Changes in phytoplankton biomass and phenology in the North Sea in response to increasing sea surface temperature

At least two major drivers of phytoplankton production have changed in recent decades in the North Sea: sea surface temperature (SST) has increased by 1.5°C between 1988 and 2014, and the nitrogen and phosphorus loads from surrounding rivers have decreased from the mid-1980’s onwards, following reduction policies. Long time series spanning four decades (1975-2015) of nutrients, chlorophyll a (Chl) and pH measurements in the Southern and Central North Sea were analysed to assess the impact of both the warming and the de-eutrophication trends. The de-eutrophication process resulted in a reduction of nutrient river loads to the sea, causing a decrease of marine nutrient concentrations in coastal areas under freshwater influence. A decline in annual mean Chl was observed across most sampling sites (coastal and offshore) in the period 1988-2015. Also, a shift in phytoplankton phenology was observed, with spring bloom formation occurring earlier in the year. A long time series of pH in the southern North Sea (Belgian Continental Shelf) shows an increase until the mid-1980’s followed by a rapid decrease, mirroring the changes in phytoplankton production related to the processes of eutrophication/de-eutrophication and warming. Analysis of the seasonal pH signal in this dataset supports the shift in phytoplankton phenology as well. We hypothesize that (i) the decline in annual mean Chl since 1988 is most likely due to the de-eutrophication process (for coastal waters) and the SST increase (for both coastal and offshore waters) and that (ii) the shift in phytoplankton phenology is very likely due to SST increase.

Submitted in: Limnology and Oceanography