Coastal darkening exacerbates eutrophication symptoms through bottom-up and top-down control modification

Coastal eutrophication due to excessive anthropogenic nutrient loading is a major threat worldwide, and especially in estuaries and semi-enclosed waterbodies, like the brackish Baltic Sea. In addition, coastal waters may become darker (coastal darkening) due to increased input of colored compounds from terrestrial run-off and sediment resuspension. Still, the effects of darkening on coastal food web responses to eutrophication are unknown. In a mesocosm experiment with benthic and pelagic habitats, we manipulated nutrient loading, presence of fish and light availability to disentangle bottom-up and top-down control of eutrophication symptoms in ambient and darkened waters. Overall, we found higher pelagic Chlorophyll a concentrations (a proxy of algal biomass) with darkening and with nutrient enrichment in both clear and dark waters. Albeit fish had a strong impact on zooplankton and zoobenthos, they had no cascading effect on algae. We conclude that coastal darkening due to changes in land use and climate change can pose an additional challenge concerning the recovery of coastal waters from eutrophication.

Automated summary

Coastal eutrophication caused by excessive anthropogenic nutrient input is a global threat, particularly in estuaries and semi-enclosed waterbodies. Additionally, coastal waters can become darker due to increased input of colored compounds. However, the effects of darkening on coastal food webs in response to eutrophication are unknown. A study using mesocosm experiments found that darkening and nutrient enrichment led to higher algal biomass, but fish had no cascading effect on algae. This highlights the additional challenge that coastal darkening poses for the recovery of coastal waters from eutrophication.