Decreasing denitrification rates poses a challenge to further decline of nitrogen concentration in Lake Taihu, China

Nitrogen (N) concentrations in many lakes have decreased substantially in recent years due to external load reduction to mitigate harmful algal blooms. However, little attention has been paid to the linkage between the lakes' nitrogen removal efficiency and improved water quality in lakes, especially the variation of denitrification rate (DNR) under decreasing N concentrations. To understand the efficiency of N removal under improving water quality and its influence on the N control targets in Lake Taihu, a denitrification model based on in situ experimental results and long-term (from 2007 to 2022) water quality and meteorological observations to estimate DNR and relate it to the amount of N removal (ANR) from the lake. The concentration of total nitrogen (TN) in Lake Taihu decreased from 3.28 mg L−1 to 1.41 mg L−1 from 2007 to 2022 but the reduction showed spatial heterogeneity. The annual mean DNR decreased from 45.6 μmol m−2 h−1 to 4.2 μmol m−2 h−1, and ANR decreased from 11.85×103 t yr−1 to 1.17×103 t yr−1 across the study years. N budget analysis suggested that the amount of N removed by denitrification accounted for 23.3% of the external load in 2007, but decreased to only 4.0% in 2022. Thus, the contribution of nitrogen removal due to internal N cycling significantly declined with improving water quality. Notably, the proportion of ANR in winter to total ANR increased from 14% in 2007 to 23% in 2022 due to warming. This may potentially lead to N deficits in spring and summer and thus N limitation to phytoplankton. A TN concentration of less than 1 mg L−1 in the lake and 1.5 mg L−1 in the inflowing lake zones in spring contribute to local N-limitation in Lake Taihu for cyanobacteria control. Our study revealed a universal pattern that N removal efficiency will decrease under improved water quality, which is instructive for eutrophic lakes in nitrogen management.