Water darkening modulates the efficiency of coastal eutrophication mitigation in a mesocosm experiment.

To mitigate coastal eutrophication, a worldwide concern to sustain ecosystem services, actions often focus on nutrient-load reductions but sometimes also aim to limit the abundance of forage fish. Adding to the eutrophication problem, changes in climate and land use lead to increased inputs of coloured dissolved organic matter (darkening). We conducted a mesocosm experiment to study the effects of water darkening on the outcome of eutrophication mitigation actions acting either bottom-up or via top-down control. We initiated eutrophication under varying water colour conditions. Then, we performed mitigation actions by stopping nutrient additions, removing forage fish, or both. Stopping nutrient additions was the most effective action in reducing pelagic algal biomass, irrespective of water colour. In contrast, the effect of fish removal varied with water colour, only having an effect in darker waters. We link these varying responses to shifts in nutrient availability and zooplankton community biomass and composition. Our experiment demonstrates that future eutrophication management may need to consider multiple concurrent pressures, including coastal darkening, as they can generate indirect effects caused by trophic interactions.