These phytoplankton blooms usually consist of a single species of microorganism. The fact that they can be seen from space gives you an idea of just how abundant they are. The blooms in the oceans can be due to diatoms or algae but by far the most common large blooms are due to cyanobacteria.
Prochlorococcus sp. and Synechococcus sp. have the largest population sizes of any species on the planet. About 30% of all oxygen production on the planet is due to marine phytoplankton and these two species account for a significant proportion.
A third genus of cyanobacteria, Trichodesmium, is mostly found off the coast of Australia. In addition to producing oxygen by photosynthesis, it is responsible for a considerable proportion of nitrogen fixation in the oceans.
[Hat Tip: Bad Astronomy]
