Submarine groundwater discharge drives nitrous oxide source/sink dynamics in a metropolitan estuary
Michael J. Reading, Douglas R. Tait, Damien T. Maher, Luke C. Jeffrey, Rogger E. Correa, James P. Tucker, Haile Arefayne Shishaye, Isaac R. Santos
Abstract
Abstract Coastal waterways can be significant sources of the potent greenhouse gas nitrous oxide (N 2 O) due to nitrogen inputs and eutrophication. Here, we quantify groundwater derived N 2 O inputs and atmospheric emissions within a modified urban embayment (Sydney Harbour, Australia). Overall, we found low N 2 O saturation (91–171%) and air–water fluxes (−2.2 to 24.6 μ mol m −2 d −1 ). Concentrations were highest in upstream brackish areas and a commercial/industrial subembayment. Dissolved inorganic nitrogen concentrations were low and inversely correlated to N 2 O throughout the harbor. N 2 O surface water dynamics were apparently driven by saline submarine groundwater discharge, as quantified by the radioisotope tracer radon‐222. Groundwater discharge was highest within the embayments and mangrove‐lined upper estuary. While groundwater was a net N 2 O source to surface waters, two upstream sub‐embayments featured groundwater N 2 O concentrations lower than surface water, suggesting a sink driven by surface waters recirculating in intertidal sediments. Surface‐water N 2 O was undersaturated within one upstream embayment, likely due to N 2 O consumption within sediments. Contrastingly, the downstream embayments featured higher groundwater N 2 O and accounted for 45% ± 21% of the groundwater N 2 O flux. Sydney Harbour was a net source of N 2 O to the atmosphere (mean 0.6 ± 0.3 μ mol m −2 d −1 ) with larger N 2 O fluxes occurring from relatively small areas. N 2 O emissions (expressed in CO 2 equilivents) were equivalent to 17% of CO 2 emission estimates from previous studies. The low N 2 O emissions in Sydney Harbour contrast with other modified estuaries which often emit higher N 2 O fluxes due to larger nitrogen inputs.