Oxygen concentrations affect aerobic and anaerobic processes, including energy metabolism and denitrification. Projected decreases in dissolved oxygen concentration  will thus impact organisms and their biogeography in ways dependent upon their oxygen requirements , which are highest for large, multicellular organisms . The upper ocean generally contains high dissolved-oxygen concentrations due to air-sea exchange and photosynthesis, but in subsurface waters, deoxygenation may impair aerobic organisms in multiple ways . Many processes contribute to lowered oxygen levels: altered ventilation and stratification; microbial respiration enhanced by nearshore eutrophication; and less oxygen solubility in warmer waters. For example, deoxygenation in highly eutrophic estuarine and coastal marine ecosystems  can result from accelerated microbial activity, leading to acute organismal responses. Under hypoxia , physiological and ecological processes are impaired and communities undergo species migration, replacement and loss, transforming community composition  . Hypoxia can lead to expanding OMZs, which will favour specialised microbes and hypoxia-tolerant organisms  . As respiration consumes oxygen and produces CO2, lowered oxygen levels are often interlinked with acidification in coastal and tropical habitats  and is an example of a compound hazard .
IPCC
Climate Change 2022: Impacts, Adaptation and Vulnerability
IPCC Report