Earth, Ocean and Atmospheric Sciences Thrust, The Hong Kong University of Science and Technology (Guangzhou), Guangzhou, PR China; School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, PR China.
Earth, Ocean and Atmospheric Sciences Thrust, The Hong Kong University of Science and Technology (Guangzhou), Guangzhou, PR China.
Water Res. 2024 Nov 1;265:122258. doi: 10.1016/j.watres.2024.122258. Epub 2024 Aug 13.
Deoxygenation in estuarine and coastal waters worldwide has been largely attributed to the increasing anthropogenic nutrient input, whereas the contribution by long-term (decadal) changes in physical forcing is less investigated. This study aims to disentangle the impacts of three-decade changes in summer river nutrient concentration and physical forcing on the deoxygenation off a large eutrophic estuary, the Pearl River Estuary (PRE) in China. Using a coupled physical-biogeochemical model, we reproduce the observed summer oxygen conditions under the historical (the 1990s) and present (the 2020s) status of river nutrient concentration, freshwater discharge, and wind forcing. We show that the bottom hypoxic (dissolved oxygen < 2 mg/L) area off the PRE in the 2020s has increased by 73 % relative to the 1990s. The expansion is a result of the increased bottom water oxygen consumption outweighing the enhanced vertical oxygen supply, with the former driven by the sharp increase in inorganic nitrogen and phosphorus concentrations (160 %) and the latter caused by the decadal decline in both freshwater discharge (38 %) and wind speed (12.5 %) in summer. Model experiments suggest that if the observed changes in physical forcing had not occurred, the dramatic increase in anthropogenic nutrient concentrations from the 1990s to 2020s could have led to a much greater expansion of hypoxic area (249 %). On the contrary, the decadal decrease in summer freshwater discharge alone (while keeping the nutrient loading the same as in the 1990s) almost eliminates hypoxia off the PRE by weakening water column stratification and limiting the offshore spread of nutrients and organic matter, whereas the declined wind speed increases the hypoxic area by 247 % mainly through enhancing water column stability. Our results reveal that long-term changes in physical forcing are confounding the effects of anthropogenic nutrient input on deoxygenation, underlining the need to consider regional forcing changes in nutrient management to meet water quality goals.
全球范围内，河口和沿海海域的脱氧现象在很大程度上归因于人为营养物输入的增加，而长期（数十年）物理强迫变化的贡献则较少受到关注。本研究旨在厘清三十年来夏季河流水体营养物浓度和物理强迫变化对中国大型富营养河口——珠江河口（PRE）脱氧现象的影响。我们使用耦合的物理-生物地球化学模型，再现了在历史时期（上世纪 90 年代）和当前时期（本世纪 20 年代）河流水体营养物浓度、淡水径流量和风力强迫的现状下观测到的夏季氧气条件。结果表明，与上世纪 90 年代相比，本世纪 20 年代 PRE 河口底部缺氧（溶解氧 < 2mg/L）面积增加了 73%。这种扩张是由于底层水体氧气消耗的增加超过了垂直氧气供应的增加，前者是由无机氮和磷浓度的急剧增加（160%）驱动的，后者是由夏季淡水径流量（38%）和风速（12.5%）的十年下降引起的。模型实验表明，如果没有观察到的物理强迫变化，从 20 世纪 90 年代到 21 世纪 20 年代人为营养物浓度的显著增加可能导致缺氧面积的大幅扩大（249%）。相反，仅夏季淡水径流量的十年下降（同时保持与上世纪 90 年代相同的营养负荷）通过削弱水柱分层作用并限制营养物质和有机物质向近海扩散，几乎消除了 PRE 河口的缺氧现象，而下降的风速通过增强水柱稳定性将缺氧面积增加了 247%。本研究结果表明，物理强迫的长期变化使人为营养物输入对脱氧的影响变得复杂，这突显了在营养物质管理中考虑区域强迫变化以实现水质目标的必要性。
