​Methane and nitrous oxide concentrations and sea-air fluxes in western Long Island Sound, a seasonally hypoxic urban estuary:​ Hourly to seasonal variability

Cara C M Manning , Anagha Payyambally, Josie L. Mottram, Kelsey Ward

We report the first water column profiles of dissolved methane (CH4) and nitrous oxide (N2O) in western Long Island Sound, an urban estuary in which seasonal hypoxia occurs due to eutrophication and restricted exchange with the ocean. We collected samples at seven stations along an 18 km transect in August 2023, October 2023, and May 2024. CH4 concentrations and sea-air fluxes were highest in August (mean concentration 101 nmol/kg and mean sea-air flux 154 μmol m−2 d−1) and lowest in May (mean concentration 32 nmol/kg and mean sea-air flux 62 μmol m−2 d−1). Conversely, N2O concentrations and sea-air fluxes were highest in May (mean concentration 12.0 nmol/kg and mean sea-air flux 4.8 μmol m−2 d−1) and lowest in August (mean concentration 10.1 nmol/kg and mean sea-air flux 2.5 μmol m−2 d−1). Surface concentrations of CH4 and N2O and sea-air fluxes were highest at the westernmost station in all three seasons. To investigate short-term variability in CH4, N2O, and oxygen (O2), we collected samples every 4 hours over 28 hours at the middle station of the transect, in all three months. Observed changes in gas distributions were linked to water provenance, driven by tidal currents. Our data suggest that there is a persistent source of surface water to western Long Island Sound with high concentrations of CH4 and N2O which drives an along-sound gradient in surface concentrations and sea-air fluxes.