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¯²d¯¹). The Johnstone River Estuary had the highest N₂O) concentrations (97–245% saturation), with mean emissions of 4 to 6 μmol m¯²d¯¹, driven by upstream groundwater and riverine sources. In the Fitzroy River Estuary, N₂O) concentrations (100–204% saturation) and emissions (1–2 μmol m¯²d¯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.