Another mechanism for carbon sequestration via plankton is deep water artificial upwellings. Lovelock and Rapley proposed this in Nature in September '07 and there has been at least one experiment off Hawaii.

One proposal is to use wave powered pumps and the other is to take advantage of the effect of ocean thermal energy conversion plants, which bring up cold water from the deep ocean as an energy source (using the temperature difference between warm surface waters and cold deep waters to run heat engines).