Structuring Zooplankton Communities Research Articles

Zooplankton vertical stratification in the East-pacific and Indian sectors of the Southern Ocean

IntroductionIn the Southern Ocean, the large-scale distribution of zooplankton, including their abundance and community composition from the epipelagic to the upper bathypelagic layers, remains poorly understood. This gap in knowledge limits our comprehension of their ecological and biogeochemical roles.MethodsTo better understand their community structure, depth-stratified zooplankton samples were collected from 0 to 1500 m during four summers in the East-Pacific and Indian sectors of the Southern Ocean. In addition, analysis of environmental drivers including temperature, salinity, dissolved oxygen, and chlorophyll a concentration, as well as water masses was conducted.ResultsOur study indicates that zooplankton diversity may be similar between the two sectors, while zooplankton abundance was higher in the East-Pacific sector during different sampling months and years. Moreover, zooplankton abundance decreased with depth in both sectors. Based on cluster analysis, zooplankton communities were generally divided by either the epipelagic or the deeper layers’ communities. In both sectors, the epipelagic layer was dominated by cyclopoid copepods, such as Oithona similis and Oncaea curvata, as well as calanoid copepods including Calanoides acutus, Rhincalanus gigas, and Ctenocalanus citer, while copepods and other taxa including Chaetognatha, Amphipoda, and Ostracoda, were important contributors to the deep layer communities.DiscussionOur analysis revealed that water masses, combined with their physical characteristics such as specific temperature and salinity ranges and depth, along with biological factors such as chlorophyll a concentration, might be the most important drivers for structuring zooplankton communities from epipelagic to upper bathypelagic layer.

Dominance of species sorting over dispersal at microgeographical scales in polluted lotic ecosystems

Global rivers, particularly those in populated urban areas, are challenged by multiple stressors such as pollution from anthropogenic activities. Dissecting the relative role of each ecological process involved in structuring biotic communities is fundamental in both theoretical and applied ecology. The ecological niche-based species sorting and stochastic dynamics-based dispersal are two major competing processes in determining community structure. Studies have reached a common realization on the environmental gradient-geographical scale relationship (EGGSR), resulting in species sorting over dispersal in determining community structure at large geographical scales where significant environmental gradients often exist. However, this relationship has been recently challenged at fine geographical scales when significant environmental gradients are formed by local anthropogenic activities. Here, we used three receiving rivers of wastewater treatment plants (WWTPs) as the promising system to test the hypothesis that species sorting plays a dominant role over dispersal in structuring zooplankton communities at microgeographical scales (∼1.2 km). After WWTP effluent discharge, we consistently detected significant environmental changes in all three receiving rivers, leading to significant variation in both community structure and taxonomic co-occurrence networks. Variation partitioning showed that environmental variables explained higher proportions of community variation than spatial ones, supporting that species sorting played a dominant role over dispersal in structuring zooplankton communities. Thus, our findings here reject EGGSR, illustrating that the effect of species sorting has been overlooked in disturbed aquatic ecosystems at fine spatial scales. More importantly, all analyses in multiple rivers here validate the “microscale species sorting” hypothesis. The validation of such hypothesis provides a novel methodology for point source pollution management by assessing environment-community interactions and functional changes of biological communities. The differed variables underlying species sorting among three rivers illustrate that ecological management should be case-specific, with the full consideration of local water quality background and pollutant composition of each point pollution source.

Which physicochemical variables should zooplankton ecologists measure when they conduct field studies?

