N2O seasonal distributions and air‐sea exchange in UK estuaries: Implications for the tropospheric N2O source from European coastal waters

[1] We report measurements of dissolved nitrous oxide (N2O), dissolved inorganic nitrogen, and turbidity in surveys of six UK inner estuaries between February 2000 and October 2002: the Humber, Forth, Tamar, Tyne, Tees, and Tay. We also present dissolved N2O data for the Wash outer estuary from May 1995 and dissolved O2 data for the Forth estuary from June 2001. N2O was always supersaturated relative to air and was highest in the Humber (range 140–6500%) and generally higher at all sites during summer. In estuaries with well defined turbidity maximum zones (TMZs) at low salinity, N2O was maximal in the TMZ, coincident with high NH4+ and/or NO3−. Inspection of the broad relationships between N2O, NH4+, NO3−, NO2−, and O2 revealed a predominantly nitrification source for the N2O in the estuaries studied; denitrification-derived N2O was apparently unimportant and denitrification did not constitute a significant NO3− sink. In the anthropogenically impacted Tees estuary N2O (saturation 140–2000%) was attributed to high NH4+ in sewage and industrial effluent. N2O emissions were thus primarily a function of NH4+ derived from internal resuspension and/or ammonification, or external inputs and were independent of river-borne NO3−. We reevaluated total UK and European estuarine N2O emissions using these and published data, based on an aerially weighted approach that separately identified inner and outer estuaries, and a downward revision of the total European estuarine area used in a recent synthesis. Our revised estimates, ∼1.9 ± 1.2 × 109 g N2O yr−1 for the UK and 6.8 ± 13.2 × 109 g N2O yr−1 for Europe (including UK) are dominated by large (area ∼200–500 km2) anthropogenically impacted macrotidal inner estuaries. By contrast large pristine macrotidal systems, small inner estuaries, and large outer estuaries appear to be comparatively minor N2O sources. The UK estuarine N2O source is <2% of the UK N2O budget. Our revised European estuarine N2O emission is around 2 orders of magnitude smaller than a recent previous estimate that set this equivalent to ∼26% of the global estuarine total. We contend that this is an overestimate due to biases in the flux calculations resulting from likely overestimates of the mean N2O saturation and mean wind speed for European estuaries, and the European estuarine area. Taking this into account reduces this estimate to be more in line with our revised synthesis.