Recovery of lakes from eutrophication: changes in nitrogen retention capacity and the role of nitrogen legacy in 10 Danish lakes studied over 30 years

Using data on 23 Danish lakes, we conducted mass balances to develop total nitrogen (TN) models for predicting annual mean TN in lakes based on external TN loading and found high predictability when including lake hydraulic retention time and mean depth in the model. We further used a unique 30-year mass balance data series from 10 Danish lakes with contrasting mean depths and hydraulic retention times to elucidate the effect of external TN loading reduction and N legacy on lake TN. We found that the TN retention percentage during the 30 years was generally not sensitive to an often major reduction in the external TN loading; it overall followed the pattern of the above model predictions, suggesting a low TN legacy effect. Moreover, the TN retention percentage was not affected by changes in TP. Our results, therefore, show a fast response to TN loading reduction, indicating that we can expect an immediate effect on lake water quality in shallow lakes suffering from internal phosphorus loading during re-oligotrophication provided that inorganic N is low enough to become a growth-limiting nutrient.