CO 2 limited conditions favor cyanobacteria in a hypereutrophic lake: An empirical and theoretical stable isotope study

Harmful algal blooms (HABs) are a global problem, exacerbated by rising temperatures, cultural eutrophication, urbanization, and agricultural development. During these HABs, phytoplankton consumption of CO 2 may result in conditions of C limitation, where algal taxa best adapted for these conditions will be at a competitive advantage. Many cyanobacteria are capable of alleviating CO 2 limitation by a variety of strategies, including the active assimilation ofHCO 3 − . In this study, we utilized a high‐resolution, month‐long time series of stable C isotopes and high‐performance liquid chromatograph‐based algal taxonomy in the hypereutrophic Lake Taihu, China, to investigate whether cyanobacteria are indeed advantaged by CO 2 limiting conditions. We employed a model of phytoplankton C acquisition to support the inferences derived from direct measurements. Diurnal cycles of production and respiration caused δ 13 C DIC to vary between −4‰ and −9‰, while δ 13 C POC varied between −29.6‰ and −19.6‰. Measured and modeled phytoplankton fractionation of DIC were positively correlated with pCO 2 and negatively correlated with cyanobacterial abundance, suggesting that CO 2 limitation preferentially favored increased cyanobacterial biomass, relative to other taxa. We propose that the ability of many cyanobacteria to access otherwise limiting pools of inorganic C is intrinsically linked with their capacity to cope with CO 2 limiting conditions, and may be a key factor in their dominance during HABs.