DOI 10.1371/journal.pone.0308803 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.
| File | Pages | Size | Access | |
|---|---|---|---|---|
Publisher's official version | 33 | 3 Mo | ||
S1 Fig. Sampling stations IBTS 2008. | - | 842 Ko | ||
S2 Fig. Empirical and theoretical variograms of species used for the determination of optimal grid cell size. | - | 663 Ko | ||
S3 Fig. Taxon-specific optimal grid cell size (Lopt). | - | 805 Ko | ||
S4 Fig. Number of sampling stations per cell. | - | 599 Ko | ||
S5 Fig. Schematic representation of the methodological approach. | - | 508 Ko | ||
S6 Fig. Statistical methods to determine optimal number of clusters (2008 big spatial extent). | - | 317 Ko | ||
S7 Fig. Highest membership values per cell for clustering using different k. | - | 802 Ko | ||
S8 Fig. Number of clusters/cut-off levels tested. | - | 790 Ko | ||
S9 Fig. Percentage of explained variance of dimensions of PCA on taxonomical data big spatial extent. | - | 599 Ko | ||
S10 Fig. PCA applied to zooplankton taxa. | - | 530 Ko | ||
S11 Fig. Mean abundance of the most important taxa per cluster. | - | 702 Ko | ||
S12 Fig. Mean abundance of taxa per cluster with individually scaled y-axis. | - | 974 Ko | ||
S13 Fig. Drivers per assemblage. | - | 414 Ko | ||
S14 Fig. Distribution of nutrients and chlorophyll a. Maps display mean per grid cell of nutrients sampled during the IBTS 2008. | - | 1 Mo | ||
S15 Fig. PCA on potential abiotic and biotic drivers. | - | 976 Ko | ||
S16 Fig. Boxplots of PCA dimensions per cluster. | - | 539 Ko | ||
S17 Fig. Relative phyto- microplankton composition per zooplankton assemblage. | - | 435 Ko | ||
S18 Fig. Statistical methods to determine optimal number of clusters (Comparison 2008 vs 2022, small spatial extent). | - | 467 Ko | ||
S19 Fig. Clusters received using different k. | - | 359 Ko | ||
S20 Fig. | - | 913 Ko | ||
S21 Fig. Difference in abundance between 2008 and 2022 per grid cell. | - | 298 Ko | ||
S22 Fig. Inter-annual differences of environmental conditions. | - | 496 Ko | ||
S1 Table. Taxa determined in 2008 sorted by taxonomic level. | - | 18 Ko | ||
S2 Table. Taxa determined in 2022 sorted by taxonomic level. | - | 18 Ko | ||
S1 File. Taxa determination mesozooplankton. | 1 | 49 Ko | ||
S2 File. Calculation of mesozooplankton abundance. | 1 | 80 Ko | ||
S3 File. Calculation of fish larvae and fish eggs, phyto-microplankton. | 1 | 38 Ko | ||
S4 File. Determination optimal grid cell size. | 3 | 97 Ko | ||
S5 File. Choice of taxa for clustering. | 1 | 41 Ko | ||
S6 File. Pilot study clustering method. | 1 | 48 Ko | ||
S7 File. Definition of optimal number of clusters. | 2 | 60 Ko | ||
S8 File. Detailed characterization of assemblages. | 3 | 72 Ko | ||
S9 File. Maps and heatmaps of inter-annual differences of taxa abundance and environmental parameters per grid cell. | 1 | 97 Ko |
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