CO2 alters community composition and response to nutrient enrichment of freshwater phytoplankton

Nutrients can limit the productivity of ecosystems and control the composition of the communities of organisms that inhabit them. Humans are causing atmospheric CO<inf>2</inf> concentrations to reach levels higher than those of the past millions of years while at the same time propagating eutrophication through the addition of nutrients to lakes and rivers. We studied the effect of elevated CO<inf>2</inf> concentrations, nutrient addition and their interaction in a series of freshwater mesocosm experiments using a factorial design. Our results highlight the important role of CO<inf>2</inf> in shaping phytoplankton communities and their response to nutrient addition. We found that CO<inf>2</inf> greatly magnified the increase in phytoplankton growth caused by the increased availability of nutrients. Elevated CO<inf>2</inf> also caused changes in phytoplankton community composition. As predicted from physiology and laboratory experiments, the taxonomic group that was most limited by current day CO<inf>2</inf> concentrations, chlorophytes, increased in relative frequency at elevated CO<inf>2</inf>. This predictable change in community composition with changes in CO<inf>2</inf> is not altered by changes in the availability of other nutrients.