Nitrous oxide: Estuarine sources and atmospheric flux

Nitrous oxide (N₂O), nitrate, nitrite and ammonium concentrations were determined along axial surface water profiles of the Tamar Estuary, South West England, on five occasions. Physical variables, including suspended solids, salinity and temperature were also measured. In addition, a temporal survey was carried out in which these variables were measured at a fixed point over a tidal cycle. Nitrous oxide was supersaturated throughout the estuary on all sampling occasions, with a maximum in the low salinity region. The N₂O maximum exhibited no significant spatial deviation with season or tidal state, in contrast to the other determined variables. Nitrous oxide supersaturation resulted predominantly from release by sediments, which was potentially enhanced by tidal resuspension and bioturbation. Nitrification in the water column associated with the turbidity maximum was considered to be a secondary seasonal source of nitrous oxide. External sources, such as terrestrial runoff and sewage input also contributed. The estuary represents a source of atmospheric nitrous oxide throughout the year.