Alleviating eutrophication by reducing the abundance of Cyanophyta due to dissolved inorganic carbon fertilization: Insights from Erhai Lake, China

The eutrophication of lakes is a global environmental problem. Regulating nitrogen (N) and phosphorus (P) on phytoplankton is considered to be the most important basis of lake eutrophication management. Therefore, the effects of dissolved inorganic carbon (DIC) on phytoplankton and its role in mitigating lake eutrophication have often been overlooked. In this study, the relationships between phytoplankton and DIC concentrations, carbon isotopic composition, nutrients (N and P), and hydrochemistry in the Erhai Lake (a karst lake) were investigated. The results showed that when the dissolved carbon dioxide (CO_2(aq)) concentrations in the water were higher than 15 µmol/L, the productivity of phytoplankton was controlled by the concentrations of TP and TN, especially by that of TP. When the N and P were sufficient and the CO_2(aq) concentrations were lower than 15 µmol/L, the phytoplankton productivity was controlled by the concentrations of TP and DIC, especially by that of DIC. Additionally, DIC significantly affected the composition of the phytoplankton community in the lake (p<0.05). When the CO_2(aq) concentrations were higher than 15 µmol/L, the relative abundance of Bacillariophyta and Chlorophyta was much higher than those of harmful Cyanophyta. Thus, high concentrations of CO_2(aq) can inhibit harmful Cyanophyta blooms. During lake eutrophication, when controlling N and P, an appropriate increase in CO_2(aq) concentrations by land-use changes or pumping of industrial CO₂ into water may reduce the proportion of harmful Cyanophyta and promote the growth of Chlorophyta and Bacillariophyta, which may provide effectively assist in mitigating water quality deterioration in surface waters.