Ocean acidification and deoxygenation are universal global environmental problems and might influence the physiology of marine organisms, element cycling and ultimately change marine ecosystem services. Along the coastal shelf and its adjacent marginal seas, human activity has induced eutrophication and enhanced both local acidification and hypoxia. For example, in the well-known hypoxic zone of the northern Gulf of Mexico, the pH has already decreased by 0.45 in the oxygen-depleted bottom water, which is much more serious than the impacts resulting from increased atmospheric CO2 sequestration (pH decreased by 0.11). Similar situations have also been observed in other marginal seas, e.g. the East China Sea hypoxic zone. Various ecosystems (e.g. urbanized estuaries, salt marshes, mangroves, etc.) are all facing the synergistic threat of hypoxia and acidification.

The driving mechanisms of acidification and hypoxia are extremely complicated in different environments. Natural and anthropogenic processes, such as anthropogenic CO2 sequestration, enhanced organic matter remineralization, coastal upwelling, the different time scale of variations in the terrestrial carbon and/or alkalinity fluxes, etc., all influence the onset and maintenance of acidification and/or hypoxia. Moreover, coastal upwelling also brings the low-pH water from below and enhances acidification in those regions.

Although acidification and hypoxia are two of the hot scientific topics in oceanography, and advances in our understanding have been made in recent decades, the status of acidification and hypoxia, the driving mechanisms, and the evolution under the framework of global change are still to be studied. How the biogeochemical processes are different between the various kinds of warm and cold marginal seas with respect to acidification and hypoxia is also an important scientific issue to be solved. The scope of this Research Topic is to cover the most recent advances in our understanding of the status of acidification and hypoxia, the coupling mechanisms of the multi-drivers, ecosystem responses, and prediction of their evolution in marginal seas.

Authors are invited to submit both original research articles and reviews. The topics covered in this Research Topic include but are not limited to:

- Development, maintenance, and present status of ocean acidification and/or hypoxia in coastal shelf and marginal seas;
- The impacts of different time-scale human activities (i.e., land-use change, nutrients runoff, sewage discharge, sewage treatment rate, sewage treatment techniques, environmental governance, etc) on acidification and/or hypoxia;
- The impacts of natural process (i.e., coastal upwelling, flood, drought, typhoon/hurricane, etc.) on hypoxia and/or acidification;
- How the biogeochemical processes change in acidified and/or hypoxic environments, such as processes of calcification, (de)nitrification or assimilation rates;
- Ecological effects of acidification and/or hypoxia, such as the impacts on the physiology of plankton, benthic organisms, fishery and aquaculture in acidified and/or hypoxic environments;
- Comparative studies of biogeochemical processes between the various kinds of warm and cold marginal seas with respect to acidification and hypoxia;
- Prediction of acidification and/or hypoxia status and ecosystem responses to global change and increasing human population.

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