Seasonal Aspects and Short-Term Variability of the Pelagic Offshore Ecosystems

In the pelagic ecosystems, food web interactions between phytoplankton and zooplankton determine the amount of energy transferred to higher trophic levels, the remineralisation of nutrients and the export of organic material to the benthic ecosystem. The variability in the strength of this coupling has important implications for the efficiency of trophic transfer and for the match-mismatch of secondary production with higher trophic level consumers such as fish larvae. This receives particular attention in recent years with regard to potential climatic alteration of the phyto- and zooplankton phenology. In the western Baltic Sea, the recurrent seasonal patterns of phytoplankton and their short- as well as long-term variation are well understood, but little is known about the coupling to zooplankton. Data from a high-frequency sampling across the salinity gradient in the western Baltic Sea shows the well-known delay in the seasonal development of phytoplankton from the Belt Sea to the southern Baltic Proper. However, while the coupling to zooplankton in the Belt Sea is relatively tight, an increasing offset in the timing occurs in the southern Baltic Proper that affects the utilization of the spring bloom and is explained by a shift in biodiversity.