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

Lijuan Kang; Mengyuan Zhu; Guangwei Zhu; Hai Xu; Wei Zou; Man Xiao; Chaoxuan Guo; Yunlin Zhang; Boqiang Qin

doi:10.1016/j.watres.2024.121565

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

Water Res. 2024 Jun 1:256:121565. doi: 10.1016/j.watres.2024.121565. Epub 2024 Apr 2.

Authors

Lijuan Kang¹, Mengyuan Zhu², Guangwei Zhu³, Hai Xu², Wei Zou², Man Xiao², Chaoxuan Guo², Yunlin Zhang², Boqiang Qin²

Affiliations

¹ 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.
² State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, PR China.
³ 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. Electronic address: gwzhu@niglas.ac.cn.

PMID: 38581985
DOI: 10.1016/j.watres.2024.121565

Abstract

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^-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 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^-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 general pattern that N removal efficiency decreases with improved water quality, which is instructive for eutrophic lakes in nitrogen management.

Keywords: Denitrification; Microcystis bloom; Nitrate; Nitrogen deficit.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

China
Denitrification*
Environmental Monitoring
Lakes*
Nitrogen*
Water Pollutants, Chemical
Water Quality

Substances

Nitrogen
Water Pollutants, Chemical