Zooplankton Size Research Articles

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.

Effects of an anticyclonic eddy on the distribution and community structure of zooplankton in the South China Sea northern slope

Abstract Mesoscale anticyclonic eddies have been frequently observed in the South China Sea (SCS) northern slope. Here we present the first multidisciplinary cruise conducted in an anticyclonic eddy to characterize zooplankton community structure in the northern SCS during March 2017. In-situ net sampling combined with a ZooScan integrated system was used to determine the zooplankton biomass, abundances, and biovolumes in four size classes (0.2–0.5 mm, 0.5–1 mm, 1–2 mm, and 2–5 mm). According to the temperature and salinity, the stations were divided into three groups, corresponding with the inside, edge, and outside of the anticyclonic eddy, respectively. Elevated zooplankton biomass, abundances, and biovolumes were clearly observed at the edge and outside of the anticyclonic eddy compared to the inner eddy area. This was associated with the significantly increased proportions of zooplankton in the 1–2 mm size class. Copepoda were the major contributors to the zooplankton, with 68.7% of the total abundance and 51.5% of the total biovolume. The analysis of similarity revealed that the zooplankton size composition and taxonomic composition were significantly different between the inner and outer areas of the anticyclonic eddy. In addition, we also found obvious diel variations for the zooplankton, particularly in the inner anticyclonic eddy area. Shrimp showed significantly diel variations (0–200 m) in this region. The anticyclonic eddy substantially influenced the zooplankton distribution and community structure in the SCS northern slope during March 2017.

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.

The life aquatic in high relief: shifts in the physical and biological characteristics of alpine lakes along an elevation gradient in the Rocky Mountains, USA

Rapidly occurring environmental changes in alpine lakes highlight the importance of better understanding the ecological structure and function associated with these systems. Previous research has identified how the physical characteristics of lakes change as a function of landscape position, but comparatively little is known about shifts in the biotic community across mountain regions. In 2016, we sampled 19 lakes across an elevation gradient (2480–3550 m a.s.l.) within the Rocky Mountains, USA, to evaluate how both the abiotic characteristics of lakes and their planktonic biological communities covaried with elevation. Based on generalized linear mixed models (GLMMs), increases in elevation were associated with decreases in most nutrient concentrations (with the exception of nitrate), dissolved organic carbon, water temperature and lake stratification. Conversely, elevation increases were positively related to nitrate concentrations and water clarity. Extending this analysis to the biological community, we found that higher-elevation lakes exhibited lower phytoplankton and zooplankton densities, whereas elevation associated positively with average zooplankton size. Our data are consistent with the hypothesis that the alpine environment acts as a strong niche filter, limiting the quantity and diversity of taxa to groups capable of tolerating the short growing season, high flushing rate, strong variation in interannual precipitation, intense ultraviolet radiation exposure, and lower resource availability associated with such habitats.

Calibration of the zooplankton community size spectrum as an indicator of change in Canadian Shield lakes

Developing the crustacean zooplankton community size spectrum into an indicator of change in lakes requires quantification of the natural variability in the size spectrum related to broad-scale seasonal, annual, and spatial factors. Characterizing seasonal patterns of variation in the size spectrum is necessary so that monitoring programs can be designed to minimize the masking effects that seasonal processes can have on detecting longer-term temporal change. We used a random effects model to measure monthly, annual, and interlake variability in the slope (i.e., relative abundance of small and large organisms) and centered height (i.e., total abundance) of the crustacean zooplankton normalized abundance size spectrum from 1981 to 2011 among eight Canadian Shield lakes. Consistent with theoretical predictions, the slope was a relatively stable characteristic of the zooplankton community compared with the height, which varied significantly among lakes. We identified a seasonal signal in height and slope and used a mixed effects model to characterize the linear rate of change from May to October; there was an overall decline in height and an overall increase in slope. Seasonal variance was greater than annual variance for both the height and the slope, suggesting that long-term monitoring of lakes and interlake comparisons using zooplankton size spectra should be based on temporally standardized sampling protocols that minimize the effects of seasonal processes. We recommend sampling the zooplankton community in midsummer because this results in size spectrum estimates close to seasonal mean values.

