Data from: Autumn and winter plankton composition and size structure in the North Sea

Plankton dynamics in temperate ecosystems have been mainly studied during productive seasons, with comparatively less research conducted during the winter, particularly on microplankton. Implementing plankton sampling during a regular fishery cruise, we investigated the North Sea micro- and mesozooplankton community composition, abundance and size structure (55-2000 µm) during autumn (Buchan/Banks area) and winter (Downs area) between 2013 and 2019. Samples were analyzed using image-based techniques. Community diversity (broad taxa) was relatively similar across years in both areas, with diatoms and tripos taxa sets dominating the microplankton community and gastropods and copepods the mesozooplankton one. The average micro- to mesoozooplankton ratio (in abundance) was 90:1 for Buchan/Banks, resulting in average Normalized Abundance Size Spectra (NASS) slopes of -1.45 ±0.18 SD. For Downs, the micro- to mesoozooplankton ratio was 235:1 and steeper NASS slopes of -1.67 ±0.20 SD due to a lower contribution of large organisms. Interannual changes in the planktonic community for each area and their potential environmental drivers were examined using a redundancy analysis (including taxonomy and size) and a correlation analysis using NASS slopes (size only). Both approaches highlighted the importance of water mass properties (e.g. salinity, temperature, turbidity) in shaping plankton dynamics, although the amount of explained variance differed between approaches (11 versus 46%). Our results contribute to a better understanding of standing stocks of plankton and their environmental drivers. Specifically, novel insights were gained into microplankton dynamics, which play an important role in supporting the growth and survival of winter-spawned fish larvae in the North Sea.