| Summary: | Summary The North Sea planktonic system is one of the most studied cases of sudden community changes in the marine environment. Continuous Plankton Recorder (CPR) data provided insight into the long-term trends and seasonal patterns of both phytoplankton and zooplankton and their relationships with hydrographical factors as well as climatological events during the last 50 years. A cold episodic event in the late 1970s and a shift towards a warmer community in the late 1980s have been thoroughly described in the literature. Both events have been related to different environmental factors, such as changes in sea surface temperature and Atlantic water inflow through the northern North Sea. This thesis was aimed to achieve a deeper understanding of long term plankton trends in the North Sea, detect more recent changes in the plankton community, and the environmental drivers behind them. The approach taken throughout this research consisted ofanalyzing extensive long term monitoring data in the open North Sea, the Dutch coastal area and the whole north-eastern North Atlantic. Here we described for the first time a change in the plankton community of the North Sea at the end of the 1990s. This change particularly affected the abundance and seasonal patterns of dinoflgellates and the dominant zooplankton group, the shelf-sea copepods (i.e. Temora longicornis, Pseudocalanus elongatus, Paracalanus sp.). Temperature changes and different water mass composition of the North Sea are suggested as main drivers behind this change. While looking more in detailed to the plankton trends in the Dutch North Sea waters, we identified an overall C:Chla increase. In coastal waters physiological adaptation to higher light and lower nutrient levels may have enhanced the C:Chlorophyll a, while different processes act in offshore waters. These findings not only indicate the rapidly changing environment in the Dutch coastal zone, but also about the validity of Chlorophyll a as an indicator of phytoplankton biomass trends. The detected ...
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