2.3 Phytoplankton assemblages
Information of phytoplankton communities during the study period was obtained from publicly available plankton monitoring data published by Wasmund et al. (2017). The compiled phytoplankton data from January through May show that the phytoplankton spring bloom in 2016 occurred almost simultaneously in the Belt Sea, Arkona Basin and Bornholm Basin during the first half of March. The bloom was dominated by diatoms in Kiel Bay and increasingly by Mesodinium rubrum (a photosynthetic ciliate that relies on chloroplasts derived from its cryptophyte symbiont (Qiu, Huang & Lin 2016)) along a western to eastern latitudinal gradient. We compiled the relative abundance of major algal groups based on the 10 most abundant phytoplankton taxa – see pie charts in Fig. 1. The most noticeable trends across the latitudinal gradient is the much greater diatom abundance in Kiel Bight than Gdansk Basin, and vice versa for the cryptophyte group. The total plankton production was smaller in the western than eastern sites; Kiel Bay: 488 µg/L, Arkona Basin: 412 µg/L, Bornholm Basin: 702 µg/L, and Gdansk Basin: 796 µg/L (averages from three cruises January-May) (Wasmund et al. 2017).