PNNL

The Science                                

The hyporheic zone (HZ) at the bottom of rivers plays an important role in the overall river ecosystem, accounting for a significant portion of carbon dioxide (CO₂) emissions into the water column. However, HZ respiration modeling studies lack quantification of how the HZ contributes to total CO₂ at the scale of the entire watershed or basin. Previous studies have also incompletely considered the contribution of anaerobic respiration. A new modeling study developed an approach to couple carbon and nitrogen cycles in river corridors as a way to quantify microbially-driven aerobic and anaerobic respiration in the HZ. This new model allowed researchers to determine the key factors controlling the spatial variability of microbially-driven respiration within the Columbia River Basin.   

The Impact

Riverbed CO₂ production accounts for a significant portion of the carbon cycle of inland waters. Previous regional and global studies that estimated stream/river CO₂ release did not include the effect of CO₂ production in the riverbed. The basin-scale coupled carbon-nitrogen model developed in this study allows researchers to quantify the spatial variation of aerobic and anaerobic respiration across the entire Columbia River Basin. This study offers an option for testing hypotheses related to microbially-driven respiration processes in river systems in other biomes and climates and can be used as a tool to design sampling schemes for large-scale experimental studies.

Summary

Microbes in riverbeds generate high amounts of CO₂, but numerical simulation models have not accurately quantified their contributions to the total CO₂ budgets across entire river basins and other large regions. In this study, a multi-institutional team of researchers used a numerical simulation model to estimate the CO₂ emissions from riverbeds into the water column in the presence and absence of oxygen. The researchers then identified important variables that explain the spatial variation of riverbed CO₂ emissions within the Columbia River Basin. The study found that CO₂ emissions from riverbeds showed high spatial variability. Within the Columbia River Basin, wetter sub-basins showed higher CO₂ emissions than drier sub-basins. Medium-sized rivers generated the highest CO₂ emissions. Most CO₂ emissions from channels occurred in the presence of oxygen. However, reaches in agricultural areas generated relatively high CO₂ emissions without oxygen. And finally, the team found that the water exchange rate between channels and riverbeds, as opposed to other physical variables, could explain the spatial variation of CO₂ emissions.

PNNL Contact

Kyongho Son, Pacific Northwest National Laboratory, kyongho.son@pnnl.gov

Funding

This research was supported by the Department of Energy (DOE), Office of Science (SC) Biological and Environmental Research (BER) program, as part of BER's Environmental System Science (ESS) program. This contribution originates from the River Corridor Scientific Focus Area (SFA) at Pacific Northwest National Laboratory. This research used resources from the National Energy Research Scientific Computing Center, a DOE SC User Facility.