Multiple global change factors and the long-term dynamics of harmful algal blooms in the North Sea

The North Sea has been identified as an area where the impacts of climate change and de-eutrophication efforts are already apparent, specifically on phytoplankton communities, with shifts in biogeography, altered species composition, and increased biomass of harmful algal bloom (HAB) species. Here, we test whether environmental changes in the Dutch North Sea are associated with changes in the abundances of HAB species and if the probability of blooms has increased over the past two decades. We do so by using generalized additive and logistic regression models, respectively. Results show that Phaeocystis globosa and potential ASP (amnesic shellfish poisoning) and DSP (diarrhetic shellfish poisoning) toxin-producing species have increased in abundance along the coast over the period 2000-2018, despite overall declines in total phytoplankton biomass over the same period. Conversely, the abundance of potential ASP producers and particularly P. globosa declined in the more central areas of the North Sea. The probability of blooms, which varied across months during the season, generally increased. Environmental factors associated with these increases in the probability of HABs included increasing sea surface temperatures (for potential ASP and DSP toxin producers), possibly related to summer stratification of the water column, and increasing total nitrogen (TN) : total phosphorus (TP) ratios (for potential ASP and DSP toxin producers and P. globosa) due to de-eutrophication efforts that more effectively reduced TP than TN. Our results demonstrate how long-term changes in HAB species abundances in the North Sea are associated with shifts in multiple global change factors that together may lead to intensified HAB development.