Variations in Hyporheic Zone Aerobic Respiration across Four Rivers as Predicted from Dissolved Organic Matter Composition

Abstract:

Hydrologic exchange between river channels and their surrounding subsurface environments including the hyporheic zone leads to enhanced mixing and stimulates biogeochemical reactions. These processes are known to contribute significantly to cycling of key nutrients (e.g., organic matter and nitrogen) in river corridors. The thermodynamic properties of dissolved organic matter (DOM) have been shown to play a key role in controlling biogeochemical rates in these systems. As part of a week-long virtual summer school hosted by the Environmental Molecular Sciences Laboratory (EMSL) in July 2020, students analyzed seven datasets from four rivers using a novel bioinformatics and reactive transport simulation pipeline. In the final step of the pipeline, we used the PFLOTRAN reactive transport simulator to predict vertical profiles of DOM concentration as a function of hydrologic exchange flux magnitude and the molecular character of the DOM pool. Characteristics of DOM in the four rivers were provided by the Worldwide Hydrobiogeochemical Observation Network for Dynamic River Systems (WHONDRS) consortium and were derived from Fourier Transform Ion Cyclotron Resonance (FTICR) Mass Spectrometry (MS) analyses of surface and pore water samples at EMSL. FTICR-MS data were used to define the elemental formulae of thousands of organic compounds found in each sample. These formulae were in turn used to define stoichiometry and rate parameters for reactive transport models using a thermodynamics-based approach. 1D model simulations were performed using the PFLOTRAN code to represent flow and transport under vertical flow through the hyporheic zone. Model predictions illustrate the potential variability of aerobic respiration rates and vertical concentration profiles in the hyporheic zone as a function of DOM composition (both surface and pore waters) and vertical exchange flux rates.