Articles | Volume 19, issue 14
https://doi.org/10.5194/bg-19-3469-2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-19-3469-2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Interannual variabilities, long-term trends, and regulating factors of low-oxygen conditions in the coastal waters off Hong Kong
Zheng Chen
School of Environmental Science and Engineering, Sun Yat-sen
University, Guangzhou, 510275, China
Department of Oceanography, Dalhousie University, Halifax, Nova
Scotia, B3H 4R2, Canada
Chuang Xu
Center for Water Resources and Environment, Sun Yat-sen University,
Guangzhou, 510275, China
Zhongren Zhang
School of Environmental Science and Engineering, Sun Yat-sen
University, Guangzhou, 510275, China
Guangdong Zhihuan Innovative Environmental Technology Co., Ltd.,
Guangzhou, 510030, China
Shiyu Li
School of Environmental Science and Engineering, Sun Yat-sen
University, Guangzhou, 510275, China
Jiatang Hu
CORRESPONDING AUTHOR
School of Environmental Science and Engineering, Sun Yat-sen
University, Guangzhou, 510275, China
Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou, 510275, China
Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai),
Zhuhai, 519000, China
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Surface ocean alkalinity enhancement, through the release of alkaline materials, is a technology that could increase the storage of anthropogenic carbon in the ocean. Halifax Harbour (Canada) is a current test site for operational alkalinity addition. Here, we present a model of Halifax Harbour that simulates alkalinity addition at various locations of the harbour and quantifies the resulting net CO2 uptake. The model can be relocated to study alkalinity addition in other coastal systems.
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Human activities are changing the coastal water environment, but the role of suspended sediments in oxygen loss is not well understood. We used a model to compare dissolved oxygen levels and related factors in the 1990s and 2010s in the Pearl River Estuary. Reduced suspended sediments and increased pollution have expanded low-oxygen areas by 1.5 times. It highlights that declining suspended sediments increase hypoxia in estuaries, especially with rising nutrients, which need urgent attention.
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In situ observations over 42 years were used to explore the long-term changes to low-oxygen conditions in the Pearl River estuary. Apparent expansion of the low-oxygen conditions in summer was identified, primarily due to the combined effects of increased anthropogenic inputs and decreased sediment load. Large areas of severe low-oxygen events were also observed in early autumn and were formed by distinct mechanisms. The estuary seems to be growing into a seasonal, estuary-wide hypoxic zone.
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We demonstrate that even sparse BGC-Argo profiles can substantially improve biogeochemical prediction via a priori model tuning. By assimilating satellite surface chlorophyll and physical observations, subsurface distributions of physical properties and nutrients were improved immediately. The improvement of subsurface chlorophyll was modest initially but was greatly enhanced after adjusting the parameterization for light attenuation through further a priori tuning.
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Short summary
Deterioration of low-oxygen conditions in the coastal waters off Hong Kong was revealed by monitoring data over two decades. The declining wind forcing and the increasing nutrient input contributed significantly to the areal expansion and intense deterioration of low-oxygen conditions. Also, the exacerbated eutrophication drove a shift in the dominant source of organic matter from terrestrial inputs to in situ primary production, which has probably led to an earlier onset of hypoxia in summer.
Deterioration of low-oxygen conditions in the coastal waters off Hong Kong was revealed by...
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