Submarine groundwater discharge drives nitrous oxide source/sink dynamics in a metropolitan estuary

• Coastal waterways can be significant sources of the potent greenhouse gas nitrous oxide (N2O) due to nitrogen inputs and eutrophication. Here, we quantify groundwater derived N2O inputs and atmospheric emissions within a modified urban embayment (Sydney Harbour, Australia). Overall, we found low N2O saturation (91-171%) and air-water fluxes (-2.2 to 24.6 mu 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 N2O throughout the harbor. N2O 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 N2O source to surface waters, two upstream sub-embayments featured groundwater N2O concentrations lower than surface water, suggesting a sink driven by surface waters recirculating in intertidal sediments. Surface-water N2O was undersaturated within one upstream embayment, likely due to N2O consumption within sediments. Contrastingly, the downstream embayments featured higher groundwater N2O and accounted for 45% +/- 21% of the groundwater N2O flux. Sydney Harbour was a net source of N2O to the atmosphere (mean 0.6 +/- 0.3 mu mol m(-2) d(-1)) with larger N2O fluxes occurring from relatively small areas. N2O emissions (expressed in CO2 equilivents) were equivalent to 17% of CO2 emission estimates from previous studies. The low N2O emissions in Sydney Harbour contrast with other modified estuaries which often emit higher N2O fluxes due to larger nitrogen inputs.

Ämnesord

NATURVETENSKAP  -- Biologi -- Ekologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Ecology (hsv//eng)

Nyckelord

Marine & Freshwater Biology

Oceanography

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