1Louisiana Universities Marine Consortium, Chauvin, LA 70344, USA
2Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, VA 23062, USA
3Integrative Oceanography Division, Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0218, USA
4Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA
5Institut Maurice-Lamontagne, Pêches et Océans Canada, 850 route de la mer, Mont-Joli, Québec, G5H 3Z4, Canada
6State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 Zhongshan Road North, Shanghai 200062, China
Abstract. Water masses can become undersaturated with oxygen when natural processes alone or in combination with anthropogenic processes create enough carbon that is aerobically decomposed faster than the rate of oxygen re-aeration. The dominant natural processes usually involved are photosynthetic carbon production and microbial respiration. The re-aeration rate is indirectly related to its isolation from the surface layer. Hypoxic water masses (<2 mg L−1, or approximately 30% saturation) can form, therefore, under "natural" conditions, and is more likely to occur in marine systems when the water residence time is extended, water exchange and ventilation is minimal, stratification occurs, and where carbon production and export to the bottom layer are relatively high. Hypoxia has occurred throughout geological time and naturally occurs in oxygen minimum zones, deep basins, eastern boundary upwelling systems and fjords. Hypoxia development and continuation in many areas of the world's coastal ocean is accelerated by human activities, especially where nutrient loading increased in the Anthropocene. This higher loading set in motion a cascading set of events related to eutrophication. Nutrient loading is likely to increase further as population growth and resource intensification rises, especially in developing countries dependent on crops using fertilizers, and it is likely that the occurrence and persistence of hypoxia will be even more widespread and have more impacts than presently observed. Climate change will further complicate the causative factors.