(2022). Winter distribution of zooplankton and ichthyoplankton assemblages in the North Sea and the English Channel. SEANOE. https://doi.org/10.17882/101042
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Winter distribution of zooplankton and ichthyoplankton assemblages in the North Sea and the English Channel
Although zooplankton was 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 shed some light 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.
Disciplines
Biological oceanography
Keywords
spatial distribution, North Sea, zooplankton assemblages, English Channel, mesozooplankton, ichthyoplankton, winter
Location
57N, 49.5S, 0E, 8W
Devices
Mesozooplankton:
In 2008 and 2022, a total of respectively 142 and 43 mesozooplankton samples were taken using a WP2 bongonet (mesh size 200 µm) which was vertically hauled from 3 m above bottom to surface. Species determination was assisted by a ZooScan. Briefly, a ZooScan allows scanning zooplankton samples as pictures, which are subsequently used by a machine-learning algorithm for species determination. The machine-learning algorithm used in 2008 was Plankton Identifier (http://www.obs-vlfr.fr/~gaspari/Plankton_Identifier/index.php) and Eco-Taxa (http://ecotaxoserver.obs-vlfr.fr) in 2022. Both algorithms are based on the ZooProcess software and produce comparable results. Samples stored in 1% formol were rinsed and specimens were divided in two size classes (A: 200-500 µm, B: >500 µm) to increase determination success of the method. To further enhance determination exactness, samples were fractionated by means of the Motoda method. Finally, specialists in zooplankton taxonomy validated ZooScan taxonomic classification output.
Mesozooplankton abundance per taxon and station/sample was calculated as described in the supplementary of the affiliated study (S2 and S3 Sections).
Fish larvae:
In 2008 and 2022, a total of respectively 130 and 103 fish larvae samples were collected. Sampling took place after sunset and was realized using a MIK (Method Isaac Kid) with a mesh size of 500 µm. Samples were analyzed for species abundance in the laboratory using a Stereo Microscope (Olympus SZX16 with a 7x – 115x zoom range).
Fish larvae abundance per taxon and station/sample was calculated as described in the supplementary of the affiliated study (S2 and S3 Sections).
Fish eggs:
Fish eggs were sampled continuously during the route of the research vessel. Seawater was pumped from three meters below sea surface by means of the Continuous Underway Fish Egg Sampler (CUFES). In total 861 samples were taken in 2008. Samples were stored in 1% formol. Taxonomic determination was realized using a Stereo Microscope (Olympus SZX16 with a 7x – 115x zoom range).
Fish egg abundance per taxon and station/sample was calculated as described in the supplementary of the affiliated study (S2 and S3 Sections).
Environmental data:
Salinity and temperature were measured at the surface by means of a thermosalinometer, while depth was measured using a bathysonde.
Using a Niskin bottle, allowed to assess the concentration of particualte organic matter (POM), chlorophyll a and nutrients and to sample phyto- and microplankton. Phyto- and microplankton taxa composition and taxa abundance were determined using inverse-microscopy (S3 Section).
Data
| File | Size | Format | Processing | Access | |
|---|---|---|---|---|---|
Data from 2008: Abundance data per station of mesozooplankton, fish larvae, fish eggs and phyto-microplankton, environmental data per station, metadata | 415 Ko | XLS, XLSX | Quality controlled data | ||
Data 2022: Abundance per station of mesozooplankton and fish larvae, environmental data, metadata | 69 Ko | XLS, XLSX | Quality controlled data |
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