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

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 mu mol/(m(2) 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. (c) 2020 Elsevier Ltd. All rights reserved.

Automated summary

Reservoirs are significant sources of nitrous oxide, but there is limited understanding of N2O emissions from the world's largest reservoirs. A study on a tributary bay of Three Gorges Reservoir found that N2O emissions were reduced after impoundment, highlighting the importance of hydraulic conditions on N2O emissions from large reservoirs.