Cyanobacterial dominance under CO₂(aq)-limited conditions increases ecological risks in karst reservoirs

The frequent occurrence of cyanobacterial blooms, especially in lakes and reservoirs, poses a significant threat to aquatic ecosystems and human health. The availability of CO₂ plays a critical role in shifting phytoplankton communities from diatom and green algae dominance to cyanobacterial dominance. The naturally high pH of surface water in karst regions, coupled with seasonal thermal stratification in reservoirs, leads to a more pronounced aqueous CO₂ (CO₂(aq)) deficiency in surface water of karst reservoirs. However, the impact of dynamic changes in inorganic carbon on phytoplankton communities in karst reservoirs remains unclear. Therefore, two typical sub-deep reservoirs in the karst region of southwestern China were selected in this study. Monthly surface water samples and seasonal vertical profile water samples were collected over a period of one year. The concentrations of nutrients, dissolved inorganic carbon (DIC), and CO₂(aq), as well as the stable isotope composition of DIC (δ¹³C_DIC) and phytoplankton species, were analyzed. The results showed that surface water CO₂(aq) concentrations were considerably lower in the warm season compared to the cold season, and cyanobacterial abundance (r= -0.80, p < 0.01) showed a significant negative correlation with CO₂(aq) concentration, while diatoms (r= -0.22, p > 0.05) and green algae (r= -0.06, p > 0.05) showed poor correlations with CO₂(aq) concentration. This indicates that CO₂(aq) deficiency conditions for phytoplankton growth favor the dominance of cyanobacteria. The CO₂(aq) concentration in the bottom water was significantly higher than that in the surface water during the period of thermal stratification. This indicates that thermal stratification in reservoirs impedes vertical CO₂ exchange, making surface water more susceptible to CO₂ limitation and more conducive to cyanobacterial dominance. In addition, the nature of high pH of the karst reservoirs may exacerbate CO₂(aq) deficiency in surface water, correspondingly enhancing cyanobacterial dominance. Furthermore, with global warming, the temperature rise and accompanying nutrient release from the sediment will increase the risk of cyanobacterial blooms in the reservoirs. This study highlights the high risk of cyanobacterial dominance in karst reservoirs in the context of global warming, and provides new insights into the ecological vulnerability of these reservoirs.