[Chondrally (OP)]

The best way to reoxygenate the Ocean is to reduce temperature.  However a recent article has pointed out that in the last twenty years,  the amount of Dissolved Oxygen in the Ocean has decreased by more than the amount due to the
increasing temperature.  So what are the mechanisms that cause this loss of oxygen in the Ocean?
Some of the mechanisms include nitrogen and phosphorus fertilizer runoff causing algae and phytoplankton blooms that prevent diffusion of oxygen into the ocean and create oxygen dead zones.  Another mechanism is the death of fish species in record numbers that exhibit eutrophication,  which is decay and they get oxidized and give off CO2.  Can you think of other mechanisms?

http://canadajournal.net/science/alarming-decrease-oceans-dissolved-oxygen-level-says-new-research-57329-2017/

The book Oxygen by Nick Lane:

https://www.amazon.ca/Oxygen-molecule-that-made-world/dp/0198784937/ref=...

Three hundred million years ago, in Carboniferous times, dragonflies grew as big as seagulls, with wingspans of nearly a metre. Researchers claim they could have flown only if the air had contained more oxygen than today - probably as much as 35 per cent. Giant spiders, tree-ferns, marine rock
formations and fossil charcoals all tell the same story. High oxygen levels may also explain the global firestorm that contributed to the
demise of the dinosaurs after the asteroid impact.

The strange and profound effects that oxygen has had on the evolution of life pose a riddle, which this book sets out to answer. Oxygen is a toxic gas. Divers breathing pure oxygen at depth suffer from convulsions
and lung injury. Fruit flies raised at twice normal atmospheric levels of oxygen live half as long as their siblings. Reactive forms of oxygen, known as free radicals, are thought to cause ageing in people. Yet if
atmospheric oxygen reached 35 per cent in the Carboniferous, why did it promote exuberant growth, instead of rapid ageing and death?

Oxygen takes the reader on an enthralling journey, as gripping as a thriller, as it unravels the unexpected ways in which oxygen spurred the evolution of life and death. The book explains far more than the size of
ancient insects: it shows how oxygen underpins the origin of biological complexity, the birth of photosynthesis, the sudden evolution of animals, the need for two sexes, the accelerated ageing of cloned animals like Dolly the sheep, and the surprisingly long lives of bats and birds.

Drawing on this grand evolutionary canvas, Oxygen offers fresh perspectives on our own lives and deaths, explaining modern killer diseases, why we age, and what we can do about it. Advancing revelatory new ideas, following chains of evidence, the book ranges through many disciplines, from
environmental sciences to molecular medicine. The result is a captivating vision of contemporary science and a humane synthesis of our place in nature. This remarkable book might just redefine the way we think about the world.

Oxford Landmark Science books are "must-read" classics of modern science writing which have crystallized big ideas, and shaped the way we think

[chris]

Some of the mechanisms include nitrogen and phosphorus fertilizer runoff causing algae and phytoplankton blooms that prevent diffusion of oxygen into the ocean and create oxygen dead zones. 

I'm not sure that I entirely believe that; the ocean is a net contributor of oxygen to the atmosphere, rather than the other way around.

[Chondrally (OP)]

Yes you are correct, the ocean IS a contributer to oxygen through phytoplankton and algae and diffusion, due to increasing temperature, however in the past,  when temperatures were cooler, diffusion from the atmosphere over eons balanced the oxygen in the water with that in the atmosphere with the surface in equilibrium.  equilibrium still remains at the surface, its just that now, the ocean can hold less oxygen!