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

In order to evaluate seasonal and tidal influence on the variability of dissolved nitrous oxide (N(2)O) 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 N(2)O and other relevant environmental par meters temperature salinity dissolved oxygen and inorganic nitrogen forms (nitrite nitrite and ammonium) were measured Dissolved N(2)O 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 N(2)O 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 N(2)O concentration in the estuary Estimated N(2)O air sea fluxes to the atmosphere reached a maximum value of similar to 10 4 mu mol 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 N(2)O it appeals to be a weaker source than other more eutrophic estuaries