CO₂ alters community composition of freshwater phytoplankton: A microcosm experiment

The effects of CO₂ and related changes in water carbonate chemistry on freshwater phytoplankton were studied in in situ microcosm experiments, performed in East Lake Taihu, in August 2013 and April 2014, respectively. The microcosm treatments represented low (270ppm), present (380ppm), and high (750ppm) pCO₂ conditions. Elevated CO₂ decreased pH and increased aqueous CO₂ concentrations, and did not change alkalinity. The maximum reduction of pH was 0.26 when CO₂ increased from current level to 750ppm at the ending of experiment in the spring. The rise of pCO₂ stimulated the growth of phytoplankton, and the net primary production could increase by 35% when CO₂ was elevated from current level to 750ppm in summer. Interestingly, our results indicated that increasing pCO₂ affected phytoplankton community structure in the microcosms. Cyanobacteria tended to be superior under high pCO₂, with two species Anabaena circinalis and Anabaena eucompacta benefitting from the rising pCO₂. The response of chlorophytes and diatoms to the rise of pCO₂ depended on taxa. In contrast, increased CO₂ did not change biomass or productivity of phytoplankton in spring, when biomass was relatively low.