The incomplete recovery of Danish coastal waters following reduced nutrient inputs and oligotrophication

Summary

Danish coastal waters were severely affected by eutrophication in the 1980s, prompting the implementation of national nutrient reduction plans. The aim of this study was to explore the recovery of Danish coastal waters and report trends for selected indicators of eutrophication. Dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), chlorophyll a, light penetration depth, eelgrass main depth limit, a benthic fauna index (DKI), and bottom water oxygen concentration. We report on the temporal and spatial trends of eutrophication in 109 coastal waterbodies from 1980 to 2023. The analysis revealed that although improvements have been observed in several waterbodies, eutrophication continues to affect large areas. The national eutrophication status improved until the early 2000s, followed by stagnation. This trend was primarily driven by changes in winter DIN and DIP concentrations, whereas oxygen concentrations, eelgrass main depth limits, and DKI remained relatively stable throughout the study period. The means of chlorophyll a and light penetration depth showed signs of recovery until 2012 and 2021, respectively, followed by trend reversals. These findings suggest that reductions in nutrient levels have been pivotal in driving Danish coastal waters towards recovery; however, current mitigation efforts are insufficient to ensure that Danish coastal waters can achieve status as ‘not affected by eutrophication’.