Effects of the 2018 European heatwave and drought on coastal biogeo-chemistry 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 affected ecosystem health. In this study, the effects of this event on the marine environment are investigate, with a focus on the biogeochemical response in the German Bight of the North Sea. Using time series data from FerryBoxes, research cruises, monitoring programs and remote sensing we com-pare conditions in 2018 to climatological values. We find that (1) the heatwave caused rapid warming of surface wa-ters, (2) the drought reduced river discharge and nutrient loads to the coast, and (3) these combined effects altered coastal biogeochemistry and productivity. During 2018, both water discharge and nutrient loads from rivers discharging into the German Bight were below the seasonally variable 10th percentile from March onward. Through-out the study domain, water temperature was near or below that threshold in March 2018, but higher than in other years during May 2018, representing not only a heat wave, but also the fastest spring warming on record. This extreme warming period saw concurrent high peaks in chlorophyll a, dissolved oxygen and pH, consistent with the develop-ment of a strong spring bloom. It appears that productivity was above 75th percentile of the 21-year record in most of the nearshore region, while offshore it was widely below the 25th percentile in 2018. The drought-related low dis-charge limited nutrient supply from the rivers, but likely increased water residence time nearshore, where a surge in primary production with efficient nutrient utilization during the spring depleted nutrients available for transport off-shore. There, the heatwave-related rapid warming of surface water resulted in the establishment of a stable thermal water column stratification, hindering vertical nutrient supply to the surface layer during the summer.

Automated summary

In 2018, Europe was hit by an unprecedented heatwave and drought, leading to reduced terrestrial production and impact on ecosystem health. This study focuses on the effects of this event on the marine environment in the German Bight of the North Sea, and finds that the heatwave caused rapid warming of surface waters, while the drought reduced river discharge and nutrient loads to the coast, resulting in altered coastal biogeochemistry and productivity.