Links between functional diversity and biochemical transformations underlie spatial scaling of riverine organic matter diversity

Riverine dissolved organic matter (DOM) plays substantial biogeochemical roles as it travels and transforms along river corridors. Specifically, a large amount of organic matter is transported to the ocean from terrestrial inputs (~0.95 Pg C per year) and releases a significant amount of CO2 as it undergoes abiotic and biotic reactions. Combined, these processes result in variable DOM chemistry (e.g., chemical characteristics like thermodynamic availability) and composition (e.g., chemodiveristy or formula richness) which have demonstrated impacts on biogeochemistry and ecosystem function. Despite this importance, however, few studies focus on examining the potential mechanisms underlying variation in DOM composition. Here, we analyzed samples collected within a stream network in the Yakima River Basin (YRB) using ultrahigh-resolution mass spectrometry (i.e., FTICR-MS) to characterize DOM. To link DOM chemistry to potential function, we identified putative biochemical transformations within each sample. We also performed a series of functional diversity analyses using molecular formula characteristics (e.g., thermodynamic favorability, degradability). We observed that the number of potential biochemical transformations increased with increasing upstream catchment area. This increase was connected to expanding functional diversity of the molecular formula. This pattern suggests that as molecular formulas become more diverse in thermodynamics or degradability, there is increased opportunity for biochemical transformations. We also observed that these patterns are, in part, connected to land cover whereby the occurrence of many land cover types (e.g., agriculture, urban, forest, shrub) could expand DOM functional diversity. We also observed that a novel functional diversity metric measuring similarity to common riverine molecular formulas (i.e., carboxyl-rich alicyclic-like molecules) was significantly related to urban coverage. These results show that DOM diversity does not decrease along stream networks, as predicted by the River Continuum Concept, but rather are influenced by the thermodynamic and degradation potential of molecular formula within the DOM, as well as land cover/use patterns.