Suffocation really isn't the big issue here. Phytoplankton generate energy via photosynthesis, so as the temperature of water – and this the concentration of gases that are dissolved inside – changes, so too will the organisms' biological fitness. Since they reproduce very rapidly, and their populations enormous, plankton will be able to mutate their enzymes to be more appropriate for the situation (down side from the genomic perspective – decreased biodiversity, especially in species without sexual reproduction).
The more pressing concern is that larger, more slowly-reproducing aquatic organisms will experience significant strain to accommodate their metabolic needs. For many fish, crustaceans, coral, and others: rapid change in dissolved content on the ocean will cause population loss.