Background
The authors say "it is usually thought that unlike terrestrial plants, phytoplankton will not show a significant response to an increase of atmospheric CO2," but they note, in this regard, that "most analyses have not examined the full dynamic interaction between phytoplankton production and assimilation, carbon-chemistry and the air-water flux of CO2," and that "the effect of photosynthesis on pH and the dissociation of carbon (C) species have been neglected in most studies."  Hence, they proceed to rectify this situation.

What was done
Schippers et al. developed "an integrated model of phytoplankton growth, air-water exchange and C chemistry to analyze the potential increase of phytoplankton productivity due to an atmospheric CO2 elevation."  To test the predictions of their model, they let the freshwater alga Chlamydomonas reinhardtii grow in 300-ml bottles filled with 150 ml of a nutrient-rich medium at enclosed atmospheric CO2 concentrations of 350 and 700 ppm that they maintained at two air-water exchange rates characterized by CO2 exchange coefficients of 2.1 and 5.1 m day^-1, as described by Shippers et al. (2004b), while periodically measuring the biovolume of the solutions by means of an electronic particle counter.

What was learned
The authors report that their experimental results "confirm the theoretical prediction that if algal effects on C chemistry are strong, increased phytoplankton productivity because of atmospheric CO2 elevation should become proportional to the increased atmospheric CO2," which means, in their words, that "productivity would double at the predicted increase of atmospheric CO2 to 700 ppm."  Although they note that "strong algal effects (resulting in high pH levels) at which this occurs are rare under natural conditions," they still predict "a potential productivity increase of up to 40%, at observed pH levels for marine species with low affinity for HCO3^-," and that effects on algal production in freshwater systems could potentially be larger, such that a "doubling of atmospheric CO2 may result in an increase of the productivity of more than 50%."

What it means
With respect to the ongoing rise in the air's CO2 content, Schippers et al. say their results suggest that "the aquatic C sink may increase more than expected," which would help to slow the rate-of-rise of the air's CO2 concentration and provide a greater food base for higher marine and freshwater organisms.  On the negative side, they note that it could possibly aggravate nuisance phytoplankton blooms.  Clearly, much more research should be directed to addressing these important matters.

Reference
Schippers, P., Vermaat, J.E., de Klein, J. and Mooij, W.M.  2004.  The effect of atmospheric carbon dioxide elevation on plant growth in freshwater ecosystems.  Ecosystems 7: 63-74.