non-invaded lakes and was directly related to Leptodora abundance although not to Leptodora size. Mean Leptodora body size declined with increasing abundance of Bythotrephes. We evaluated three possible explanations for these patterns in Leptodora—(a) competition with Bythotrephes for zooplankton prey, (b) direct predation by Bythotrephes, and (c) size-selective predation by fish. While we were unable to unequivocally distinguish among these hypotheses, our observations are most consistent with predation by Bythotrephes changing zooplankton community composition and size structure in a manner that is detrimental to Leptodora. Our results indicate that Bythotrephes invasion may trigger more complex and subtle changes in food webs than previously thought.