I doubt we'll get an anoxic event because of the rapid-response mechanism of coastal-shelf phytoplankton. Currently iron is the limiting resource, ironically because there's plenty of phosphate from eutrophication.

Disease will be a big killer as well, don't forget. Plagues thrive in situations of highly stressed organisms. Never mind AIDs — I mean the real nasty killers like cholera and diptheria and influenza and the black death.