The identification of nutrient limitations on eutrophication in Dianchi Lake, China

Due to rapid urbanisation and economic development, Dianchi Lake has been eutrophic since the 1980s. Control of nutrient loading is regarded as the primary restoration method. Two independent approaches: analysis of 13 years of Dianchi Lake monitoring data using Support Vector Machines (SVM) and a nutrient‐enrichment experiment, identified phosphorus and nitrogen as simultaneously limiting. Total phosphorus (TP) has a clear, stable correlation with chlorophyll‐a (Chl‐a) concentrations in Dianchi Lake. Compared to total nitrogen (TN), TP shows a stronger correlation when Chl‐a concentration is below 0.051 mg/L and an almost equal correlation when it is above this level. An excessive phosphorus to nitrogen ratio may stimulate compensatory fixation of atmospheric N2 by cyanobacteria; therefore, only reducing the external nitrogen input may not necessarily result in lower total nitrogen and chlorophyll concentrations. We recommend that reducing the lake TP load to the lowest economically‐feasible level is the most cost‐effective restoration measure.