Scientific References

Peer Reviewed Literature: Methane and nitrous oxide porewater concentrations and surface fluxes of a regulated river

Abstract

Greenhouse gas (GHG) emissions from rivers are a critical missing component of current global GHG models.
Their exclusion is mainly due to a lack of in-situ measurements and a poor understanding of the spatiotemporal
dynamics of GHG production and emissions, which prevents optimal model parametrization. We combined simultaneous
observations of porewater concentrations along different beach positions and depths, and surface
fluxes of methane and nitrous oxide at a plot scale in a large regulated river during three water stages: rising, falling,
and low. Our goal was to gain insights into the interactions between hydrological exchanges and GHG emissions
and elucidate possible hypotheses that could guide future research on the mechanisms of GHG production,
consumption, and transport in the hyporheic zone (HZ). Results indicate that the site functioned as a net source
of methane. Surface fluxes of methane during river water stages at three beach positions (shallow, intermediate
and deep) correlated with porewater concentrations of methane. However, fluxes were significantly higher in
the intermediate position during the low water stage, suggesting that low residence time increased methane
emissions. Vertical profiles of methane peaked at different depths, indicating an influence of the magnitude 
and direction of the hyporheic mixing during the different river water stages on methane production and consumption.
The site acted as either a sink or a source of nitrous oxide depending on the elevation of the water column.
Nitrous oxide porewater concentrations peaked at the upper layers of the sediment throughout the
different water stages. River hydrological stages significantly influenced porewater concentrations and fluxes
of GHG, probably by influencing heterotrophic respiration (production and consumption processes) and transport
to and from the HZ. Our results highlight the importance of including dynamic hydrological exchanges
when studying and modeling GHG production and consumption in the HZ of large rivers.