The majority of dams in the US are small, low-head dams that are less than 5 m in height. Reservoirs created by larger dams are known to serve as sinks for nitrogen and sources of methane. However, data demonstrating if similar trends exist with low-head dam reservoirs is lacking. Our research hypothesis is that small dam reservoirs will be greater sinks for nitrogen and sources of methane than free-flowing river systems. To test this hypothesis, we collected sediment and water samples from three locations in five paired reservoir-reference sites. Sampling occurred during three seasons in Central Ohio, USA. To characterize nitrogen processing, we measured potential nitrification and denitrification in sediments and nitrate/ammonium concentrations in water. We also measured potential methane oxidation rates and porewater methane levels of sediments, and the degree of methane saturation of water. Two-factor ANOVAs using a block design were completed for all parameters – block (each reservoir-reference pair), factors: treatment (reservoir or reference) and season (spring, summer, autumn).
Results/Conclusions
Rates of potential denitrification and porewater methane levels of sediments were higher in reservoir than reference sites (p < 0.05). Rates of sediment potential nitrification and methane oxidation and water concentrations of nitrate, ammonium, and methane did not differ (p > 0.05). The data gives mixed results. While higher rates of potential denitrification and sediment porewater methane levels support the concept of reservoirs being stronger sinks for nitrogen and sources of methane than river systems, the water data are non-supportive. We conclude that the studied low-head dam reservoirs are neither strong sinks of nitrogen nor sources of methane.
