Spatiotemporal variability of nitrous oxide in a large eutrophic estuarine system: The Pearl River Estuary, China

Abstract

The spatiotemporal variations of nitrous oxide (N2O) in the Pearl River Estuary, a large perturbed estuary, were investigated via six cruises covering both wet and dry seasons during 2007–2011. Significant spatial and temporal variabilities in N2O concentrations and N2O saturations were detected. Spatially, N2O was oversaturated in the entire estuary; ranging from 328nmolL−1, or 38 times saturation in the O2-depleted Upper Estuary, down to 11–79nmolL−1 in the Middle Estuary (163–905% saturation), and to ~7nmolL−1 (slight supersaturation) in the Lower Estuary. Temporally, increased N2O up to 182±82nmolL−1 (1800±750% saturation) was observed in the Upper Estuary during winter at low river discharge in comparison to 76±19nmolL−1 (1163±287% saturation) in summer at high river discharge; whereas no significant seasonal difference was detected within the Middle and Lower Estuaries. The N2O fluxes decreased by 2 orders of magnitude from upstream to downstream (733 to lower than 5μmolm−2d−1). Seasonally, the higher N2O fluxes integrated across the estuary were in spring and winter, and lower fluxes were exhibited in summer and autumn. The annual water–air N2O flux was estimated to be 37±15μmolm−2d−1. This rendered a total emission of (1.67±0.89)×109gN2Oyr−1, which is equivalent to the revised total emission from 19 European inner estuaries (1.35×109gN2Oyr−1). Moreover, this amount of N2O emission equals approximately 30% of reported CO2 emission from the Pearl River Estuary in terms of greenhouse warming potential. The N2O production was predominantly modulated by nitrification in the Upper Estuary while in the Middle and Lower Estuaries, estuarine mixing appeared to dominate the N2O behavior.