Tidal and spatial variability of nitrous oxide (N2O) in Sado estuary (Portugal)

• This study reveals tidal/spatial variability of nitrous oxide (N2O) in Sado estuary.
• Besides Sado river, upwelling represents an additional N2O source in the lower/mid Sado estuary, in summer.
• The estuary functions as a source of N2O to the atmosphere.
• N2O fluxes from Sado estuary represent a minor contribution in a global perspective of estuarine systems.

The estimate of the nitrous oxide (N2O) fluxes is fundamental to assess its impact on global warming. The tidal and spatial variability of N2O and the air-sea fluxes in the Sado estuary in July/August 2007 are examined. Measurements of N2O and other relevant environmental parameters (temperature, salinity, dissolved oxygen and dissolved inorganic nitrogen – nitrate plus nitrite and ammonium) were recorded during two diurnal tidal cycles performed in the Bay and Marateca region and along the estuary during ebb, at spring tide. N2O presented tidal and spatial variability and varied spatially from 5.0 nmol L−1 in Marateca region to 12.5 nmol L−1 in Sado river input. Although the Sado river may constitute a considerable N2O source to the estuary, the respective chemical signal discharge was rapidly lost in the main body of the estuary due to the low river flow during the sampling period. N2O varied with tide similarly between 5.2 nmol L−1 (Marateca) and 10.0 nmol L−1 (Sado Bay), with the maximum value reached two hours after flooding period. The influence of N2O enriched upwelled seawater (∼10.0 nmol L−1) was well visible in the estuary mouth and apparently represented an important contribution of N2O in the main body of Sado estuary. Despite the high water column oxygen saturation in most of Sado estuary, nitrification did not seem a relevant process for N2O production, probably as the concentration of the substrate, NH4+, was not adequate for this process to occur. Most of the estuary functioned as a N2O source, and only Marateca zone has acted as N2O sink. The N2O emission from Sado estuary was estimated to be 3.7 Mg N–N2O yr−1 (FC96) (4.4 Mg N–N2O yr−1, FRC01). These results have implications for future sampling and scaling strategies for estimating greenhouse gases (GHGs) fluxes in tidal ecosystems.