Why Do Phytoplankton Evolve Large Size in Response to Grazing?

Phytoplankton are among the smallest primary producers on Earth, yet they display a wide range of cell sizes. Typically, small phytoplankton species are stronger nutrient competitors than large phytoplankton species, but they are also more easily grazed. In contrast, evolution of large phytoplankton is often explained as a physical defense against grazing. Conceptually, this explanation is problematic, however, because zooplankton can coevolve larger size to counter this size-dependent escape from grazing. Here, we hypothesize that there is another advantage for the evolution of large phytoplankton size not so readily overcome: larger phytoplankton often provide lower nutritional quality for zooplankton. We investigate this hypothesis by analyzing an eco-evolutionary model that combines the ecological stoichiometry of phytoplankton-zooplankton interactions with coevolution of phytoplankton and zooplankton size. In our model, evolution of cell size modifies the nutrient uptake kinetics of phytoplankton according to known allometric relationships, which in turn affect the nutritional quality of phytoplankton. With this size-based mechanism, the model predicts that low grazing pressure or nonselective grazing by zooplankton favors evolution of small phytoplankton cells of high nutritional quality. In contrast, selective grazing for nutritious food favors evolution of large phytoplankton of low nutritional quality, which are preyed on by medium- to large-sized zooplankton. This size-dependent change in food quality may explain the commonly observed shift from dominance by small picophytoplankton in oligotrophic waters with low grazing pressure to large phytoplankton species in nutrient-rich waters with high grazing pressure.

Spatial patterns of zooplankton size structure in relation to environmental factors in Jiaozhou Bay, South Yellow Sea.

The spatial variation in size structure of mesozooplankton and its relationship with environmental factors were studied from January 2006 to December 2010 in Jiaozhou Bay. The spatial body-size patterns of zooplankton differed significantly from the inner to the outer bay, and have a significant correlation with environmental variables, especially nutrients, salinity and water depth. The abundance of total zooplankton and small size ranks presented a decreasing gradient from the inner to the outer part, and was positive correlated with nutrients and negative with salinity, while the abundance of large size ranks exhibited an opposite gradient, and was positive correlated with water depth and negative with nutrients. The size spectra slope and size diversity were generally low in the inner part and closely related to body size ranks. Our results suggest that size structure patterns of zooplankton showed a clear spatial pattern and significantly associated with environmental factors in marine ecosystems.

Responses of zooplankton body size and community trophic structure to temperature change in a subtropical reservoir.

Understanding the effects of global warming on trait variation and trophic structure is a crucial challenge in the 21st century. However, there is a lack of general patterns that can be used to predict trait variation and community trophic structure under the ongoing environmental change. We investigated the responses of body size and community trophic structure of zooplankton to climate related factors (e.g., temperature). Isotopic niche breadth was applied to investigate the community trophic structure across a 1‐year study from a subtropical reservoir (Tingxi Reservoir) in southeastern China. Body size and community isotopic niche breadth of zooplankton were larger during water mixing than stratification periods and correlated significantly with water temperature change along the time series. The contributions of intra‐ and intertaxonomic components to body size and community trophic structure variation showed significant relationships with the temperature change going from the mixing to stratification periods. Water temperature imposed direct effect on body size, while direct and indirect effect on the community trophic structure of zooplankton occurred through trophic redundancy along time series. Water temperature and community properties (e.g., body size, trophic redundancy, or trophic interaction) showed complex interactions and integrated to influence community trophic structure of zooplankton. Our results can expand the knowledge of how elevated temperature will alter individual trait and community trophic structure under future climate change.

