Effects of the 2018 European heatwave and drought on coastal biogeochemistry in the German Bight

In 2018, Europe experienced an unprecedented heatwave and drought, especially in central and northern Europe, which caused decreased terrestrial production and ecosystem health. The effects on the marine environment are yet unclear. Here we investigate the biogeochemical response to the event in the German Bight of the North Sea. Using time series data from FerryBoxes, research cruises, monitoring programs and remote sensing we compare conditions in 2018 to climatological values. We hypothesize that (1) the heatwave caused unusual warming of surface waters, (2) the drought reduced river discharge and nutrient loads to the coast, and (3) the combined effects altered coastal biogeochemistry and productivity. During 2018, both water discharge and nutrient loads from rivers discharging into the German Bight were extremely low from March onward. Throughout the study domain, water temperature was unusually low in March 2018, but extremely high in May 2018, representing the fastest spring warming on record. This extreme warming period saw concurrent unusually high peaks in chlorophyll a, dissolved oxygen and pH values, consistent with the development of a strong spring bloom. It appears that productivity was unusually strong in near-shore regions, while offshore it was unusually low in 2018. The drought related low discharge limited nutrient supply from the rivers, but likely enhanced near-shore water residence time, where a burst of effective nutrient utilization during the spring depleted nutrients available for transport off shore. There, the heatwave related rapid surface water warming resulted in the establishment of a stable thermal water column stratification, hindering vertical nutrient supply to the surface layer during the summer.