Hydrologic Variability Drives Differential Methane Dynamics in Agricultural Reservoirs of the Northern Great Plains

Abstract

Climate variability can regulate aquatic methane fluxes as increasing temperatures can elevate microbial metabolic rates, including methanogenesis. It is less well known how climate‐induced variability in seasonal precipitation and runoff might affect methane concentrations and fluxes in aquatic ecosystems. Here, we measured seasonal methane concentrations and calculated diffusive fluxes from 20 agricultural reservoirs in the northern Great Plains in contrasting wet and dry summers. Relative to the dry year, water column depths increased 65% (from 1.7 to 2.6 m) in the wet year and was associated with stronger stratification and increased anoxia at depth. Solute concentrations also declined during the wet year, with sulfate concentrations less than half that observed in the dry year (645 mg SO42 L 1 vs. 1620 mg SO42 L 1). Together, the more profound anoxia combined with lower sulfate concentrations resulted in significantly higher hypolimnetic CH4 concentrations in the wet year (40.3 μM) compared with the dry year (18.1 μM), particularly in August (30‐fold higher). Despite these patterns, surface CH4 concentrations and estimated diffusive emissions did not significantly increase in wet summers (1.13 μM and 2.31 mmol m 2 yr 1) relative to dry summers (3.78 μM and 5.71 mmol m 2 yr 1), likely owing to offsetting mechanisms of increased CH4 storage and oxidation through the deeper water column. Climate‐driven changes in precipitation and runoff are expected to modify the physical factors controlling methanogenesis and methanotrophy. Our findings show corresponding minimal effects on diffusive fluxes of methane, but future studies should also address ebullition and seasonal turnover to capture the full CH4 budget of inland waters.