Deoxygenation of oceans is an increasing problem with effets on sealife

The deoxygenation of the seas has increased markedly during the last 100 years. The areas with reduced oxygen have increased ten times between 1900 and 2000. There have always been oxygen-minimum zones in oceans, but their volume has increased markedly in the recent past, because of decreased ocean circulation and as a result of increased respiration following elevated temperature. In addition to the climate change-caused increase in hypoxic seas, the eutrophication of coastal areas because of human actions have caused pronounced low-oxygen areas especially in the traditionally industrialized western countries.

Spreading hypoxia is a major problem, as it decreases the populations of fish and other organisms. It further affects the species distributions with more preferred species decreasing and decreases biodiversity. The effects of reduced oxygen level as such are aggravated by an increased water temperature, i.e. climate change,  because the oxygen consumption of fish and other poikilothermic animals increases with temperature increase. Simultaneously, the oxygen solubility in water decreases. Even this isn’t enough, but the oxygen binding by haemoglobin is reduced at a given oxygen tension with increased temperature. This reduces the capability of fish and other animals to survive in hypoxic conditions. This makes it more difficult of animals to tolerate increased temperature.

So, climate change and the pollution of the seas together cause deoxygenation. The pollution further decreases  the capability of microscopic algae to produce oxygen by photosynthesis. To combat the deoxygenation problem we need to stop eutrophication, and sea pollution by wastewater cleaning. Further, we need to combat climate change much more effectively than we have hitherto done. We need healthy seas to be able to feed the world, and the current increase in ocean deoxygenation is not doing that.

The ocean deoxygenation problem is the subject of an IUCN report, downloadable from https://www.iucn.org/resources/publications. (p.s. I have been studying hypoxia responses in fish from1980).