Oxygen – Dynamics and Biogeochemical Consequences

Dissolved oxygen is a keystone molecule that is involved in the production, consumption, and cycling of organic matter and inorganic elements in the aquatic environment. The majority of multicellular plants and animals in coastal aquatic ecosystems require oxygen for proper physiological functioning, and the biogeochemical cycling of many important inorganic elements is highly oxygen dependent. Consequently, spatial and temporal variations in oxygen, and the forces that control such variation, are of central importance in the study of coastal ecology and biogeochemistry. Human-associated increases in the extent and severity of oxygen depletion (i.e., hypoxia and anoxia) in coastal waters worldwide have substantially altered both the distribution of aquatic organisms and the related biogeochemical cycling of organic matter and inorganic elements. Changes in anthropogenic nutrient management and realizations of climate change scenarios have already exacerbated low-oxygen conditions and the associated effects on coastal biogeochemistry. In this chapter, we review the distribution, dynamics, and controls on oxygen in coastal aquatic ecosystems, with special emphasis on the implications of oxygen dynamics for the cycling of organic and inorganic materials in water columns and sediments. We include an overview of eutrophication and other factors driving oxygen depletion in coastal waters and the resulting changes to the cycling of carbon, oxygen, nitrogen, phosphorus, sulfur, iron, and manganese. Concluding discussions emphasize the central role that oxygen plays in modulating responses of coastal ecosystems and associated biogeochemical processes in relation to eutrophication and global climate change.