The combined effects of warming and densities of overwintering copepods on the spring succession of Baltic Sea plankton were investigated using indoor mesocosms. Three zooplankton densities (1.5, 4 and 10 copepods l-1) and 2 temperature levels (Δ0°C and Δ6°C; 0°C and 6°C above present-day temperatures in the Kiel Bight) were chosen. Both the timing and the duration of the protozooplankton (PZP) bloom were significantly affected by temperature, but not by copepod density. In contrast, the bloom intensity of PZP was highly affected by the factors temperature and copepod density and their interaction. This suggests that under elevated temperature conditions PZP grows faster, but, at the same time, is subject to higher top-down control by copepods. At low temperatures and low copepod densities, PZP, in turn, fully escaped from copepod predation. Further changes in copepod overwintering densities resulted in a strong suppression of ciliates, of which small-sized ciliates (<30 μm) were especially vulnerable to copepod predation, while other PZP size classes remained unaffected. In conclusion, results point at a pivotal regulating role of overwintering copepods under future warming conditions. Further, warming was shown to cause a distinct match between phytoplankton and PZP, thus strengthening trophic pathways through PZP. Our findings are discussed in the context of the 'trophic link-sink' debate by considering potential alterations in the flux of matter and energy up the food web.
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