Density currents reduce nitrous oxide emissions in a tributary bay of Three Gorges Reservoir

Abstract

Reservoirs are a significant source of the potent greenhouse gas nitrous oxide (N₂O), but there are few data on N₂O in the world's largest reservoirs and limited understanding of the factors controlling their emission rates. Here we analyzed high-resolution measurements of dissolved N₂O concentrations and fluxes in a typical tributary bay of Three Gorges Reservoir. The surface water was oversaturated in N₂O during both low and high water level (8.6 -16.4 nmol/L, 107% - 180% saturation) and N₂O fluxes varied nearly tenfold (0.2 and 1.6 μmol/(m² h)). Dissolved N₂O concentrations were characterized by pronounced vertical gradients, which were controlled by bidirectional density currents. The river water with high concentrations entered the bay as an underflow along the riverbed, the upper part of the water column was formed by intrusive backwater of Three Gorges Reservoir having significantly lower N₂O concentrations. In consequence, the N₂O emission potential of the impoundment was reduced compared to pre-impoundment conditions. These results reveal the importance of hydraulic conditions on N₂O emission from large reservoirs and suggest that flow regulation can be a potential tool for mitigating greenhouse gas emissions from manmade impoundments.

Keywords: Bidirectional density currents; Dissolved N(2)O; Greenhouse Gas; N(2)O emission; Three Gorges Reservoir.