Nutrient dynamics following 50 years of reduced loading in the large and shallow Lake Arresø

Abstract Motivated by a citizens group to improve the ecological quality in Denmark’s largest lake, we examined 50 years of measurements to evaluate the scope for improvement. We found that external inputs in the early 2020s contained only 10% of phosphorus (P) compared to the mid-1980s, mainly due to effective purification of urban wastewater. Although in-lake P and chlorophyll concentrations decreased four–fivefold, summer Secchi depths are only 40–60 cm, but not because large sediment nutrient pools causes high internal loading and long-term delays upon load reductions. Rather, frequent sediment resuspension ensures intimate contact between nutrient pools in surface sediments and the water and close correspondence between P inputs and in-lake concentrations. Thus, costly attempts to remove surface sediments or add chemicals to bind P will not have long-term effects. Instead, predictions show that reducing the external P input twofold would increase Secchi depths to about 1 m and might bring waterplants back to Lake Arresø. For this to happen would require appreciable reduction in nutrient runoffs from the farmland by means of larger uncultivated lowland areas, wider buffer strips along the streams and reduced fertilizer application.