Deep convection in the Ocean depends on the water temperature, but also on the salinity (the "saltiness") of the water. The colder and the saltier the water is, the more oxygen it can take up. Now that the Earth is warming up, snow, glaciers, and the polar ice caps may melt. This is particularly bad in the Polar regions, because the fresh water from this melting ice flows into the sea and forms a layer of water that is far less salty than the seawater. This may lead to less oxygen being taken up by the ocean, which means there will be less oxygen for life in the oceans.
Another reason that the Ocean is losing its breath is that, if the surface layer of water becomes warmer, it does not mix that well with deeper water layers. When the layers stop mixing, the oxygen that is produced by photosynthesis and by exchange with the air cannot get into deeper waters anymore.

License information: CC BY 4.0
MPAA: G
Go to source: https://kids.frontiersin.org/article/10.3389/frym.2018.00075