The synergistic effect of elevated CO₂ and phosphorus on reservoir eutrophication

Winston B, Scott JT, Pollock E. 2016. The synergistic effect of elevated CO₂ and phosphorus on reservoir eutrophication. Lake Reserve Manage. 32:373–385. Increased delivery of carbon (C) and phosphorus (P) as a result of climate change is predicted to significantly increase algal biomass and alter algal nutrient content. We conducted a 2-phased experiment to determine the following: (1) whether carbon dioxide partial pressure (PCO₂) in the water column of reservoirs dropped low enough to limit algal biomass, (2) whether increasing the PCO₂ in the water column could alleviate C limitation and increase algal biomass, and (3) whether the simultaneous increase in PCO₂ and P alter algal nutrient content. In the first phase we reconstructed PCO₂ in the water column of several northwest Arkansas reservoirs on timescales ranging from days to decades. During fall, winter, and spring, PCO₂ was greater than atmospheric levels, suggesting the reservoirs were sources of C released to the atmosphere. During summer, PCO₂ was less than atmospheric levels, suggesting that reservoir PCO₂ fell low enough to potentially limit algal biomass. In the second phase, we conducted an enrichment experiment along a PCO₂ gradient representative of preindustrial (250 µatm), current (400 µatm), and 2050 projected levels from the Intergovernmental Panel on Climate Change (550 µatm) along a P gradient. Our results showed that relative to preindustrial and current levels, elevated PCO₂ had no significant effect on algal biomass and nutrient content. Instead, biomass increased significantly with increasing levels of P, supporting predictions that even under elevated PCO₂ and global climate change scenarios, P might still be the major nutrient limiting algal growth in these reservoirs.