The Importance of Mesozooplankton Diel Vertical Migration for Sustaining a Mesopelagic Food Web

We used extensive ecological and biogeochemical measurements obtained from quasi-Lagrangian experiments during two California Current Ecosystem Long-Term Ecosystem Research cruises to analyze carbon fluxes between the epipelagic and mesopelagic zones using a linear inverse ecosystem model (LIEM). Measurement constraints on the model include 14C primary productivity, dilution-based microzooplankton grazing rates, gut pigment-based mesozooplankton grazing rates (on multiple zooplankton size classes), 234Th:238U disequilibrium and sediment trap measured carbon export, and metabolic requirements of micronekton, zooplankton, and bacteria. A likelihood approach (Markov Chain Monte Carlo) was used to estimate the resulting flow uncertainties from a sample of potential flux networks. Results highlight the importance of mesozooplankton active transport (i.e., diel vertical migration) for supplying the carbon demand of mesopelagic organisms and sequestering carbon dioxide from the atmosphere. In nine water parcels ranging from a coastal bloom to offshore oligotrophic conditions, mesozooplankton active transport accounted for 18% - 84% (median: 42%) of the total carbon supply to the mesopelagic, with gravitational settling of POC (12% - 55%; median: 37%) and subduction (2% - 32%; median: 14%) providing the majority of the remainder. Vertically migrating zooplankton contributed to downward carbon flux through respiration and excretion at depth and via consumption loses to predatory zooplankton and mesopelagic fish (e.g. myctophids and gonostomatids). Sensitivity analyses showed that the results of the LIEM were robust to changes in nekton metabolic demands, rates of bacterial production, and mesozooplankton gross growth efficiency. This analysis suggests that prior estimates of zooplankton active transport based on conservative estimates of standard (rather than active) metabolism should be revisited.

Organic Contaminants in Zooplankton of Italian Subalpine Lakes: Patterns of Distribution and Seasonal Variations

Zooplankton is a key node in many trophic webs, both for food that for persistent organic contaminants that can accumulate in biota. Zooplankton of different size was seasonally sampled for two years in three deep Italian subalpine lakes (Maggiore, Como, Iseo) with the aim of determining the concentrations of perfluoroalkyl substances (PFAS), DDT, and PCB, and assessing the seasonality impacts on contaminants concentrations. In general, Lake Maggiore showed the highest concentrations for each group of contaminants, with mean values of 7.6 ng g−1 ww for PFAS, 65.0 ng g−1 dw for DDT, and 65.5 ng g−1 dw for PCB. When considering the composition pattern, perfluorooctane sulfonate (PFOS) was detected in 96% of the samples and it was the predominant PFAS compound in all of the lakes. pp’ DDE was the most detected congener among DDTs and their metabolites, while for PCBs, the prevalent group was hexa-CB that constituted 35.4% of the total PCB contamination. A seasonal trend was highlighted for all contaminant groups with concentrations in colder months greater than in spring and summer; it was evident that the contaminant concentrations were more dependent from seasonality than from size, trophic levels, and taxa composition of zooplankton. Principal component analysis showed that one of the main driver for the accumulation of most of the studied contaminants is their lipophilicity, except for perfluorooctanoic acid (PFOA) and octachlorobiphenyl.

New net zooplankton geographical information system in the Far East seas and adjacent waters of the Pacific Ocean

AbstractMotivationI describe the GIS that is based on a new database of zooplankton collected by Juday net with a 0.1 m2 opening (0.168 mm mesh).Main types of variables containedThe average density (in milligrams per cubic metre) of plankton and their different constituents in 1° trapeziums.LocationChukchi, Bering, Okhotsk, Japan/East seas and Pacific Ocean.Time period1984–2013.Taxa studiedAll mesofauna; > 214 species of holo‐ and meroplankton.MethodsGIS creation, data analysis and literature review.Software formatAny that is capable of working with shapefiles.ResultsMaps of the spatio‐temporal distribution of plankton with various taxonomic groups and dimensions were compiled and analysed. Based on these maps and on the literature, a hypothesis was made regarding the negative correlation of the zooplankton size with temperature. It was also revealed that some fluctuations in the abundance of plankton in the Bering Sea and the ocean are in phase, whereas in the Sea of Okhotsk and the Sea of Japan the fluctuations are fully out of phase. In particular, during the transition from the light to the dark time of the day in the Sea of Okhotsk and the Sea of Japan, the density of plankton almost everywhere throughout the epipelagic zone increases; however, in the Bering Sea and the ocean, over large parts of the water area, it decreases. This means that the common practice by trophologists of attempting to replace the day‐time catch in plankton nets with the night‐time catches to assess the food reserves for fish will yield significantly different results in these waters.Main conclusionThis unique GIS could be useful to planktonologists, ichthyologists, hydrobiologists, trophologists, ecologists, biogeographers and modellers for understanding patterns and drivers of plankton diversity and biomass variations at large scales. Unfortunately, owing to lack of funding, it has not been brought to the levels of species, size classes and developmental stages that could be available in the final release.

