GREENHOUSE GAS EMISSIONS FROM A TEMPERATE AGRICULTURAL RESERVOIR

Reservoirs are being built at an increasing rate each year to provide humans with resources such as hydroelectric power and drinking water. These man made systems have provided society with important services but these have come at the cost of enhanced greenhouse gas (GHG) emissions. Recent estimates suggest reservoirs are a globally significant source of GHG emissions but these estimates are largely based on studies of oligotrophic boreal and tropical reservoirs. Reservoirs draining agricultural basins are common throughout much of the developed and are subject to high nutrient loading rates from the watershed. Excess nutrient loading stimulates algae blooms and degrades water quality in these reservoirs but surprisingly little is known about how nutrients and algal blooms affect GHG dynamics. To assess GHG dynamics in an agricultural reservoir we measured GHG emission rates, dissolved concentrations and nutrient chemistry in William H. Harsha Lake, an agricultural reservoir located in southwestern Ohio on a monthly basis since October 2011.

Average daily emissions of methane (CH₄) were 50 mg CH₄-C m^-2 d^-1 . The highest emissions rates of CH₄ were observed during the summer months and during fall turnover, and the lowest emissions were observed during the winter. Depth profiles of dissolved CH₄ throughout the summer show an accumulation of CH₄ in the hypolimnium while the lake is thermally stratified.

Average daily nitrous oxide (N₂O) emissions were 0.80 mg N₂O-N m^-2 d^-1 . The highest emissions were during fall turnover. During late summer, parts of the lake became a sink for N₂O, and depth profiles of N₂O show a similar trend with the water column becoming undersaturated with N₂O during this time. Without N₂O accumulation in the water column during thermal stratification, it is likely that the source of N₂O during fall turnover is nitrification of remineralized ammonium.