The responses of phytoplankton communities to elevated CO2 show seasonal variations in the highly eutrophic Lake Taihu

Abstract

From April 2012 to January 2013 (over four seasons), in situ microcosm experiments were conducted in Lake Taihu, perturbed over a range of pCO2 scenarios (270, 380, and 750 μatm; 1 atm = 101.325 kPa). The influence of CO2 level on microcosms was greatest during the spring because of the high growth rate of phytoplankton. In this season, rising CO2 levels caused a pH reduction, and the maximum reduction was 0.6 units when CO2 level was enhanced from the present level to 750 μatm. The doubling of CO2 level could increase the net primary production (NPP) by 65% during spring when the concentrations of other nutrients were maintained. The rise of NPP could cause a decline of dissolved inorganic carbon (DIC) concentration, and CO2 enrichment might mitigate the extent of this decline. Meanwhile, higher CO2 may slow or prevent a loss of diversity of phytoplankton in microcosms in this season. During the other three seasons, Microcystis predominated, and the percentage of cyanobacteria did not alter with the change of CO2. We did not observe a significant increase in the abundance of any taxa with the rise of CO2 during the in situ microcosm experiments.