Climate Change, the Oceans, and the Oxygen Cycle
Two Thirds of Earth's Atmospheric Oxygen is Estimated to come from the Oceans
The ocean absorbs most of the heat and a large amount of the carbon dioxide (CO2) resultant from climate change, leading to the disruption of vertical and horizontal flows of water, oxygen, carbon dioxide, salinity, and other molecules and nutrients. This happens on a global scale. Oxygen and other gases is lost more easily from warmer water into warmer air, and increased CO2 in water raises levels of carbonic acid (H2CO3), some of which dissociates into carbonate and bicarbonate and protons (charged hydrogen ions, H+) raising the acidity of the waters, thus making life more difficult, particularly for shell forming organisms.
Meanwhile, all this, and the related massive algae blooms play into the increase in 'dead zones' of waters depleted of oxygen. The sudden fallout of huge amounts of deceased organisms then uses up more oxygen and releases methane, a potent greenhouse gas, and CO2 as they decay.
It is thought of much less, since the plants, algae, diatoms, seaweeds, and other photosynthesizing organisms are net oxygen producers, but photosynthesizers ALSO require environmental oxygen to breathe (operate their metabolic structures) and live, not only CO2 and sunlight and water. So dead zones also destroy oxygen producers.
While it's hard for some people to imagine any individual or local action having a meaningful influence in mitigating climate change issues, many of the oxygen depleted dead zones are driven by anthropogenic factors locally very strongly. Namely, nutrient runoff from agriculture, aquaculture, landscaping, sewage, and such like. These are things we very much CAN do something about at the individual, household, business, local, state, and national levels.
It is important to note that major photosynthetic groups (aerobic microbes such as phytoplankton, algaes, and plants) are also those responsible for by far the greatest fraction of carbon fixation on this planet. This means that they convert carbon dioxide directly into organic molecules, thus removing the CO2 from the atmosphere and/or ocean. So, a collapse in their population from climate change reduces oxygen, reducing their population further from hypoxia, and as a result the amount of CO2 removed from the atmosphere then falls driving more climate change in a vicious cycle. The recent 2018 IPCC report states that there is now at most ten years left to reduce and begin to reverse greenhouse gas accumulation before it will produce utterly catastrophic change.
The research demonstrating the essential problem:
Global warming disaster could suffocate life on planet Earth, research shows - University of Leicester Press Office, UK - December 2015 - 400 words
The team developed a new model of oxygen production in the ocean that takes into account basic interactions in the plankton community, such as oxygen production in photosynthesis, oxygen consumption because of plankton breathing and zooplankton feeding on phytoplankton.
A link to the paper ‘Mathematical Modelling of Plankton–Oxygen Dynamics Under the Climate Change’ published in the Bulletin of Mathematical Biology is at the end of the article.