Abstract Over the last century, a rich literature has developed describing how the physical and chemical environment influences zooplankton communities, but there is little guidance on the suite of limnological variables that should be measured by zooplankton ecologists. We performed a literature review to assess (i) which variables were measured most often by zooplankton ecologists, (ii) which of these variables were consistently related to zooplankton abundance and richness and (iii) whether key variables were overlooked by investigators. Our results show that there is a core group of nine limnological variables that are measured most frequently, including lake surface area, pH, phosphorus, nitrogen, dissolved oxygen, conductivity, chlorophyll-a, maximum depth and temperature. These variables were among those most often associated with variation in zooplankton, but several others, including dissolved organic carbon, alkalinity and nitrate, were sampled infrequently, despite showing promise as important explanatory variables. The selection of variables in past studies did not correlate with how often those variables were significant in the literature, but instead, might have been related to their ease of measurement. Neglecting to measure important variables could have implications for fundamental and applied studies that aim to understand the factors structuring zooplankton communities and their response to environmental change.

Zooplankton in a continuous waterscape: environmental and spatial factors shaping spring zooplankton community structure in a large canyon reservoir at the tropic of cancer

Reservoirs are continuous waterscapes that exhibit hydrodynamic characteristics of both lakes and rivers with water flow. These dynamic habitats provide a novel chance to explore the effects of metacommunity dynamics on community structure in relation to environmental, spatial, and temporal processes. We investigated the spatial distribution of spring zooplankton communities in a large reservoir in southern China. We intensively sampled along a longitudinal gradient during early and full spring. We compared the relationship of environmental and spatial variables to the community structure of three zooplankton groups: rotifers, cladocerans, and copepods. The overall composition and abundance of zooplankton groups across longitudinal zones became more heterogeneous with increasing environmental heterogeneity across sampling periods. Multi-level ANOVA and PERMANOVA results showed that rather than homogenize the environment, water flow in late spring increased spatial heterogeneity in the environment and increased the importance of species sorting in shaping the zooplankton community. However, variation partitioning analysis showed that the influence of environmental and spatial variables also varied across the three zooplankton groups likely due to their differences in both physiological and morphological adaptation. Our study shows that metacommunity processes are important in structuring zooplankton communities in reservoir waterscapes that are influenced by flow.

Seasonal strengths of the abiotic and biotic drivers of a zooplankton community

Abstract Multiple factors operate simultaneously and interact in complex ways to shape community assembly. Understanding the organisation of communities including species interactions and the importance of driving factors (food, temperature, nutrients, oxygen concentration, etc.) is a central topic in ecology. Intra‐annual variation in zooplankton and the factors that drive them are described in the Plankton Ecology Group (PEG) conceptual model. However, there has been little empirical evidence quantifying the relative contribution of abiotic and physical factors to plankton seasonality in zooplankton communities. Using a long‐term time series of zooplankton abundances and environmental conditions, we investigated which physical, chemical, and biotic factors structure zooplankton communities. We quantified their importance in seasonal changes in species composition by exploring networks of interacting species in the Římov Reservoir (Czech Republic). We investigated how these factors were associated with seasonal and intra‐annual (across all seasons) patterns of temporal variation in zooplankton. We predicted that: (1) the zooplankton community in the Římov Reservoir follows the general seasonal PEG pattern; (2) abiotic factors play an important role in the temporal distribution of zooplankton at the intra‐annual scale, but their influence on the community declines at the seasonal scale; and (3) the network of interacting species is more complex at the seasonal scale than at intra‐annual scale, so biotic drivers most strongly influence zooplankton patterns at seasonal scales. The intra‐annual pattern of zooplankton abundance was mainly determined by the physical environment. As expected, the roles of abiotic drivers of community assembly changed between seasons. There were more interactions between abiotic factors and species structuring the community at the intra‐annual scale than at the seasonal scale. Additionally, these interactions were weaker at the seasonal scale than the intra‐annual scale. The intra‐annual zooplankton community of the Římov Reservoir was the most stable, while the summer zooplankton community was the most variable. Our results provide evidence that the network of interacting zooplankton taxa is composed of positive and negative relationships, which suggests facilitative, competitive, and predator–prey interactions. Although zooplankton species interactions have an influence year‐round, the network of interacting species is more complex during the vegetative period and especially in summer, when species richness and abundance are high. Our results suggest that the zooplankton community is structured by both the biotic network at the seasonal scale and by the physical environment at the intra‐annual scale. Seasonal succession in zooplankton communities of temperate lakes is a well‐known pattern. However, the influence of interspecific interactions within seasons on this pattern has not been clearly shown until this study. Exploring the community in only one season may provide incomplete knowledge of the factors and species interactions driving the whole community.

