Factors Regulating Sediment Methane and Nitrous Oxide Production and Consumption in Northern Temperate Estuaries

Shallow coastal systems are hotspots of methane (CH₄) and nitrous oxide (N₂O) release to the atmosphere. Sediments regulate the production and consumption of these two greenhouse gases in coastal systems, yet the conditions responsible for controlling the magnitude and direction of net sediment CH₄ and N₂O fluxes remain poorly constrained. In this study, we measured net fluxes of CH₄ and N₂O between the sediment and water column in three estuaries with a broad range of physical and chemical characteristics the northeastern United States. From 2016-2018, we measured seasonal sediment fluxes throughout Narragansett Bay. In summer 2018, we took advantage of enhanced variation in sediment properties due to oyster aquaculture activity at sites in Narragansett Bay, Duxbury Bay, and Ninigret Pond by measuring fluxes directly beneath aquaculture and outside aquaculture areas. During each sampling event we also measured ambient water column conditions (temperature, salinity, dissolved nutrient concentrations) and sediment physical (porosity and density) and chemical (%C, %N, % organic matter, chlorophyll-a) properties to identify factors that can control whether or not sediments produce or consume CH₄ and N₂O, and whether they can be used to predict flux magnitude. In Narragansett Bay, sediments were a net source of CH₄ to the water column (788.07 ± 177.41 nmol CH₄ m^-2 hr^-1) and CH₄ release was highest in spring and summer. Narragansett Bay sediments were also a net N₂O sink (-39.44 ± 7.47 nmol N₂O m^-2 hr^-1), with the highest rates of sediment N₂O consumption occurring in the summer (-51.61 ± 7.59 nmol N₂O m^-2 hr^-1). N₂O flux was significantly related to water column dissolved nitrate (NO₃^-) concentration (p < 0.001, R² = 0.48), and we only recorded N₂O release in the spring when NO₃^- concentrations were high. Preliminary data from summer 2018 indicates that biodeposition from oyster aquaculture stimulates enhanced sediment N₂O consumption, but does not influence net CH₄ flux.