Seasonal agricultural activities and monsoon shifts drive fluctuations in nitrogen levels in eutrophic coastal waters: A case study of Xiangshan Bay, China

Zhi Yang; Jianfang Chen; Xiaoqun Xu; Lihua Ran; Haiyan Jin; Bin Wang; Qianna Chen

doi:10.1016/j.marpolbul.2025.117535

Seasonal agricultural activities and monsoon shifts drive fluctuations in nitrogen levels in eutrophic coastal waters: A case study of Xiangshan Bay, China

Mar Pollut Bull. 2025 Jan 9:212:117535. doi: 10.1016/j.marpolbul.2025.117535. Online ahead of print.

Authors

Zhi Yang¹, Jianfang Chen², Xiaoqun Xu³, Lihua Ran⁴, Haiyan Jin⁴, Bin Wang⁴, Qianna Chen³

Affiliations

¹ Key Laboratory of Marine Ecosystem Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China; State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China. Electronic address: yangz@sio.org.cn.
² Key Laboratory of Marine Ecosystem Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China; State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China. Electronic address: jfchen@sio.org.cn.
³ Key Laboratory of Marine Ecosystem Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China.
⁴ Key Laboratory of Marine Ecosystem Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China; State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China.

PMID: 39793175
DOI: 10.1016/j.marpolbul.2025.117535

Abstract

Xiangshan Bay, one of China's most eutrophic semi-enclosed bays, was studied to examine the seasonal distributions of salinity, temperature, nutrients, and nitrate isotopes (δ¹⁵N and δ¹⁸O) to elucidate seasonal variations in nitrate sources and the key factors driving nitrogen level fluctuations. Based on nitrate δ¹⁵N (6.1-8.9 ‰) and δ¹⁸O (0-5.7 ‰) values, the primary nitrogen sources in Xiangshan Bay were identified as domestic sewage and agricultural runoff, influenced by seasonal agricultural activities and monsoon-driven Changjiang Diluted Water (CDW) dynamics. Nitrate levels peaked in autumn (53.1 ± 1.6 μmol/L), due to intensive agricultural activity and strong CDW intrusion driven by the northeast monsoon. Conversely, the southwest monsoon in summer, coupled with higher nitrogen uptake due to warmer temperatures and increased light availability, resulted in the lowest nitrate levels (35.2 ± 5.4 μmol/L). In winter, reduced nitrate inputs were observed, reflecting minimal agricultural activity and weaker Changjiang discharge. Differences in nitrate levels between the upper and lower bay were attributed to weaker mixing and stronger local inputs in the upper bay.

Keywords: China; Nitrate sources eutrophication; Nitrogen cycle; Nitrogen isotopes; Oxygen isotopes; Xiangshan Bay.