Calanoid copepod zooplankton density is positively associated with water residence time across the continental United States

Inherent differences between naturally-formed lakes and human-made reservoirs may play an important role in shaping zooplankton community structure. For example, because many reservoirs are created by impounding and managing lotic systems for specific human purposes, zooplankton communities may be affected by factors that are unique to reservoirs, such as shorter water residence times and a reservoir’s management regime, compared to natural lakes. However, the environmental factors that structure zooplankton communities in natural lakes vs. reservoirs may vary at the continental scale and remain largely unknown. We analyzed data from the 2007 U.S. Environmental Protection Agency’s National Lakes Assessment and the U.S. Army Corps of Engineers’ National Inventory of Dams to compare large-bodied crustacean zooplankton communities (defined here as individuals retained by 0.243 mm mesh size) in natural lakes and reservoirs across the continental U.S. using multiple linear regressions and regression tree analyses. We found that large-bodied crustacean zooplankton density was overall higher in natural lakes compared to reservoirs when the effect of latitude was controlled. The difference between waterbody types was driven by calanoid copepods, which were also more likely to be dominant in the >0.243 mm zooplankton community in natural lakes than in reservoirs. Regression tree analyses revealed that water residence time was not a major driver of calanoid copepod density in natural lakes but was one of the most important drivers of calanoid copepod density in reservoirs, which had on average 0.5-year shorter water residence times than natural lakes. Reservoirs managed for purposes that resulted in shorter residence times (e.g., hydroelectric power) had lower zooplankton densities than reservoirs managed for purposes that resulted in longer residence times (e.g., irrigation). Consequently, our results indicate that water residence time may be an important characteristic driving differing large-bodied zooplankton dynamics between reservoirs and natural lakes.

Testing clustering strategies for metabarcoding-based investigation of community-environment interactions.

The degradation of freshwater ecosystems has become a common ecological and environmental problem globally. Owing to the complexity of biological communities, there remain tremendous technical challenges for investigating influence of environmental stressors (e.g., chemical pollution) on biological communities. High-throughput sequencing-based metabarcoding provides a powerful tool to reveal complex interactions between environments and biological communities. Among many technical issues, the clustering strategies for operational taxonomic units (OTUs) which are crucial for assessing biodiversity of communities, may affect final conclusions. Here, we used zooplankton communities along an environmental pollution gradient in the Chaobai River in Northern China to test different clustering strategies, including nonclustering and clustering with varied thresholds. Our results showed that though the number of OTUs estimated by nonclustering strategies and clustering strategies with divergence thresholds of 99%-97% largely varied, they were able to identify the same set of significant environmental and spatial variables responsible for geographical distributions of zooplankton communities. In addition, the ecological conclusions obtained by clustering thresholds of 99%-97% were consistent with nonclustering strategies, where for all eight clustering scenarios we detected that species sorting predicted by environmental variables overrode dispersal as the dominant factor in structuring zooplankton communities. However, clustering with the divergence thresholds of <95% affected the environmental and spatial variables identified. We conclude that both newly developed nonclustering methods and traditional clustering methods with divergence thresholds ≥97% were reliable to reveal mechanisms of complex community-environment interactions, although different clustering strategies could lead to largely varied biodiversity estimates such as those for α-diversity.

Are zooplankton communities structured by taxa ecological niches or by hydrological features?

