01 Aug 2022
01 Aug 2022

Hyporheic Zone Respiration is Jointly Constrained by Organic Carbon Concentration and Molecular Richness

James C. Stegen1, Vanessa A. Garayburu-Caruso1, Robert E. Danczak1, Amy E. Goldman2, Lupita Renteria1, Joshua M. Torgeson2, and Jacqueline R. Wells2 James C. Stegen et al.
  • 1Earth and Biological Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA, USA
  • 2Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA, USA

Abstract. River corridors are fundamental components of the Earth system, and their biogeochemistry can be heavily influenced by processes in subsurface zones immediately below the riverbed, referred to as the hyporheic zone. Within the hyporheic zone, organic matter (OM) fuels microbial respiration, and OM chemistry heavily influences aerobic and anaerobic biogeochemical processes. The link between OM chemistry and respiration has been hypothesized to be mediated by OM molecular diversity, whereby respiration is predicted to decrease with increasing diversity. Here we test the specific prediction that aerobic respiration rates will decrease with increases in the number of unique organic molecules (i.e., OM molecular richness, as a measure of diversity). We use publicly available data across the United States from crowdsourced samples taken by the Worldwide Hydrobiogeochemical Observation Network for Dynamic River Systems (WHONDRS) consortium. Our continental-scale analyses rejected the hypothesis of a direct limitation of respiration by OM molecular richness. In turn, we found that organic carbon (OC) concentration imposes a primary constraint over hyporheic zone respiration, with additional potential influences of OM richness. We specifically observed respiration rates to decrease nonlinearly with the ratio of OM richness to OC concentration. This relationship took the form of a constraint space with respiration rates in most systems falling below the constraint boundary. A similar, but slightly weaker, constraint boundary was observed when relating respiration rate to the inverse of OC concentration. These results indicate that maximum respiration rates may be governed primarily by OC concentration, with secondary influences from OM richness. Our results also show that other variables often suppress respiration rates below the maximum associated with the richness-to-concentration ratio. An important focus of future research efforts will identify factors that suppress hyporheic zone respiration below the constraint boundaries observed here.

James C. Stegen et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-613', Frederick Colwell, 15 Oct 2022
  • RC2: 'Comment on egusphere-2022-613', Anonymous Referee #2, 29 Nov 2022

James C. Stegen et al.

James C. Stegen et al.


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Short summary
Chemical reactions in river sediments influence how clean the water is and how much greenhouse gas comes out of a river. Our study investigates why some sediments have higher rates of chemical reactions than others. We find that to achieve high rates, sediments need to have two things: only a few different kinds of molecules, but a lot of them. This result spans about 80 rivers such that it could be a general rule helpful for predicting the future of rivers and our planet.