A Localized Study of Tidal Influence on Air-Sea Methane Flux in the Gulf of Maine

Air-sea flux of methane influences global climate but there remain large uncertainties in estimations of natural sources of methane to the global methane budget. With a considerably higher global warming potential than carbon dioxide, investigating methane flux dynamics at local and regional scales is important for understanding present and future contributions to warming and climate change. This study aimed to quantify methane air-sea flux in the temperate, coastal waters of the estuarine Damariscotta River in the Gulf of Maine. A custom built, floating, dynamic flux chamber linked to a laser-based absorbance spectroscopy methane-analyzer (Li-COR 7810) was used to quantify methane flux rates in situ. Air flow rate through the chamber had a non-linear effect on flux rate estimations; for consistency, our flux rate measurements were run at a through-chamber rate of 5 L/min but maximum flux rates were observed at flow rates as high as 15 L/min. How air flow rate through the chamber relates to wind speed at the air-sea interface remains to be established. Nonetheless, we observed a distinct influence of the tidal cycle on methane flux. For instance, air concentrations of methane in the vicinity of the estuary varied from 1938 to 2022 ppb between high and low tide over the course of a single tidal cycle. Over the course of the study, chamber-derived flux rates of methane ranged from 77 to 1591 ng/m^-2/min, all from seawater to air. High and low tide flux varied significantly; for instance, the mean flux rates for high and low tide over one tidal cycle were 304 and 741 ng/m^-2/min, respectively. We hypothesize that the influx of coastal waters into the estuary during the flooding tide reduced the dissolved methane concentrations at low tide that resulted from exchange with sediments sources. Our data demonstrates that the estuarine river is a consistent source of methane to the atmosphere during the summer but that the large tidal amplitude in the Gulf of Maine must be considered when modeling the source-strength for methane in these estuarine systems.