AbstractThis study aimed to clarify the mechanisms that structure zooplankton communities in a lowland river network. Fourteen stations on the Scheldt upper river basin (France/Belgium) were sampled during 4 campaigns in 2014–2015.Firstly, a niche analysis of zooplankton taxa was conducted for April and September 2014 using the outlying mean index (OMI) ordination technique. Then, application of Hutchinson's niche and of Hubbell's neutral theories to zooplankton community of the river network was tested for April and June 2015. We subsequently tested whether the taxa ecological preferences revealed were reflected in the April and June 2015 zooplankton spatial distributions.In April 2015, zooplankton community dissimilarity (Bray–Curtis distance) was more influenced by environmental conditions than by hydrological distance, in accordance with niche theory. In June 2015, a stronger influence of neutral processes was suggested by an important role of hydrology. In 2014 samples, 17 over 32 taxa presented significant marginality relying on the considered environmental parameters. This specialization was reflected in the 2015 community patterns, particularly in April. During this month, local environmental parameters were more influencing than hydrology, probably related to a higher environmental heterogeneity than in June 2015.Although zooplankters are often considered to be generalists and opportunists, mainly dependent of river hydrology, these results showed that even for passive dispersers in highly connected systems, niche processes contribute to community structuring by local filtering. Determination of species niche parameters is thus important to understand observed community patterns but also changes in species distribution relative to environmental and hydrological changes.

Impact of eastern mosquitofish,Gambusia holbrooki,on temporary ponds: insights on how predation may structure zooplankton communities

AbstractEcological and evolutionary processes can conveniently be explored through the simplified communities of temporary ponds. In order to investigate the potent roles of predation in structuring zooplankton communities, we used a manipulative study to analyze the impact of eastern mosquitofish, Gambusia holbrooki, on a set of 24 temporary ponds. Fish were introduced at different densities (0, 8, 16 and 80 mosquitofish), and zooplankton assemblages were monitored monthly between January and May. Fish predation decreased markedly the overall abundance of zooplankton. Large crustaceans (Anostraca and Calanoida) were rapidly eliminated, while Cyclopoida and Cladocera peaked in March before in turn disappearing in the later stages of the hydroperiod. These results are congruent with the theory supporting the influence of both predation and competition as major interacting forces shaping freshwater communities.

Motile behavior and predation risk in planktonic copepods

AbstractPredation is an important source of mortality in zooplankton but factors governing predation risk in marine food webs are still not well understood. Here, we examine the role of zooplankton behavior in determining predation risk. We first quantified motility of copepods with different feeding behaviors (ambush feeding, cruising, and feeding‐current feeding). Second, we estimated remote predator detection and escape characteristics of the studied copepods. Third, we proposed a simple behavior‐dependent encounter model to predict copepod predation risk from rheotactic predators. Finally, we compared our predictions with predation risk previously determined experimentally. For similar sized copepods, predicted predation risks were similar between feeding‐current feeders and cruising feeders, whereas predation was up to 8.5 times lower (range: 1.5–8.5) for ambush feeders. Predicted predation risks further differed between males and females depending on feeding behavior: in ambush feeders males actively search for non‐motile females and their predation risk was up to 6 times higher (range 1.2–6) than for females. In contrast, feeding current‐ and cruising feeders showed small differences in predation risk between genders. In all cases, predicted relative predation risks between particular behaviors were confirmed by empirical data from previous predation experiments. Our results demonstrate that prey behavior of zooplankton may lead to a predictable variation in predation risk from rheotactic predators of up to an order of magnitude, and therefore that individual behavior is an important factor in structuring zooplankton communities.