Zooplankton Size as a Factor Determining the Food Selectivity of Roach (Rutilus Rutilus) in Water Basin Outlets

Fish occurring in the outlets of water basins reduce the abundance of zooplankton. The study was performed at the outlet sections of the lake and waste stabilization pond of a sewage treatment plant. The aim of the study was to determine which zooplankton is chosen more often by the roach (Rutilus rutilus), those drifting from the waste stabilization pond or from the lake. The zooplankton from the pond was dominated by Daphnia pulex while zooplankton from the lake was dominated by small planktonic rotifers. We observed that the larger the plankter-victim’s size, the faster the reduction of its number. The fish were more likely to feed on zooplankton drifting from the waste stabilization pond than from the lake. It was influenced by D. pulex individuals, attractive for fish due to their largest body size among the analyzed zooplankton. The significance of riverine zooplankton in the downstream food web may render this data even more important.

Spatial distribution of zooplankton in the Kolsaiskie mountain lakes (Kungey Ala Tau, south-east Kazakhstan) and its connection with environment factors

The values of Clark W-statistics, ∆-Shannon-Weaver, and an average individual mass of specimen were used to characterize the spatial distribution of zooplankton in the Kolsay lakes. With a relatively constant species composition, the interannual dynamics of the size and quantity variables of zooplankton testified to an increase in the eutrophication processes of three of the four Kolsay lakes. Interannual changes in the structure of the zooplankton community were mostly determined by the introduction of allochthonous species of fish, reduction of the water level, and enhancement of biogenic load due to recreational use of the lakes. The effect of very low concentrations of heavy metals on the dimensional variables of zooplankton can be connected with the oligotrophic status of Kolsaiskie lakes.

Trophic cascade strength is influenced by size frequency distribution of primary consumers and size-selective predation: examined with mesocosms and modeling

Understanding variation in trophic cascades is critical for developing management strategies for invasive and introduced species as well as in designing biomanipulation experiments. We evaluated, in fish driven trophic cascades, the responses of invertebrate biomass, body size, and production both experimentally and through a model. Experimental mesocosm treatments of small (Moina), large (Daphnia pulex), and mixed size zooplankton communities were characterized with and without size-selective fish (bluegill, Lepomis macrochirus). Experimental responses were typical of trophic cascades showing reduced biomass, body size and production of zooplankton and generally enhanced chlorophyll a. Although zooplankton biomass was similarly reduced in all fish treatments, production for the fish D. pulex treatment was lower than the other fish treatments because larger taxa have a lower ratio of production to biomass (P/B). As would be expected based on zooplankton production among fish treatments, response of chlorophyll a in treatments with D. pulex was faster and final concentration higher than in Moina or mixed size treatments. A model of changes in production under a range of reductions in body size and/or biomass, from no reduction to 99% reduction, supported experimental results indicating invertebrate production can be resilient to reduction of biomass when body size is reduced. Strength of trophic cascades may therefore, in part, be related to interactions among reduction in biomass, size-selective predation, and size frequency distribution and plasticity of prey community. Because size-selective predation is a common trait in trophic cascades, some level of compensation is likely in most trophic cascades.

Zooplankton Size Structure in the Kolsay Mountain Lakes (Kungei Alatau, Southeastern Kazakhstan) and Its Relationships with Environmental Factors

The size structure of zooplankton in Kolsay Lakes was characterized with the use of the values of Clarke W-statistic, ∆-Shannon–Weaver, and the average individual mass of the organism. The species composition remaining relatively stable, the year-to-year dynamics of the size and abundance characteristics of zooplankton suggested more rapid eutrophication processes in three out of four Kolsay Lakes. The year-to-year changes in the structure of zooplankton communities were largely determined by the immigration of an allochthonous fish species, a drop in water level, and an increase in the biogenic load caused by the recreation use of the lakes. The effect of very low concentrations of heavy metals on the zooplankton size characteristics can be due to the oligotrophic status of the Kolsay Lakes.

