| Summary: | During 2019–20, the most severe wintertime marine heatwave since 2000–01 swept through the Baltic Sea. Sea ice was practically non-existent during this time, which is unusual for the northern Baltic Sea. The consequences for the phytoplankton community due to winter-time heatwaves are still unknown. However, it is likely that not just biomass, but also community composition during winter and subsequent spring are altered. For example, dinoflagellates may receive a head start during spring blooms after warm winters and their relative abundance could increase compared to the abundance of diatoms. During cold winters, sea ice has a strong influence on the pelagic phytoplankton community in spring. Seeding from the ice would give the dominating sympagic phytoplankton group a head start during spring blooms. Dinoflagellate cysts are better at binding carbon than diatom spores. In the long-term dinoflagellates could potentially alleviate eutrophication if they are not resuspended and germinated during cold winters. The aim of the study was to compare the pelagic phytoplankton community during the ice-free cold-water period 2019–20 to the ice-covered cold-water period 2021–22, as well as investigate the potential seeding effect of the sympagic to the pelagic. Sampling occurred in the shallow bay Sjöviken, in the northwest of Åland. Phytoplankton were identified through microscopy using an inverted phase-contrast microscope. Environmental and biotic variables between the two cold-water periods were modelled using a generalized additive model. Total community biomass was significantly higher during 2021–22. Ice cover and a significantly higher total phosphorus concentration in the pelagic during 2021–22 might be the reason for this. The biomass was strongly dominated by dinoflagellates during 2021–22, while the low biomass of 2019–20 consisted of virtually no dinoflagellates. Diatoms dominated during 2019–20. Results are in contradiction to earlier studies, where warmer winters resulted in a dominance of dinoflagellates ...
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