Marked changes in diatom and dinoflagellate biomass, composition and seasonality in the Belgian Part of the North Sea between the 1970s and 2000s.

In the last decades, the North Sea has undergone intense environmental changes which have led to regime shifts that affected all trophic levels. Since the 1970s, both increases and decreases in phytoplankton biomass and production have been reported from different parts of the North Sea. Such conflicting observations may be partly caused by methodological differences, but also reflect regional differences related to bathymetry, hydrodynamics, climate, riverine and Atlantic influence. The Belgian part of the North Sea (BPNS) is a hydrodynamically and bathymetrically complex area under strong human influence, which has been characterized by eutrophication (up to the 1980s) and de-eutrophication (1990s onwards), and pronounced long-term changes in turbidity and water temperature. We used a newly recovered and standardized historic dataset, the Belgian Phytoplankton Database (Nohe et al., 2018), to compare the biomass, composition and seasonality of diatom and dinoflagellate assemblages, two key components of the plankton in the BPNS, between the 1970s and 2000s. Diatoms, especially large-sized taxa, showed an increase from late winter to summer, resulting in a more intense and extended growing season in the 2000s. Dinoflagellates increased year-round but especially in summer. Both diatom and dinoflagellate blooms showed a clear shift towards an earlier bloom start. In addition, while in the 1970s distinct seasonal community types were present, a striking seasonal homogenization in community structure had occurred by the 2000s. Finally, we observed a pronounced increase in the abundance of harmful diatom and dinoflagellate genera. The observed changes are most likely due to an increase in sea surface temperature and water transparency, and changes in nutrient loads and ratios. Our study underscores the importance of recovering previously inaccessible historic data as they can offer unprecedented insights into long-term change in marine ecosystems, which are essential for properly evaluating the impact of human activities on these ecosystems.