What is it about?
The Bohai Sea, adjacent to the Northwest Pacific, is a semi-enclosed shallow-water marginal sea that was considered on a critical path of eutrophication and environmental degradation. To better understand the Bohai Sea metabolism-induced summertime dissolved oxygen (DO) decline, five field surveys were conducted between July 2019 and July 2021 to investigate the seasonal/interannual and spatial variations in DO, dissolved inorganic carbon (DIC), and stable isotopic composition of DIC (δ13CDIC). Although the water-mixing scheme was subject to spatial variation, a uniform apparent ratio of δ13CDIC versus apparent oxygen utilization was estimated at −0.0122‰ per μmol O2 kg−1 in the Bohai Sea in summer. Based on a three-endmember water-mixing model and the mass balance of DIC and its stable isotopic composition, the assumed uniform δ13C values of oxygen-consuming organic matter in the Bohai Sea DO-deficient areas was estimated to be −19.47 ± 1.85‰ in 2020 and between −20.6‰ and − 18.1‰ in 2021. This isotopic composition is very similar to the δ13C value of organic matter from marine diatoms, but different to that of terrestrial organic matter sources surrounding the Bohai Sea. Our results indicate that nearly all the organic matter consumed by community respiration in the Bohai Sea is produced in situ by marine plankton. To mitigate the seasonal DO shortage in the Bohai Sea, reduction of allochthonous nutrients is crucial.
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Why is it important?
Nearly all the organic matter consumed by community respiration in the Bohai Sea is produced in situ by marine plankton. To mitigate the seasonal DO shortage in the Bohai Sea, reduction of allochthonous nutrients is crucial.
Perspectives
In the context of terrestrial organic matter dynamics over global shelves, the case off the Yellow River Estuary (also the Yangtze River Estuary, with high inorganic carbon loads but relatively low organic matter loads) is different from some other estuaries such as those estuaries of Siberian rivers that emptying into the Arctic shelf (with high organic matter loads but relatively low inorganic carbon loads). In surface waters across the Laptev and the East Siberian seas, approximately 50% of riverine loads of DOC are removed during water mixing in the estuaries and across the shelf. Consequently, the terrigenous-organic-matter-induced DIC addition and CO2 degassing are observed over the shelf, leading to δ13C_DIC dynamics. In the Yellow River Estuary (also the Yangtze River Estuary), however, both DIC and δ13C_DIC exhibit almost conservative behaviors across the estuarine water mixing. According to the detection of naturally occurring radiocarbon (C-14) for Yellow River DOC and POC, the riverine organic matter is 400–1000 years old for DOC or even 4000–8000 years old for POC. Consumption of such old terrigenous organic matter is likely difficult on the seasonal time scale. A large amount of terrigenous organic matter discharged from the Yellow River is mostly buried after entering the sea, thereby participating in relatively long-term biogeochemical cycles.
Dr Wei-Dong Zhai
Shandong University
Read the Original
This page is a summary of: Exploring origin of oxygen-consuming organic matter in a newly developed quasi-hypoxic coastal ocean, the Bohai Sea (China): A stable carbon isotope perspective, The Science of The Total Environment, September 2022, Elsevier,
DOI: 10.1016/j.scitotenv.2022.155847.
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