Temporal and Spatial Changes in Organic Matter Thermodynamics across Globally-Distributed River Corridors

Abstract:

Thermodynamic properties of water soluble organic matter (OM) are increasingly recognized to strongly influence biogeochemical functions (e.g., respiration rates) in river corridors. Emerging evidence for causal links between thermodynamics and biogeochemical rates includes a combination of field observations, manipulative lab experiments, development of new theory, and dynamic simulation modeling. Here we determine how OM thermodynamics vary spatiotemporally across surface and pore waters of globally distributed river corridors. OM datasets of four river corridors from the Worldwide Hydrobiogeochemical Observation Network for Dynamic River Systems (WHONDRS) consortium were derived from Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS) analyses at the Environmental Molecular Sciences Laboratory (EMSL).OM from surface and pore waters across four rivers were sampled every 3 hours over a 48-hour time period. Elemental formulae of thousands of organic compounds in each sample were inferred from FTICR-MS data and used to estimate thermodynamic favorability using the parameter lambda (λ). We find variation in λ within and across all rivers. Because each sample contains numerous λ values, it is possible to study sample to sample changes in the distribution of λ values, which are further connected to other aspects of OM chemistry, such as elemental stoichiometric ratios. We find conserved aspects of λ distributions through time and across rivers, surface water, and pore water, but also biogeochemically relevant variation. In some cases the distributions are subtly different, but the exponential dependence of respiration on λ suggests that small variations can have large biogeochemical consequences. In addition, we find relatively consistent λ distributions can be observed despite significant variation in stoichiometry, suggesting that variation in underlying OM chemistry leads to relatively consistent distributions of λ and thus also of respiration rates. This result is consistent with recent work in river corridors, suggesting a consistent property across globally distributed river corridors.