A sea of change Impacts of reduced nitrogen and phosphorus loads on coastal phytoplankton communities

The aim of this thesis was to better understand the ways in which changes in nutrient loads impact phytoplankton communities. As nutrients entering coastal systems become more and more influenced by anthropogenic activities, so too comes the responsibility to understand what potential consequences may result. The North Sea is a key example of a coastal system experiencing large shifts in nutrient loading, with a particularly strong reduction in phosphorus (P) loads in recent decades. Bioassay experiments conducted during research cruises revealed that this has resulted in a spatial gradient from P limitation of phytoplankton growth in nearshore waters to nitrogen (N) limitation in the central North Sea. Effects of reduced P loads varied among phytoplankton groups, which may lead to shifts in community composition and a potential rise of harmful dino- and nanoflagellates. Field samples and controlled chemostat experiments showed that P limitation reduced the nutritional quality of phytoplankton, which may have negative impacts on the productivity of higher trophic levels in the food web. Chemostat experiments using North Sea phytoplankton were conducted to investigate the role of competition for resources. The results showed that both changes in N:P ratios and changes in absolute nutrient loads affected the phytoplankton species composition, in agreement with theoretical predictions of the nutrient-load hypothesis. This work supports calls for continued and improved monitoring of the North Sea. Furthermore, this research demonstrates that future de-eutrophication efforts in any aquatic system should be done with more balanced reductions of N and P as a core tenet.