CO2 alters community composition of freshwater phytoplankton: A microcosm experiment

- The rise of pCO2 stimulated the growth of phytoplankton and increased net primary production, particularly in summer.
- Cyanobacteria including Anabaena circinalis and Anabaena eucompacta benefit from the rising pCO2.
- The response of chlorophytes to the rise of pCO2 depended on taxa, while the response of diatoms to the change in CO2 differed between strains.

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 (270 ppm), present (380 ppm), and high (750 ppm) 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 750 ppm 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 750 ppm 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.

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