Lakes and human-made impoundments are important to regional carbon budgets. Because their watersheds are large relative to water body size, impoundments can receive large quantities of terrestrial organic carbon from their watersheds that may ultimately be sequestered in impoundment sediments. At the same time, many freshwaters have been shown to be net heterotrophic (i.e., a source of CO2 to the atmosphere). It is unclear to what extent impoundments are sources or sinks of carbon after simultaneously accounting for burial into sediments and CO2 fluxes. It is also unknown the extent to which watershed land use and phytoplankton productivity regulate impoundment net carbon budgets. Agricultural watersheds typically yield larger quantities of sediments than do forested watersheds. Productive systems generally have greater phytoplankton production and sedimentation. Taken together, we predict that productive impoundments in agricultural landscapes will be sinks of carbon and that less productive impoundments in forested landscapes will be sources of carbon to the atmosphere. For our study, we quantified carbon budgets for two impoundments of varying watershed land use over two years. Acton is hypereutrophic with watershed land use dominated by agriculture (89%), while Burr Oak is mesotrophic, with land use dominated by forest (81%). 
Results/Conclusions

We found that pCO2 in Acton and Burr Oak varied seasonally and yearly with pCO2 ranging from 90-2670 ppm in Acton and 111-2406 ppm in Burr Oak. Both impoundments were remarkably similar in how often pCO2 concentrations during the growing season (April – October) were in excess of atmospheric concentrations (45% for Acton, 47% for Burr Oak). This reveals a similarity in the frequency at which the impoundments were outgassing CO2 to the atmosphere. In contrast, we found that retention rates of particulate organic carbon per unit reservoir area were much higher in Acton than Burr Oak (237, 17 gC/m2/yr, respectively), indicating higher terrestrial organic carbon subsidies and/or greater within-reservoir production in the agricultural reservoir. Taken together, these results suggest that Acton is likely sequestering more carbon than Burr Oak despite the similarity in pCO2 trends, in agreement with our hypothesis that impoundments in agricultural watersheds are more likely than those in forested watersheds to be carbon sinks.