Controls on nitrous oxide production and consumption in reservoirs of the Ohio River Basin

Aquatic ecosystems are a globally significant source of nitrous oxide (N 2 O), a potent greenhouse gas, but estimates are largely based on studies conducted in streams and rivers with relatively less known about N 2 O dynamics in reservoirs. Due to long water residence times and high nitrogen (N) loading rates, reservoirs support substantial N processing and therefore may be particularly important sites of N 2 O production. Predicting N 2 O emissions from reservoirs is difficult due to complex interactions between microbial N processing in the oxygen‐poor hypolimnion and oxygen‐rich epilimnion. Here we present the results of a survey of N 2 O depth profiles in 20 reservoirs draining a broad range of land use conditions in four states in the U.S. Nitrous oxide was supersaturated in the epilimnion of 80% of the reservoirs and was undersaturated in only one, indicating that reservoirs in this region are generally a source of N 2 O to the atmosphere. Nitrous oxide was undersaturated in the hypolimnion of 10 reservoirs, supersaturated in 9, and transitioned from supersaturation to undersaturation in 1 reservoir that was monitored periodically from midsummer to fall. All reservoirs with a mean hypolimnion nitrate concentration less than 50 µg N L −1 showed evidence of net N 2 O consumption in the hypolimnion. All reservoirs sampled during lake turnover supported N 2 O production throughout the water column. These results indicate that N 2 O dynamics in reservoirs differ widely both among systems and through time but can be predicted based on N and oxygen availability and degree of thermal stratification.