Seasonal and tidal influence on the variability of nitrous oxide in the Tagus estuary, Portugal

Abstract

In order to evaluate seasonal and tidal influence on the variability of dissolved nitrous oxide (N₂O) in the Tagus estuary, Portugal, water sampling was carried out along the salinity gradient (May and November 2006) and during several tidal cycles (February and April 2007) at a fixed site. N₂O and other relevant environmental parameters, temperature, salinity, dissolved oxygen and inorganic nitrogen forms (nitrate, nitrite and ammonium) were measured. Dissolved N₂O concentrations showed strong tidal and seasonal variability, with the highest values occurring in February 2007 (13.7 nM, spring tide) and November 2006 (18.4 nM, upper estuary), apparently related to major Tagus river discharge. The existence of N₂O sources was noticed in middle estuary. During spring tide, the input from external sources may be augmented by water column nitrification, making this process a contributor to the enhancement of N₂O concentration in the estuary. Estimated N₂O air-sea fluxes to the atmosphere reached a maximum value of ~10.4 μmol m^-2 d^-1 in February 2007 during spring tide and in May 2006 in the upper and lower (left bank) estuary. Although the Tagus estuary behaves predominantly as a source of atmospheric N₂O, it appears to be a weaker source than other, more eutrophic estuaries.