Environmental drivers of the fine-scale distribution of a gelatinous zooplankton community across a mesoscale front

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 510:129-149 (2014) - DOI: https://doi.org/10.3354/meps10908 Environmental drivers of the fine-scale distribution of a gelatinous zooplankton community across a mesoscale front Jessica Y. Luo1,*, Benjamin Grassian1, Dorothy Tang1, Jean-Olivier Irisson2, Adam T. Greer1,3, Cedric M. Guigand1, Sam McClatchie4, Robert K. Cowen1,5 1Marine Biology and Fisheries, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, USA 2Ocean Observatory of Villefranche-sur-Mer, University of Pierre and Marie Curie, 181 Chemin du Lazaret, 06230 Villefranche-sur-Mer, France 3College of Engineering, University of Georgia, 200 D.W. Brooks Dr., Athens, Giorgia 30602, USA 4Southwest Fisheries Science Center, NOAA Fisheries, 8901 La Jolla Shores Drive, La Jolla, California 92037, USA 5Hatfield Marine Science Center, Oregon State University, 2030 SE Marine Science Drive, Newport, Oregon 97365, USA *Corresponding author: jessica.luo@rsmas.miami.edu ABSTRACT: Mesoscale fronts occur frequently in many coastal areas and often are sites of elevated productivity; however, knowledge of the fine-scale distribution of zooplankton at these fronts is lacking, particularly within the mid-trophic levels. Furthermore, small (<13 cm) gelatinous zooplankton are ubiquitous, but are under-studied, and their abundances underestimated due to inadequate sampling technology. Using the In Situ Ichthyoplankton Imaging System (ISIIS), we describe the fine-scale distribution of small gelatinous zooplankton at a sharp salinity-driven front in the Southern California Bight. Between 15 and 17 October 2010, over 129000 hydromedusae, ctenophores, and siphonophores within 44 taxa, and nearly 650000 pelagic tunicates were imaged in 5450 m3 of water. Organisms were separated into 4 major assemblages which were largely associated with depth-related factors. Species distribution modeling using boosted regression trees revealed that hydromedusae and tunicates were primarily associated with temperature and depth, siphonophores with dissolved oxygen (DO) and chlorophyll a fluorescence, and ctenophores with DO. The front was the least influential out of all environmental variables modeled. Additionally, except for 6 taxa, all other taxa were not aggregated at the front. Results provide new insights into the biophysical drivers of gelatinous zooplankton distributions and the varying influence of mesoscale fronts in structuring zooplankton communities. KEY WORDS: Gelatinous zooplankton · Jellyfish · Fronts · Community dynamics · Aggregations · Environmental drivers · Imaging systems · Southern California Bight Full text in pdf format Supplementary material PreviousNextCite this article as: Luo JY, Grassian B, Tang D, Irisson JO and others (2014) Environmental drivers of the fine-scale distribution of a gelatinous zooplankton community across a mesoscale front. Mar Ecol Prog Ser 510:129-149. https://doi.org/10.3354/meps10908 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 510. Online publication date: September 09, 2014 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2014 Inter-Research.

Predatory demands of Bythotrephes and Leptodora in Saginaw Bay, Lake Huron

Predatory invertebrates can be a major factor in structuring zooplankton communities. We collected zooplankton and the predatory cladocerans Bythotrephes longimanus and Leptodora kindtii at 4 stations in inner Saginaw Bay, Lake Huron during May–October 2009 and April–November 2010. Production of zooplankton prey was estimated using temperature and biomass based empirical models. Population consumption rates of Bythotrephes and Leptodora were estimated using a combination of conversion efficiency, bioenergetics and experimental based models. Bythotrephes biomass peaked in July of 2009 and 2010, and in 2010, the biomass rebounded in the fall after a late summer decline. Leptodora biomass was generally lower than that of Bythotrephes except in June 2009 and August 2010. Zooplankton biomass, production and composition showed strong seasonal patterns, with biomass more than tripling between May and June each year, largely due to an increase in biomass of Daphnia in June. By contrast, total zooplankton biomass declined by over 70% between June and July owing to declines in Daphnia biomass. Population consumption by predatory cladocerans was a large percentage or even exceeded prey production during July and August in Saginaw Bay regardless of which consumption models were used. This short term (<2months) but pronounced predation capacity of predatory cladocerans in Saginaw Bay was mainly due to consumption by Bythotrephes rather than Leptodora. Bythotrephes likely play an important role in food web function and should be accounted for when evaluating the flow of energy within the Laurentian Great Lakes systems.

