According to this latest study if carbon dioxide levels will continue to rise there will be more carbon dioxide in the water as well which will lead to tremendous increase in algae population, and so will ocean bacteria too. Ocean bacteria feed on the algae, and in the process takes oxygen from the water creating ideal conditions for creation of ocean dead zones.
Tropical ocean dead zone will significantly increase in years to come if we continue with current levels of CO2 emissions
If this scenario happens many marine ecosystems will be severely affected. Currently most fish avoid dead zones but with the rise of 50 % many fish species will be put in jeopardy. Since world still hasn't done anything to reduce constant increase of carbon emissions this scenario unfortunately looks very possible.