PNNL

The Science                                 

Biochemical transformations of organic matter (OM) are a primary driver of river corridor biogeochemistry, thereby regulating ecosystem processes at local to global scales. The research community, however, lacks knowledge of how the diversity of OM transformation varies across surface and subsurface components of river corridors. A new study has found that OM transformations in surface water are decoupled from the OM transformations occurring in sediments just a few centimeters away. Also, the composition of transformations in sediments was not linked with transformation composition in adjacent surface waters. Furthermore, the results indicate that OM transformations in surface water are an integrated signal of diverse processes occurring throughout upstream areas (10s to 1000s of kilometers). In contrast, OM transformations in sediments reflect a narrower range of processes within the centimeter-scale sampled volume.

The Impact

The results from this work indicate a decoupling between surface water and sediment OM. The result surprised researchers, given that river water is constantly flowing through sediments and returning to the river water column. This flow of water (known as hydrologic exchange) could homogenize OM between river water and sediments. The results show that homogenizing influences of hydrologic exchange are insufficient to overcome processes that researchers believe more directly impact OM transformations. They infer that the processes influencing OM transformations and the scales at which they operate diverge between surface water and sediments. This is important as OM has a strong influence over greenhouse gas production from rivers. There are efforts to build better models of rivers that account for variation in the mechanisms impacting OM. These improved models are needed to properly represent rivers in larger-scale models of the integrated Earth system and models applied at local scales to address water quality issues.

Summary

OM and its associated transformations are the heart of river biogeochemistry. Rivers, in turn, are a major component of the Earth system, with influences over global biogeochemistry. Local water quality issues are also inherently linked to river processes. OM transformations are driven by diverse biotic and abiotic processes, but the scientific community lacks knowledge of how the diversity of those processes varies across river water and sediments. To fill this knowledge gap, this study quantified the number of biotic and abiotic OM transformations across diverse river corridors. Researchers used instruments at the Environmental Molecular Science Laboratory to obtain the measurements. The number of unique transformations provide a proxy for the diversity of biochemical processes impacting OM. Using public data spanning the contiguous United States from the Worldwide Hydrobiogeochemical Observation Network for Dynamic River Systems (WHONDRS) consortium, the team learned that water OM had more biotic and abiotic transformations than OM from sediments. Also, there was no relationship between the number of transformations in surface water and sediments, and the composition of transformations in sediments was not linked with OM transformations in adjacent waters. With these results, researchers infer that OM transformations in surface water are an integrated signal of diverse processes occurring throughout the upstream catchment. In contrast, OM transformations in sediments likely reflect a narrower range of processes within the sampled volume. All of this indicates decoupling between surface water and sediment OM, which was unexpected given the potential for hydrologic exchange to homogenize OM.

Funding

This work was supported by the Department of Energy Office of Science Early Career program at Pacific Northwest National Laboratory. PNNL is operated by Battelle for DOE under Contract DE-AC05-76RL01830.