Methane and nitrous oxide concentrations and sea-air fluxes in western Long Island Sound, a eutrophic 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^-1 and mean sea-air flux 154 μmol m⁻² d⁻¹) and lowest in May (mean concentration 32 nmol kg⁻¹ 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⁻¹ and mean sea-air flux 4.8 μmol m⁻² d⁻¹) and lowest in August (mean concentration 10.1 nmol kg⁻¹ and mean sea-air flux 2.5 μmol m⁻² d⁻¹). Surface concentrations and sea-air fluxes of CH₄ and N₂O were highest at the westernmost station (closest to New York City) 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. By evaluating the relationship between the dissolved gas concentrations, tidal stage and sampling time relative to solar noon, we concluded that there is measurable short-term variability in all three gases, and that the drivers of variability in these dissolved gases are complex and not dominated by tides or light levels alone. 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.