Shifting nitrous oxide source/sink behaviour in a subtropical estuary revealed by automated time series observations

The oceans are a major source of the potent greenhouse gas nitrous oxide (N₂O) to the atmosphere. However, little information is available on how estuaries and the coastal ocean may contribute to N₂O budgets, and on the drivers of N₂O in aquatic environments. This study utilised five time series stations along the freshwater to marine continuum in a sub-tropical estuary in Australia (Coffs Creek, Australia). Each time series station captured N₂O, radon (²²²Rn, a natural submarine groundwater discharge tracer), dissolved nitrogen, and dissolved organic carbon (DOC) concentrations for a minimum of 25 h. The use of automated time series observations enabled spatial and tidal-scale variability of N₂O to be captured. Groundwater was highly enriched in N₂O (up to 306 nM) compared to the receiving surface water. Dissolved N₂O supersaturation as high as 386% (27.4 nM) was observed in the upstream freshwater and brackish water areas which represented only a small (∼13%) proportion of the total estuary area. A large area of N₂O undersaturation (as low as 53% or 3.9 nM) was observed in the mangrove-dominated lower estuary. This undersaturated area likely resulted from N₂O consumption due to nitrate/nitrite (NO_x) limitation in mangrove sediments subject to shallow porewater exchange. Overall, the estuary was a minor source of N₂O to the atmosphere as the lower mangrove-dominated estuary sink of N₂O counteracted groundwater-dominated source of N₂O in the upper estuary. Average area-weighted N₂O fluxes at the water-air interface approached zero (0.2–0.7 μmol m⁻² d⁻¹, depending on piston velocity model used), and were much lower than nitrogen-rich Northern Hemisphere estuaries that are considered large sources of N₂O to the atmosphere. This study revealed a temporally and spatially diverse estuary, with areas of N₂O production and consumption related to oxygen and total dissolved nitrogen availability, submarine groundwater discharge, and uptake within mangroves.