We are currently experiencing a dramatic and continued increase in atmospheric CO2 concentration that leads to higher aqueous CO2 concentrations and necessitates a re-evaluation of the role of carbon (CO2) relative to other nutrients in freshwater systems. These changes in atmospheric CO2 concentration are occurring at the same time as increases in the availability of phosphorus and nitrogen in ecosystems throughout the world. These changes are bound to affect phytoplankton, a key element of the global carbon cycle and the base of most aquatic food webs. We performed lake mesocosm experiments designed to investigate the effect of elevated CO2 and eutrophication on phytoplankton productivity and community composition, with a particular interest in their interaction. Our experiment was designed to explore the extremes of the range of nutrient and CO2 availability, with high concentrations of nutrients and CO2that provided nutrient-replete conditions, similar to nutrient-rich media in the laboratory. Our study was conducted across different periods of a growing season to account for seasonal differences in physical and biotic conditions.
Results/Conclusions
We found that elevated CO2 weakly stimulated total phytoplankton growth in the absence of fertilizer. There were pronounced changes in community composition, however: in particular, chlorophytes increased in frequency at elevated CO2 independent of nutrient status. This corroborates our laboratory findings, which indicated that phytoplankton community composition could be influenced by CO2 concentration in a predictable way. Furthermore, we found that CO2 greatly magnified the increase in phytoplankton standing stock caused by fertilization. These changes may have substantial impacts on nutrient and CO2 cycling and may alter whole freshwater ecosystems through bottom-up trophic effects. Furthermore, these findings may highlight the relation between the rate of provision of a nutrient, either as an inexhaustible flux or an exhaustible pulse, and their importance in limiting growth rate and yield respectively may be generalizable beyond phosphorus, nitrogen and CO2. This research is a first step towards predicting the combined effect of CO2 and eutrophication on whole communities in freshwater systems, from plankton to fish, and the potential changes in the role of these systems in global geo-chemical cycles.