I am not associated with any of the following, but there are organizations attempting to build devices that will bring colder, nutrient rich waters to the surface to cause phytoplankton blooms (or use the same nutrients for seaweed farming):
It's the temperature, not the CO2. I have fresh water shrimp and some fish. If you raise the temperature over 30°C, less oxygen dissolves into the water and there's not enough for the critters to breathe. It's basically asphyxiation. Labyrinth fish are adapted, since they can breathe air as well, but they do it merely for survival, they too become stressed and die after a while. Also some types of algae grow faster when the water is warmer. More algae = more O2 uptake during daytime.
1) ocean-based.com 2) http://www.climatefoundation.org/
We should of course do everything we can to stop emitting carbon. But projects like these may an important part in our future.