As water warms up, its ability to hold oxygen decreases. This is unfortunate for the plants and wildlife that live in these waters as they cannot survive without oxygen.
Unfortunately in a recent study it became clear that this is occurring in lakes around the world.
It is true that this impact is not being had at all levels of lakes to the same degree. Unfortunately, the majority water life is very depth sensitive and are uncapable of changing their habits to survive this change.
This is a new, and less considered threat of global warming. Unfortunately it could have significant negative effects on the world.
It is unfortunately possible that in the short term this is irreversible, it is merely one more signpost to require us to change our ways faster.