Respiration rates of tropical Atlantic copepods in relation to the oxygen minimum zone

Zooplankton respiration plays an important role in the carbon cycling of pelagic ecosystems. The rate of oxygen consumption in zooplankton is affected by the physical environment, vertical distribution range and species-specific behavior. Especially in tropical oceans, oxygen minimum zones (OMZs) may influence zooplankton metabolic processes and vertical distribution and thus structure zooplankton communities. Here we present respiration rates of tropical Atlantic copepods in relation to environmental factors, especially O2 concentration, and species-specific characteristics. Copepods were sampled during two research stays on the Cape Verde Island São Vicente in March/April and May/June 2010. Minimum O2 concentrations of 51μmolkg−1 (pO2 of 4.25kPa) at 400m depth were recorded within the OMZ. Respiration rates of epi- and mesopelagic calanoid copepods were measured by optode respirometry at three different ambient temperatures (13, 18, and 23°C) to establish the effect of temperature on metabolic rates. Mass-specific oxygen consumption ranged from 27μmolO2gDM−1h−1 in copepodids C5 of Lophothrix sp. at 13°C to 774μmolO2gDM−1h−1 in Pleuromamma xiphias copepodids C5 at 18°C and was mainly controlled by body mass and temperature. Mass-specific respiration rates were highest in surface-dwelling organisms and decreased with increasing depth. To allow for a comparison of shallow and deep-living copepods, respiration rates were standardized to a common temperature of 18°C and a mean body dry mass of 0.5mg, applying a Q10 of 2.0 and a body mass exponent of −0.56. Temperature- and body mass-corrected respiration rates did not decrease with increasing depth indicating that neither depth of occurrence, nor current hypoxic conditions within the OMZ had a fundamental, persistent effect on zooplankton respiration.

Molecular Taxonomy of a Phantom Midge Species (Chaoborus flavicans) in Korea

The larvae of Chaoborus are widely distributed in lakes, ponds, and reservoirs. These omnivorous Chaoborus larvae are crucial predators and play a role in structuring zooplankton communities, especially for small-sized prey. Larvae of Chaoborus are commonly known to produce predator-induced polyphenism in Daphnia sp. Nevertheless, their taxonomy and molecular phylogeny are very poorly understood. As a fundamental study for understanding the role of Chaoborus in predator-prey interactions in a freshwater ecosystem, the molecular identification and phylogenetic relationship of Chaoborus were analyzed in this study. A molecular comparison based on partial mitochondrial cytochrome oxidase I (COI) between species in Chaoborus was carried out for the identification of Chaoborus larvae collected from 2 localities in Korea. According to the results, the Chaoborus species examined here was identified as C. flavicans, which is a lake-dwelling species. Furthermore, partial mitochondrial genome including COI, COII, ATP6, ATP8, COIII, and ND3 were also newly sequenced from the species and concatenated 5 gene sequences excluding ATP8 with another 9 dipteran species were compared to examine phylogenetic relationships of C. flavicans. The results suggested that Chaoborus was more related to the Ceratopogonidae than to the Culicidae. Further analysis based on complete mitochondrial DNA sequences and nuclear gene sequences will provide a more robust validation of the phylogenetic relationships of Chaoborus within dipteran lineages.

