Climate warming is predicted to increase the water surface temperature of the Baltic Sea by 2-3°C and was already up 2°C higher in 2016 compared to the long-term average (HELCOM 2017) in the Baltic Proper (Arkona and Gotland Sea). Since the late 1980s a dramatic decrease of the diatom standing stock and the change to a dominance of autotrophic dinoflagellates in the Baltic Proper have been observed. According to Wasmund et al. (2017), the only reason should be climate warming i.e. higher water temperatures. It is hypothesized that earlier development of the mesozooplankton (copepods, rotifers) and microzooplankton (ciliates, heterotrophic dinoflagellates) due to milder winter temperatures lead to intensified grazing on the early stages of the spring bloom, which are represented by small diatoms, e.g. Skeletonema marinoi, and explain its decreased abundances during the last decades. However, the described interrelations are still poorly understood and are totally unclear for the Baltic Proper. Notably, microzooplankton is not monitored in detail in the Baltic Proper and the infrequent monitoring cruises to the Baltic Proper in February, March and May still leave large gaps in the plankton biomass knowledge during spring bloom, all of which do not allow to confirm the proposed top-down scenarios. Thus, I will combine high-frequency monitoring of the whole plankton community with ship-based temperature-manipulated field experiments and culture studies to a) provide high resolution data of plankton community dynamics and b) assess the mechanistic link between increased water temperature, zooplankton community changes and the significant decrease in diatom biomass during the spring bloom in the Baltic Proper.

In ship-based experiments during project cruise (April 2020), the effects of grazing will be researched by including a natural zooplankton community of only microzooplankton in one part of the experiment as well as by a community comprising micro- and mesozooplankton (natural and doubled abundance) in a second set-up under ambient and +3°C enhanced temperature regimes. A sub dilution experiment will be done to identify and select microzooplankton grazing rates. Further, with the help of monitoring (part B), the spring bloom plankton community dynamics and species composition will be researched. It should be proved if microzooplankton and mesozooplankton are already abundant in quantities able to control the diatom standing stock during phytoplankton spring bloom. Phytoplankton, microzooplankton and mesozooplankton samples will be taken at our own project cruise in the 1^st half of April 2020, during IOW regular monitoring cruises (February, March, May) and in co-operation with a cruise of GEOMAR in the 2^nd half of April of the same year. In laboratory experiments (part C), the effects of climate warming and key grazers will be analysed in more detail on species level using plankton cultures.