Retention ponds as sinks for particulate carbon

As the amount of CO₂ in the atmosphere continues to increase, many believe a system of carbon trading and credits will be implemented to combat global warming. In order for this system to be effective, accurate carbon (C) budgets must be developed, in which C sources and sinks are quantified. Recent studies suggest that although freshwater ecosystems cover less than 3% of the earth’s surface, on a global scale sediments in these ecosystems may sequester as much C as ocean sediments. However, such comparisons are hindered by the relative paucity of research on the role freshwater ecosystems have in the global C cycle. Millions of small ponds have been built in the last few decades; the area of freshwater ponds in the United States increased by more than 12% between 1998 and 2004. We quantified sediment accumulation rates and organic carbon (OC) burial rates in three retention ponds in Southwestern Ohio. These ponds were all constructed in the last ten years, and thus present an opportunity to quantify C sequestration. We found an average annual sediment accumulation rate of 2.3 cm per year, while % OC in the 3 ponds was similar, averaging 1.88% of dry mass. OC burial rates were 241, 292, and 209 g/m2/yr in the 3 ponds, rates that are somewhat lower than the previously reported global mean of 400 g/m2/yr. Nevertheless, our results show that freshwater ecosystems can be significant net sinks for C, and must be considered to provide an accurate global C budget.