Planktivory in the changing Lake Huron zooplankton community: Bythotrephes consumption exceeds that of Mysis and fish

1. Oligotrophic lakes are generally dominated by calanoid copepods because of their competitive advantage over cladocerans at low prey densities. Planktivory also can alter zooplankton community structure. We sought to understand the role of planktivory in driving recent changes to the zooplankton community of Lake Huron, a large oligotrophic lake on the border of Canada and the United States. We tested the hypothesis that excessive predation by fish (rainbow smelt Osmerus mordax, bloater Coregonus hoyi) and invertebrates (Mysis relicta, Bythotrephes longimanus) had driven observed declines in cladoceran and cyclopoid copepod biomass between 2002 and 2007. 2. We used a field sampling and bioenergetics modelling approach to generate estimates of daily consumption by planktivores at two 91-m depth sites in northern Lake Huron, U.S.A., for each month, May–October 2007. Daily consumption was compared to daily zooplankton production. 3. Bythotrephes was the dominant planktivore and estimated to have eaten 78% of all zooplankton consumed. Bythotrephes consumption exceeded total zooplankton production between July and October. Mysis consumed 19% of all the zooplankton consumed and exceeded zooplankton production in October. Consumption by fish was relatively unimportant – eating only 3% of all zooplankton consumed. 4. Because Bythotrephes was so important, we explored other consumption estimation methods that predict lower Bythotrephes consumption. Under this scenario, Mysis was the most important planktivore, and Bythotrephes consumption exceeded zooplankton production only in August. 5. Our results provide no support for the hypothesis that excessive fish consumption directly contributed to the decline of cladocerans and cyclopoid copepods in Lake Huron. Rather, they highlight the importance of invertebrate planktivores in structuring zooplankton communities, especially for those foods webs that have both Bythotrephes and Mysis. Together, these species occupy the epi-, meta- and hypolimnion, leaving limited refuge for zooplankton prey.

Discriminating zooplankton communities in lakes with brook trout (Salvelinus fontinalis) and in fishless lakes

Natural geographic barriers and escarpments inhibited the post-glacial colonization of many lakes by fish on the eastern Canadian Boreal Shield. Hence, fishless lakes containing freshwater communities that evolved without fish predation are commonly found on the Boreal Shield next to fish-present lakes. The aim of this study was thus to assess how different top-down control from fish or invertebrate predation structure zooplankton communities in eastern Canadian Boreal Shield lakes throughout the ice-free season. We examined zooplankton community characteristics using univariate indices such as abundance, richness (S), evenness (J'), diversity (H'), and multivariate species assemblages (Bray—Curtis dissimilarity) in 5 lakes with a single fish population, brook trout, Salvelinus fontinalis, and in 5 naturally fishless lakes throughout the ice-free season. The total abundance of zooplanktonic organisms was significantly higher in fishless lakes over the whole sampling period; this was related to the presence of small herbivorous species such as rotifers. Species assemblages of zooplankton were also significantly different between the 2 types of lakes throughout the season, principally due to the high abundance of rotifers and the scarcity of immature and mature daphnids in fishless lakes. Our results suggest that the low intensity of selective predation by brook trout on large herbivores and the heavy predation by Chaoborus americanus larvae on daphnids in fishless lakes appear to be the key factors for structuring the zooplankton community. Multivariate indicators identified patterns that were not revealed with the usual univariate indices, showing the importance of selecting appropriate indicators for the assessment of community structure.

Population dynamics of Chaoborus flavicans and Daphnia spp.: effects on a zooplankton community in a volcanic eutrophic lake with naturally high metal concentrations (L. Monticchio Grande, Southern Italy)

