Nitrous oxide (N₂O) in Macquarie Harbour, Tasmania

Abstract. Fjord-like estuaries are hotspots of biogeochemical cycling due to steep physicochemical gradients. The spatiotemporal distribution of nitrous oxide (N₂O) within many of these systems is poorly described, especially in the southern hemisphere. The goal of this study is to describe the spatiotemporal distribution of N₂O within a southern hemisphere fjord-like estuary, describe the main environmental drivers of this distribution, the air/sea flux of N₂O, and the main drivers of N₂O production. Cruises 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 at one depth at system end members, and at 5 depths at 4 stations within the harbour.

Results indicate that N₂O is 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 AOU 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 subhalocline 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 / autumn efflux of N₂O to the atmosphere.

This study is the first to report observations of N₂O distribution, generation processes, and estimated diapycnal / surface N₂O fluxes from this system.