Re: Global warming help?
More phytoplankton in the water would be a bad thing. They might take up some additional carbon from the atmosphere and create more clouds - but - they'd kill the oceans in the process.
When the plankton decay their bodies are consumed by oxygen-metabolising bacteria. Which is fine in a normal, ventilated ocean. However, as plankton populations increase and oceans warm, you run into a hard limit on the availability of oxygen. Not just that more is being consumed, but also that warmer surface waters hold less oxygen, but that there is less overturning and mixing of oxygenated surface waters because of increased temperature driven stratification. In high latitudes where most deep sea ventilation takes place you get a double hit from increased temperatures driving greater productivity and fresher waters from ice melt refusing to overturn.
As oxygen levels fall in deeper waters, conditions favour sulfur metabolising bacteria whose byproduct is hydrogen sulfide - highly toxic to bottom-dwelling communities, and who have the effect of allowing phosphorus and nitrogen to remain in the water column rather than being trapped in sediments. These two elements allow for increased productivity which keeps pushing oxygen levels to the floor.
These are called euxenic conditions which are like the eutrophied ponds you find at this time of year, they are are nowadays found in restricted bodies of water such as strongly stratified lakes, fjords and the Black Sea; but the geological record shows a very strong correlation between global euxenia, high temperatures (from delta 18-O) and high carbon dioxide in the atmosphere and oceans (from CCD reconstructions and 12-C/13-C). The last big one was during the Palaeocene-Eocene Thermal Maximum when it appears a good part of the world's oceans were anoxic at depth and in places euxenia may have reached the surface creating dead zones for larger organisms.
There's some approachable papers here if you'd like to know the details:
Diaz, R. J. and Rosenberg, R. (2008) ‘Spreading dead zones and consequences for marine ecosystems’, Science, 321(5891), pp. 926–929.
Meyer, K. M. and Kump, L. R. (2008) ‘Oceanic Euxinia in Earth History: Causes and Consequences’, Annual Review of Earth and Planetary Sciences, 36(1), pp. 251–288.