Rivers and streams contribute greatly to global carbon dioxide (CO2) emissions, but there are large uncertainties associated with global fluxes from these systems and the drivers of CO2 fluxes in streams appear to be system dependent. Here, we aim to investigate differences in CO2 dynamics between natural and artificial waterways by comparing the magnitude and drivers of CO2 concentrations and fluxes from the Logan River and manmade canals in northern UT. CO2 flux samples were collected using the floating chamber method, and linear mixed models were used to assess relationships between concentrations/fluxes and environmental variables. We found that, on average, the canals had higher fluxes of CO2, but there was no difference in CO2 concentrations. We also found that the environmental factors associated with CO2 concentrations and fluxes differed between the artificial and natural sites, providing evidence that these waterbody types should be considered separately in GHG models and budgets.
We then conducted a secondary study to investigate the effects of salinity, a growing threat to freshwater ecosystems, on the carbon use efficiency (CUE) of sediment microbes, which are key players in GHG production in streams. CUE refers to the portion of C assimilated by microbes that is incorporated in biomass, rather than being respired as CO2. Higher microbial CUE could enhance the C sink capacity of stream ecosystems, potentially mitigating rising atmospheric CO2 levels. We investigated the effects of freshwater salinity on microbial CUE by incubating sediments from the Logan River at environmentally relevant salinities, ranging from ambient to 100 uS/cm above ambient. After 24 hours, we processed the sediments and estimated microbial CUE using two methods: stoichiometric modeling and 13C glucose tracing. Our results from the 13C tracing experiment surprisingly suggest that salinity has a positive effect on CUE, indicating that microbial growth may be stimulated under moderate osmotic stress. We found no significant relationship between salinity and CUE using the stoichiometric modeling technique. This study illustrates important differences in CUE estimation methods and provides evidence that salinity may increase the C storage capacity of the Logan River.