Winter distribution of zooplankton and ichthyoplankton assemblages in the North Sea and the English Channel

Although zooplankton were extensively studied in the North Sea, knowledge about winter zooplankton assemblages is still scarce, despite potential influence of zooplankton overwintering stocks on seasonal plankton succession and productivity. Furthermore, several economically and ecologically important fish species reproduce during winter contributing to the zooplankton community as passive members (eggs) or predators (larvae). To elucidate on winter zooplankton distribution, abundance and composition in the Southern North Sea and Eastern English Channel, we defined assemblages based on mesozoo- and ichthyoplankton data sampled between January and February 2008 using fuzzy-clustering and indicator species. Mesozoo- and ichthyoplankton (eggs+larvae) were integrated in a common analysis by using a spatial grid adapted to the datasets and defined by means of a geostatistical method developed in agronomics. Potential environmental drivers of assemblage distribution were evaluated by means of GLMM and comparison with data from 2022 facilitated insight about the inter-annual representativeness of the assemblages. Five zooplankton assemblages were found varying with regard to total zooplankton abundance, dominant and indicator taxa. Spatial variability of abiotic (dissolved nutrients, salinity, depth, temperature, organic matter in suspension, chlorophyll a), biotic variables (phyto- and microplankton composition), water masses and fish spawning grounds were revealed as potential drivers of assemblage distribution. Assemblages off the Rhine-Scheldt estuary and in the German Bight harbored the biggest zooplankton overwintering stocks that might influence the grazing pressure on phytoplankton spring production. Assemblages off the Rhine-Scheldt estuary and covering the English Channel and the Southern Bight were found to be of high importance for herring and plaice larvae. Although further analyses suggested inter-annual representativeness of the assemblages found (2008 vs 2022), the assessment of further years would be necessary to account for potential inter-annual variability. Future studies could profit from the assessment of microzooplankton facilitating insight in fish larvae feeding potential and zooplankton overwintering strategies.

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S1 Fig. Sampling stations IBTS 2008.
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S2 Fig. Empirical and theoretical variograms of species used for the determination of optimal grid cell size.
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S3 Fig. Taxon-specific optimal grid cell size (Lopt).
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S4 Fig. Number of sampling stations per cell.
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S5 Fig. Schematic representation of the methodological approach.
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S6 Fig. Statistical methods to determine optimal number of clusters (2008 big spatial extent).
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S7 Fig. Highest membership values per cell for clustering using different k.
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S8 Fig. Number of clusters/cut-off levels tested.
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S9 Fig. Percentage of explained variance of dimensions of PCA on taxonomical data big spatial extent.
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S10 Fig. PCA applied to zooplankton taxa.
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S11 Fig. Mean abundance of the most important taxa per cluster.
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S12 Fig. Mean abundance of taxa per cluster with individually scaled y-axis.
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S13 Fig. Drivers per assemblage.
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S14 Fig. Distribution of nutrients and chlorophyll a. Maps display mean per grid cell of nutrients sampled during the IBTS 2008.
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S15 Fig. PCA on potential abiotic and biotic drivers.
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S16 Fig. Boxplots of PCA dimensions per cluster.
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S17 Fig. Relative phyto- microplankton composition per zooplankton assemblage.
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S18 Fig. Statistical methods to determine optimal number of clusters (Comparison 2008 vs 2022, small spatial extent).
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S19 Fig. Clusters received using different k.
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S20 Fig.
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S21 Fig. Difference in abundance between 2008 and 2022 per grid cell.
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S22 Fig. Inter-annual differences of environmental conditions.
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S1 Table. Taxa determined in 2008 sorted by taxonomic level.
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S2 Table. Taxa determined in 2022 sorted by taxonomic level.
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S1 File. Taxa determination mesozooplankton.
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S2 File. Calculation of mesozooplankton abundance.
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S3 File. Calculation of fish larvae and fish eggs, phyto-microplankton.
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S4 File. Determination optimal grid cell size.
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S5 File. Choice of taxa for clustering.
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S6 File. Pilot study clustering method.
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S7 File. Definition of optimal number of clusters.
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S8 File. Detailed characterization of assemblages.
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S9 File. Maps and heatmaps of inter-annual differences of taxa abundance and environmental parameters per grid cell.
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How to cite
Neven Carolin Julie, Giraldo Carolina, Girardin Raphael, Lefebvre Alain, Lefebvre Sebastien, Loots Christophe, Meunier Cédric Leo, Marchal Paul (2024). Winter distribution of zooplankton and ichthyoplankton assemblages in the North Sea and the English Channel. Plos One. 19 (10). e0308803 (33p.). https://doi.org/10.1371/journal.pone.0308803, https://archimer.ifremer.fr/doc/00914/102587/

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