The long-term change in phytoplankton nutrient limitation following external nutrient loading reduction in a subtropical shallow lake

Eutrophication caused by excess inputs of nitrogen (N) and phosphorus (P) is a global environmental problem for lakes. External nutrient control may change the relative influence of N and P on phytoplankton biomass. For Lake Wuli, a subtropical shallow lake in China, suffering from severe eutrophication since the early 1980s, a restoration program aimed to control external nutrient loading was implemented in 2002. Long-term data (1998–2019) revealed that annual mean phytoplankton biomass (Chl a concentration), total nitrogen (TN), and total phosphorus (TP) concentrations decreased significantly following external nutrient loading reduction. Both summer and winter TN concentrations declined markedly, whereas TP and Chl a decreased significantly only in winter. The decline in the concentrations of TN was larger than that of TP, resulting in a significant reduction in the TN/TP ratio from 33.3 to 15.5 on an annual basis, from 23.3 to 10.9 in summer, and from 44.3 to 21.2 in winter. A high TN/TP ratio and low SRP concentration (<10 μg/L) indicate that Lake Wuli was P-limited in winter. The changes in the summer TN/TP ratio indicate a shift from P limitation of phytoplankton growth to N and P co-limitation, which was confirmed by a nutrient addition experiment conducted in summer in 2023. Our results indicate that, at the present environmental conditions, inputs of either N or P, and especially both nutrients, will increase the risk of phytoplankton blooming in summer in this subtropical shallow lake.