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 changes in the planktonic system are not the only pelagic changes in the North Sea. We related the changes in plankton abundance and distribution with reported changes in recruitment of North Sea herring, particularly through the predator-prey relationship of some plankton species with pre-metamorphosis larvae. Even though spawning stock biomass has been high during the last decade, recruitment of North Sea herring has decreased since 2002. There were already indications that the early larval stage could be the critical point of development, with reduced survival and growth rates during the 2000s. The analyses presented here showed the abundance of Pseudocalanus sp. during winter to have a strong relationship with larval distribution and abundance later in herring life cycle, suggesting that predator-prey processes, and potentially starvation of first-feeding larvae, are behind the low recruitment in recent years.
These changes in the North Sea pelagic ecosystem are not self-contained, but part of an even larger scale process taking place all across the northeastern Atlantic region. In thisthesis we showed how the changes detected in the North Sea, occurred synchronously in different Atlantic regions. This synchronicity suggests common global trends affecting marine ecosystems. We suggest that rising temperature and changes in oceanic circulation are behind this synchronicity, and that local circumstances, or atmospheric patterns with more local influences, such as North Atlantic Oscillation in the North Sea, modulate the responses of marine ecosystems. In the synthesis the main conclusions of each chapter are put in context together and the importance of zooplankton as a link between primary productivity and higher trophic level consumers is discussed. Furthermore, the importance of monitoring and the correct selection of biological and environmental indicators is also discussed.
The knowledge provided by this doctoral thesis increase our understanding of the processes regulating the plankton community composition in the North Sea (Chapter 1 & 2) and the northeastern Atlantic region (Chapter 4), the potential relationship of plankton community with fish early-stage larvae (Chapter 3) and how environmental changes might affect the relevance of the indicators used to assess the state of the biological system (Chapter 2).