Nitrous oxide (N ₂ O) in Macquarie Harbour, Tasmania

Abstract. Fjord-like estuaries are hotspots of biogeochemical cycling due to their steep physicochemical gradients. The spatiotemporal distribution of nitrous oxide (N2O) within many of these systems is poorly described, especially in the Southern Hemisphere. The goals of this study are to describe the spatiotemporal distribution of N2O within a Southern Hemisphere fjord-like estuary, the main environmental drivers of this distribution, the air–sea flux of N2O, and the main drivers of N2O production. Sampling surveys were undertaken in Macquarie Harbour, Tasmania, to capture N2O concentrations and water column physicochemical profiles in winter (July 2022), spring (October 2022), summer (February 2023), and autumn (April 2023). N2O samples were collected from middle water depths in the ocean (5 m), minor river (1 m) endmembers, the major river (10 m) endmember at 2 m from the bottom, and at five depths through the water column at four stations within the main harbour body. Results indicate that N2O was consistently supersaturated (reaching 170 % saturation) below the system's freshwater lens where oxygen concentrations are often hypoxic but infrequently anoxic. In the surface lens, levels of N2O saturation vary with estimated river flow and with proximity to the system's main freshwater endmember. The linear relationship between apparent oxygen utilisation and ΔN2O saturation indicates that nitrification is the process generating N2O in the system. When river flow was high (July and October 2022), surface water N2O was undersaturated (as low as 70 %) throughout most of the harbour. When river flow was low (February and April 2023) N2O was observed to be supersaturated at most stations. Calculated air–sea fluxes of N2O indicated that the system is generally a source of N2O to the atmosphere under weak river flow conditions and a sink during strong river flow conditions. The diapycnal flux was a minor contributor to surface water N2O concentrations, and sub-halocline N2O is intercepted by the riverine surface lens and transported out of the system to the ocean during strong river flow conditions. In a changing climate, western Tasmania is expected to receive higher winter rainfall and lower summer rainfall, which may augment the source and sink dynamics of this system by enhancing the summer and autumn efflux of N2O to the atmosphere. This study is the first to report observations of N2O distribution, generation processes, and estimated diapycnal and surface N2O fluxes from this system.