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

Western Long Island Sound is a seasonally hypoxic urban estuary undergoing anthropogenic change, which may influence concentrations and emissions of the potent greenhouse gases methane (CH₄) and nitrous oxide (N₂O). To provide a benchmark of current gas distributions in this system, enabling detection of future changes and comparison with data from other estuaries worldwide, we obtained the first water column profiles of dissolved CH₄ and N₂O concentrations in western Long Island Sound. 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, suggesting that there is a persistent source of surface water to western Long Island Sound with high concentrations of CH₄ and N₂O. 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. 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 tidal advection or light levels alone.