Macroscale Patterns of Dissolved Organic Matter Thermodynamic Properties Across Diverse River Systems

Abstract

Abstract Thermodynamic properties of dissolved organic matter (DOM) influence river biogeochemistry. Using these properties to predict biogeochemical rates across rivers requires knowledge of how they vary. We employed mass spectrometry to quantify three DOM thermodynamic properties mechanistically linked to microbial respiration and biomass growth. We estimated these properties for each identified molecule in over 500 water and sediment samples from freshwater and saline rivers spanning diverse biomes. DOM thermodynamic properties exhibited continental-scale spatial gradients associated with climate and land-cover. Theory predicts relationships among the three properties. Observed inter-property relationships diverged from these predictions, but were consistent across continents. We infer that while there is variation in DOM thermodynamic properties across rivers--associated with climate and land cover--quantitative shifts in any one property are mechanistically linked to shifts in the other properties, leading to highly constrained inter-property relationships that persist across diverse rivers. Given the broad extent of sampled rivers, we suggest the observed inter-property relationships may hold across all rivers. Models that predict river biogeochemistry via DOM thermodynamic properties can use patterns observed here as inputs and constraints.