Long-Term Effects of Nutrient Shifts and Warming on Chlorophyll-a in a Temperate Coastal Environment

Abstract Anthropogenically driven nutrient shifts and warming are key stressors affecting phytoplankton in marine coastal environments. The German Bight, a coastal region in the North Sea, has undergone substantial nutrient reductions and rising sea surface temperature (SST) over recent decades. Despite extensive research, the combined long-term effects of nutrient shifts and warming, particularly in the river-influenced coastal area, remain unclear. Here, we aimed to evaluate trends and explore the partial and interactive effects of winter nutrient alterations and seasonal warming on spring chlorophyll- a (Chl- a ) levels. For this purpose, we compiled a comprehensive spatiotemporal dataset (1980–2019) and applied generalized linear mixed models. Results showed that spring Chl- a concentrations peaked in the 1980s and 1990s, with values surpassing 30 µg L −1 in the Elbe estuary. Concentrations declined to < 15 µg L −1 in the following decades, although values > 15 µg L −1 persisted in the southern inner coastal waters. Decreasing winter dissolved inorganic nitrogen (DIN) emerged as the primary driver of declining spring Chl- a in coastal waters (estimate = 0.40, p < 0.01), while winter phosphorus and nitrogen-to-phosphorus ratios were not significant predictors. Spring SST had a weak positive effect on Chl- a (estimate = 0.09, p = 0.03), suggesting that warming during spring may enhance phytoplankton growth. Together, winter DIN and spring SST explained 30% of the variance in spring Chl- a . Additional factors, such as light availability and grazing, likely contribute to unexplained variability. This study provides evidence that nutrient reductions have successfully lowered chlorophyll levels, but persistent hotspots in the inner coastal waters highlight the need for targeted management under continued warming.