Experimental evidence of the role of nitrogen for eutrophication in shallow lakes: A long-term climate effect mesocosm study.

The effectiveness of controlling nitrogen (N) loading (in addition to phosphorus [P]) to manage the eutrophication of aquatic ecosystems has been debated despite the role of N in producing algal biomass and toxins. Long-term, controlled tests of the efficacy of N loading reductions are largely missing from the scientific record, perhaps due to the historical focus on P control. To address this knowledge gap, we examined the results from a unique, long-term study conducted in 24 flow-through (2.5-month retention time) lake ecosystem-scale mesocosms in Denmark, operating since 2003 at two contrasting nutrient loading levels crossed with three temperature scenarios (ambient, IPCC (Intergovernmental Panel on Climate Change) A2 scenario, and A2+50%). For 1 year, the N loading, apart from groundwater inputs, was stopped in high nutrient loading mesocosms, while P loading was maintained. We followed the changes in key environmental variables and system metabolism for 5 years, including the 2 years prior to N loading reduction and 2 years after N loading resumption. The low nutrient loading treatments, which only received N and P from groundwater, were used as a reference. We found a strong effect of N loading on total N (TN), N oxides (NO + NO), and N:P ratios. After reducing the excess external N loading, which had lasted for 15 years, TN and N oxides declined to similar levels as those in the low nutrient treatments at all temperature scenarios and increased quickly when N loading was resumed. Algal biomass (as chlorophyll ) and ecosystem production and respiration were also affected. The results showed (1) a rapid response of water N concentrations to external N loading, (2) major ecosystem effects, including reduced algal biomass and system metabolism, and (3) overall low sensitivity in response to the IPCC temperature scenarios. This study was conducted under semi-natural conditions, providing strong experimental support for the key role of N at the ecosystem level in shallow lakes. Our results have profound implications for lake management and suggest that external N loading reductions may strengthen the recovery of shallow lakes from eutrophication.