Temporal variability in zooplankton community in the western Yellow Sea and its possible links to green tides.

Large-scale macro-algal blooms of Ulva prolifera (also called green tides) have appeared each summer since 2008 in the western Yellow Sea. In this study, we investigated the temporal variability in zooplankton community in the western Yellow Sea and its possible links to green tides using data from a long-term plankton survey off the coast of Qingdao, China. Environmental conditions observed in the study area during the green tide period (GTP: June–August, 2008–2013) were compared to the non-green tide period (NGTP: June–August, 2005–2007), to support the contention that variations observed in zooplankton community may be attributed to the green tides, as opposed to natural climatic or environmental variations. Zooplankton assemblage structure observed during the GTP was then compared to the NGTP. Significant variations were detected both in zooplankton abundance and assemblage structure between the two defined periods. The abundance of zooplankton, mainly copepods, was significantly decreased during the GTP. Meanwhile, the relative abundance of copepods decreased by approximately 10% and that of gelatinous zooplankton, including appendicularians, chaetognaths, and medusae, almost doubled (ca. increased by 6.4%). The dominant species of meroplankton completely changed, specifically, polychaeta, and echinoderm larvae were more dominant than decapod and bivalve larvae. With regard to zooplankton size structure, the NGTP showed a higher size diversity with more small-sized organisms, while the GTP showed a lower size diversity in the community. According to general linear models, the interannual variation in summer zooplankton abundance was significantly correlated with green tides. These results indicate that the temporal changes in zooplankton community may have a close link to the green tides.

Terrestrial organic matter increases zooplankton methylmercury accumulation in a brown-water boreal lake

Increases in terrestrial organic matter (tOM) transport from catchments to boreal lakes can affect methylmercury (MeHg) accumulation in aquatic biota both directly by increasing concentrations of aqueous MeHg, and indirectly through effects on MeHg bioavailability and on energy pathways in the lower food web. We carried out a detailed seasonal study of water chemistry, zooplankton diet, and MeHg accumulation in zooplankton in two lakes with contrasting tOM concentrations. Between-lake differences explained 51% of the variability in our water chemistry data, with no observed effect of season or sampling depth, contrary to our expectations. Higher tOM was correlated with higher aqueous Hg concentrations, lower areal pelagic primary productivity, and an increased contribution of terrestrial particles to pelagic particulate organic matter. Based on dietary marker analysis (δ13C, δ15N, and fatty acid [FA] composition), zooplankton diet was strongly linked to feeding mechanism, with dietary reliance on phytoplankton highest in the selective-feeding calanoid copepods, and lowest in filter feeding cladocerans. Zooplankton dietary reliance on phytoplankton and their concentrations of high-quality lipids, including polyunsaturated fatty acids, were higher in the clear-water lake than in the brown-water lake, where bacterial and terrestrial food sources were more prevalent. MeHg was highest in zooplankton from the brown-water lake, with highest concentrations in the 200–500 μm zooplankton size fraction for both lakes. Contrary to our expectations, there was no effect of season on zooplankton dietary markers or MeHg. Our results suggest that, overall, higher tOM results in higher MeHg concentrations in water and zooplankton, and reduces zooplankton dietary reliance on phytoplankton. Increased tOM thus leads to a decrease in the nutritional quality of zooplankton (i.e. higher MeHg concentrations, and lower concentrations of essential fatty acids), which may cascade up the food web with negative implications for higher trophic levels.

