Carbon dioxide concentrations in eutrophic lakes: undersaturation implies atmospheric uptake

Understanding concentrations and contributions of carbon dioxide (CO2) in aquatic ecosystems is an important part of a comprehensive global carbon budget. Current dogma suggests that world lakes are important emitters of CO2 to the atmosphere. We estimated the partial pressure of carbon dioxide (rho CO2) in 131 agriculturally eutrophic lakes over a 7 year sampling period. Values of rho CO2 in these lakes ranged from 0.1 to 40 392 mu atm with a median of 322 mu atm (n = 3049). In contrast to previous analyses of CO2 in lakes, 60% of the eutrophic lake samples were undersaturated with CO2. Correlation analysis implied that nutrient-driven primary production, reflected by high oxygen concentrations, drives CO2 concentrations below atmospheric equilibrium. Multiple regression analysis showed several limno-logical and catchment characteristics that explained a statistically significant amount of variability in rho CO2 (R-2 = 0.32). Important variables included chlorophyll a concentration and the ratio of total nitrogen to total phosphorus. Our estimated rho CO2 values were significantly (p < 0.0001) lower than a previously published dataset of world lake rho CO2 values derived primarily from oligotrophic-mesotrophic lakes. High-nutrient lakes, especially those that are small and rich in oxygen from primary production, could act as net atmospheric CO2 uptake sites.