CARBON METABOLISM, UPTAKE KINETICS, AND EXPORT: HOW WATERSHED FORM INFLUENCES CARBON MOBILIZATION AND IN-STREAM TRANSFORMATIONS IN HEADWATER CATCHMENTS

Abstract:

Previous research has demonstrated the highly dynamic nature of hydrologic connectivity, and the vertical and spatial expansion of the active watershed area during wet periods. While activation of variable DOM and solute sources during expansion and contraction periods has been well documented in a number of systems, changes in nutrient loading to streams have rarely been linked explicitly to in-stream function.

To this end, we investigated the linkages between terrestrial mobilization of DOC and DIC, in-stream biogeochemical cycling, and downstream transport across scales in two geomorphically contrasting watersheds located in Tenderfoot Creek Experimental Forest, Montana. We deployed a network of in-situ high frequency sensors with a focus on CO₂, dissolved oxygen, fluorescent DOM, nitrate, and a suite of supporting chemical constituents every 30 minutes beginning with the onset of snowmelt and through summer baseflow recession.

Our results suggest that DOM and DIC fluxes, as well as ecosystem processes such as metabolism, were coupled to watershed scale carbon accumulation and mobilization. In both watersheds, metabolism tracked the temporal trends of DOM loading from the terrestrial landscape, indicating that the streams are actively transforming allochthonous organic materials during transport. Headwater stream reaches in the watershed with more hydrologically connected riparian source areas exhibited elevated metabolism, carbon uptake, and carbon export as compared to streams in the watershed with less riparian connectivity, suggesting that the degree of riparian connectivity may explain spatial variation in metabolism and in-stream carbon cycling within and across stream networks.

Ultimately, this study highlights the tight coupling between terrestrial uplands and in-stream ecosystem processes in headwater catchments, and identifies spatio-temporal variation in hydrologic connectivity as a key driver of in-stream metabolic variation. We posit that the balance between biologically and physically driven in-stream fluxes (i.e. relative rates of metabolic components and downstream transport) across watersheds, stream network position, and time can provide new insight into the linkages of watershed and in-stream processes.