Assessing the relevance of top-down and bottom-up effects as phytoplankton structure drivers in a subtropical hypereutrophic shallow lake

Although several studies explain the trophic cascade in water systems, we lack knowledge about top-down–bottom-up effects on phytoplankton from eutrophic lakes. In this study, we tested the importance of trophic cascades on phytoplankton structure, predicting that environmental variations are the main drivers. We performed a monthly sampling during a year to measure environmental variables, phytoplankton and zooplankton, plus two samplings (winter and summer) to assess fish structure. Furthermore, we analyzed zooplanktivorous fish stomach-gut contents and completed a hatching zooplankton resting egg experiment to assess the effect of fish on dormant populations. We ran a partial redundancy analysis (pRDA) for phytoplankton using zooplankton, nutrient availability and environmental variables as predictor variables. We finally calculated several ratios of the zooplankton:phytoplankton biovolume to assess potential predation effects. Phytoplankton was correlated with variations in temperature and conductivity plus nutrients (pRDA: 63.4%, F = 4.6, P = 0.001) and was dominated alternatively by diatoms and cyanobacteria. Zooplankton was dominated by microphagous rotifers (> 45% of the total biovolume), and only the ratio of microphagous rotifer:small chlorophytes was significant during summer and autumn (F = 10.6, P = 0.005). The fish community was dominated by insectivorous-planktivorous fish (> 65% of total density), yet a negative selection of zooplankton items (Ivlev’s index < 0) was found. Nevertheless, the zooplankton resting stage analysis showed that microphagous Rotifera were dominant (29 species emerged), suggesting a structuring effect of fish on the zooplankton size. We conclude that phytoplankton was mainly controlled by environmental variations plus nutrient availability, while top-down had a less evident effect.

Biochemical composition and energy content of size-fractionated zooplankton east of the Kerguelen Islands

Food quality is recognized as a key parameter of food web functioning in which zooplankton plays a crucial role not only in linking lower to upper trophic levels but also in transforming the quality of the organic matter available to predators. The influence of size and taxonomic group composition of zooplankton in these processes was assessed in eastern Kerguelen waters (Southern Ocean) at the onset of the spring bloom in 2011. Biochemical (lipids, proteins and carbohydrates) and elemental (carbon and nitrogen) composition were measured in five size—fractions of bulk zooplankton ranging from 80 µm to > 2000 µm and in large copepods, euphausiids, annelids and salps, and energy content was derived from biochemical contents. Proteins were the dominant component of zooplankton dry weight (21.5% dw), followed by lipids (8.9% dw), soluble carbohydrates (2.2% dw) and insoluble carbohydrates (1.0% dw). A concentration increase with zooplankton size for all biochemical components was observed, particularly stronger for proteins and lipids. Copepods and salps provided, respectively, the highest and the lowest amount of lipids and energy. A four-fold increase in energy content was observed from the smallest to the largest fraction inducing a significant increase (> 10 kJ g−1 dw) in the quality of zooplankton matter. This may explain why large zooplankton represent a major food resource for numerous fish, seabirds and marine mammals in the Southern Ocean. Such unique results are required to better quantify energy dynamics in polar food webs.

Surface zooplankton size and taxonomic composition in Bowdoin Fjord, north-western Greenland: A comparison of ZooScan, OPC and microscopic analyses

In Greenland, tidewater glaciers discharge turbid subglacial freshwater into fjords, forming plumes near the calving fronts. To evaluate the effects of this discharge on the zooplankton community in the fjords, we collected sea surface zooplankton samples in Bowdoin Fjord in north-western Greenland during the summer of 2016 and made microscopic, OPC and ZooScan analyses. Within the three quantitative methods, ZooScan has advantages that can evaluate various parameters (e.g., abundance, biomass, size and taxonomic information) simultaneously and has the ability to eliminate abiotic particles, such as silt and sediment, which are abundant in samples. Based on taxonomic biomass data, the zooplankton community is clustered into three groups, which varied spatially: inner, middle and outer fjord groups. Jellyfish dominated the outer fjord group, and barnacle cypris larvae dominated the middle fjord group. For the inner fjord group, large-sized Calanus spp. and chaetognaths were abundant. Since these species are characterized with oceanic taxa, they would intrude through the deep fjord water and subsequently be upwelled through entrainment of glacially modified plume water. From the NBSS analysis on zooplankton size spectra, the steep slope of NBSS in the middle fjord community suggests that the high productivity was caused by the addition of meroplanktonic cypris larvae.