An article on Nature News talks about how rising temperatures and increase in CO2 in the atmosphere effects the worlds oceans by depleting O2 levels. The increase in CO2 will feed the algae growth and increase bacteria grow as it feeds on the increase in biomass, which will decrease the O2 level. Any one who has ever had an aquarium you now you need to make sure you have an air source in the aquarium or you could kill your fish because of exactly what is described in this article. If our severely depleted fishing stocks start dying because of lack of O2 on top of our over fishing we will have a collapse in the fishing stocks. Not to mention how boring it will be to scuba dive in a dead ocean!
You can find the article here: http://www.nature.com/news/2008/081114/full/news.2008.1230.html