State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, PR China; University of Chinese Academy of Sciences, Beijing 100049, PR China.
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, PR China.
Water Res. 2024 Jun 1;256:121565. doi: 10.1016/j.watres.2024.121565. Epub 2024 Apr 2.
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 was developed and long-term (from 2007 to 2022) water quality and meteorological observations were used 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 to 1.41 mg L from 2007 to 2022 but the reduction showed spatial heterogeneity. The annual mean DNR decreased from 45.6 μmol m h to 4.2 μmol m h, and ANR decreased from 11.85×10 t yr to 1.17×10 t yr during 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 N removal by internal N cycling decreased significantly as water quality improved. Notably, the proportion of ANR in winter to total ANR increased from 14 % in 2007 to 23 % in 2022 due to warming. This could potentially lead to N deficiencies in spring and summer, thus limiting the availability of N to phytoplankton. A TN concentration of less than 1.0 mg L in the lake and 1.5 mg L in the inflowing lake zones in spring contribute to local N-limitation in Lake Taihu for cyanobacteria control. Our study revealed a general pattern that N removal efficiency decreases with improved water quality, which is instructive for eutrophic lakes in nitrogen management.
近年来，由于减轻富营养化藻类水华的外部负荷，许多湖泊的氮（N）浓度大幅下降。然而，人们很少关注湖泊氮去除效率与水质改善之间的联系，特别是在 N 浓度下降的情况下反硝化速率（DNR）的变化。为了了解水质改善下的 N 去除效率及其对太湖氮控制目标的影响，本研究基于原位实验结果建立了反硝化模型，并利用长期（2007 年至 2022 年）水质和气象观测数据来估算 DNR，并将其与湖泊 N 去除量（ANR）相关联。太湖总氮（TN）浓度从 2007 年的 3.28mg/L 下降到 2022 年的 1.41mg/L，但下降存在空间异质性。年平均 DNR 从 45.6μmol m h下降到 4.2μmol m h，而 ANR 从 2007 年的 11.85×10 t yr下降到 2022 年的 1.17×10 t yr。氮收支分析表明，2007 年反硝化去除的氮量占外部负荷的 23.3%，但到 2022 年降至仅 4.0%。因此，随着水质的改善，内部氮循环去除氮的贡献显著下降。值得注意的是，由于变暖，2022 年冬季 ANR 占总 ANR 的比例从 2007 年的 14%增加到 23%。这可能导致春夏季氮缺乏，从而限制了浮游植物对氮的利用。因此，太湖春夏季湖水 TN 浓度小于 1.0mg/L，入湖区 TN 浓度小于 1.5mg/L，有助于控制蓝藻水华。本研究揭示了一种普遍模式，即随着水质的改善，N 去除效率降低，这对富营养化湖泊的氮管理具有指导意义。
