Seasonal and spatial controls on N2O concentrations and emissions in low-nitrogen estuaries: Evidence from three tropical systems

Abstract

Nitrous oxide (N2O) production and emissions are observed in estuary waters, yet little is known about estuary N2O fluxes under low dissolved inorganic nitrogen (DIN) conditions. We present high-resolution spatial surveys of N2O concentrations in three low-DIN (NO3− < 30 μmol L−1) tropical estuaries in Queensland, Australia (Johnstone River, Fitzroy River, Constant Creek), during consecutive wet and dry seasons. Constant Creek had the lowest concentrations of dissolved inorganic nitrogen (DIN; 0.01 to 5.4 μmol L−1 of NO3− and 0.09 to 13.6 μmol L−1 of NH4+) and N2O (93–132% saturation), and associated low mean N2O emissions (0.02–3 μmol m−2 d−1). The Johnstone River Estuary had the highest N2O concentrations (97–245% saturation), with mean emissions of 4 to 6 μmol m−2 d−1, driven by upstream groundwater and riverine sources. In the Fitzroy River Estuary, N2O concentrations (100–204% saturation) and emissions (1–2 μmol m−2 d−1) were associated with wastewater inputs, which had a larger effect during the dry season. Estuary freshwater flushing time was an important factor: when freshwater was relatively stagnant, in-situ N2O emissions were larger than riverine N2O inputs. Conversely, fast freshwater flushing times diminished the role of in-situ emissions, and dissolved N2O largely flushed through the estuary directly to the ocean. Overall N2O emissions from the three tropical estuaries were low compared to previous studies, but were reasonably predicted by DIN concentrations: at low water column NO3− concentrations (i.e. <5 μmol L−1) estuaries can exhibit negative water-air N2O fluxes.