The response of Daphnia populations to invertebrate predators involves morphological or behavioural changes. Few studies suggest that contaminant aqueous metals, like Cu or Ni at environmentally relevant concentrations, interfere with invertebrate chemical communication systems, such as that which operates between Daphnia and Chaoborus. The objective of our study was to determine if this interference could be also observed in lakes naturally rich in dissolved metals, such as volcanic lake (Lago Grande di Monticchio). This study aimed to assess if natural dissolved metals (e.g., Fe, Mn and Sr) could impair the ability of Daphnia pulex and D. galeata × hyalina × cucullata 'complex' populations to respond to Chaoborus kairomones by producing morphological defenses against potential predation, and to understand how Chaoborus predation might affect zooplankton community composition and overall zooplankton density. The predator impact did not result in: i) any morphological changes; ii) any apparent shift in body size pattern of the prey population; iii) any shift in life history traits. Chaoborus accounted for high mortality rates in Cladocera and strongly reduced the chance of individuals to reach maturity. Moreover, highly significant negative correlations between abundance of dominant taxa of zooplankton and C. flavicans were found. The last larval instars of C. flavicans seem to reduce the number of crustaceans, particularly cladocerans and copepod adults and could play an important role in structuring zooplankton communities. Our results suggest that metal inhibition of defence strategies induction probably occurs along the signal transduction pathway in Lake Grande di Monticchio. Impairment of chemosensory response to predatory chemical cues may have widespread ecological consequences in aquatic systems. Chaoborus predation effects can greatly affect both zooplankton biomass and community composition, impact interactions at lower trophic levels and generate an ecological cascade leading to a rapid eutrophication.

Zooplankton assembly in mountain lentic waters is primarily driven by local processes

The influence of local (i.e. physico-chemical constraints) and regional (i.e. dispersal) processes in determining the structure of zooplankton communities in lentic waters was analysed in a mountain district of 4500 km 2 (northern Apennines, Italy). In the 47 lakes and ponds considered in this study, a total of 139 taxa were found. Surface area (SA) and maximum depth ( Z max) significantly influenced species richness and nestedness, stressing the importance of habitat availability for zooplankton diversity and composition. Species richness and nestedness did not correlate with site isolation, indicating low influence of dispersal for these community properties. A negative relationship between the similarity in species composition among sites and their mutual geographic distance was also found. The relative importance of different sources of compositional variation was quantified by direct ordination. Local environmental conditions (i.e. Z max, altitude, pH and conductivity) accounted for 65.9 % of the variation in rotifer assemblages. The importance of environmental constraints in structuring zooplankton communities was best documented by the microcrustacean distribution in the study area, which was mainly correlated (73.4 % of the variation in the species data) with water ionic content, surface area and hydroperiod of the habitats. Although previous studies on processes shaping zooplankton assemblages that operate at different scales have yielded contrasting results, our data provide clear evidence for a major role of local constraints.

Are jellyfish increasing in response to ocean acidification?

Ocean acidification may negatively affect calcifying plankton, opening ecological space for noncalcifying species. Recently, a study of climate‐forcing of jellyfish reported the first analysis suggesting that there were more jellyfish (generally considered a noncalcifying group) when conditions were more acidic (lower pH) from one area within the North Sea. We examine this suggestion for a number of areas in the North Sea and beyond in the Northeast Atlantic using coelenterate records from the Continuous Plankton Recorder and pH data from the International Council for the Exploration of the Sea for the period 1946‐2003. We could find no significant relationships between jellyfish abundance and acidic conditions in any of the regions investigated. We conclude that the role of pH in structuring zooplankton communities in the North Sea and further afield at present is tenuous.

Feeding behaviour, efficiency and food preference in yabbies Cherax destructor

The influence of body size on the consumption of live zooplankton (Daphnia spp.) by freshwater crayfish was examined using yabbies (Cherax destructor) ranging from 5 to 45 g. Food preference between live zooplankton and inert pellets was also assessed under experimental conditions. In experimental tanks, yabbies of four size classes ( 0.05). Capture efficiency of live Daphnia by yabbies less than 15 g was significantly lower (76%, P = 0.008) than the three larger size classes (93.6%). Yabbies less than 15 g consumed a significantly (P < 0.001) higher percentage (5.2%) of their body weight than the other size classes (1.1%, 0.8%, and 0.6%, respectively). In the presence of both live zooplankton and a pellet diet, yabbies spent significantly (P = 0.005) more time feeding on zooplankton (85%) than on inert pellets (15%). This was the first study to quantify zooplankton consumption by yabbies and the results provide insights into understanding the trophic role of freshwater crayfish in structuring zooplankton communities and the husbandry management of crayfish farming.