The Science

Dissolved organic matter (DOM) is a large mix of carbon-based molecules in natural ecosystems. Within river corridors, scientists have struggled to understand and predict which molecules tend to associate strongly with river water versus which associate with riverbed sediments. A multi-institutional team of researchers addressed this by creating a quantitative “habitat affinity” score that measures whether each molecule is overrepresented in water or in sediments. The scores were combined to describe whole samples and global-scale patterns. Across 93 paired river corridor field sites sampled by the Worldwide Hydrobiogeochemistry Observation Network for Dynamic River Systems (WHONDRS) consortium, most molecules showed little affinity for either habitat. This suggests frequent exchange between water and sediments, as expected in dynamic river corridors. Water-associated molecules tended to be larger and harder to break down, while molecules that were more sediment-associated tended to be easier to break down but less favorable in terms of energy gained via microbial oxidation.

The Impact

This work provided a general framework for linking molecular traits to the degree to which specific types of DOM have a strong affinity toward water or sediment components of river corridors. The study showed that only about one-third of detected molecules had strong, statistically distinct affinities for either water or sediments, while most molecules appeared more interchangeable across habitats. The study clarified which kinds of molecules are most likely to drive differences between water and sediment DOM in river corridors and which types of molecules are most likely to be shared. By pairing molecular habitat affinity with environmental variables, the researchers also identified conditions associated with weaker separation between water and sediment DOM. This helps reveal conditions in which river corridor components (or habitats) are likely to look chemically similar or distinct in terms of their DOM. This can inform studies in biogeochemistry, microbial ecology, and hydrology that track how organic molecules move and transform throughout river corridors.

Summary

Researchers analyzed DOM from paired river water and sediment samples collected across 93 sites on three continents, sampled by the WHONDRS consortium. They used Fourier transform ion cyclotron resonance mass spectrometry, an ultrahigh-resolution method that detects thousands of distinct molecular formulas, and introduced two linked metrics. The first metric quantifies molecule-by-molecule habitat affinity using an effect-size comparison between water and sediments, identifying molecules with high affinity to a specific habitat. The second metric averages those molecule-level affinities within each sample to summarize the overall tendency of a DOM mixture to be dominated by water-associated or sediment-associated molecules. Across all detected molecules, about 65 percent showed no strong habitat-specific affinity, and these low-affinity molecules tended to be more thermodynamically favorable and associated with higher sediment respiration rates. By contrast, water-associated molecules were enriched in lignin- and tannin-like classes, while sediment-associated molecules were enriched in protein- and lipid-like classes. At the sample level, habitat affinity varied with elevation and correlated with different molecular traits in water versus sediments. The analysis also linked weaker habitat affinity (i.e., similar molecules in water and sediment) to higher temperatures and several indicators of human influence, including nitrogen fertilizer use and impervious surface area. The researchers extended the analysis to a projection of global patterns in habitat affinity, revealing a peak in affinity for sediments near the equator. These global projections can serve to guide additional field sampling across global river corridors to evaluate the projections and reveal governing mechanisms.

The initial draft of the text above was created using ChatGPT (version 5.5 or lower, OpenAI). The language and content were subsequently edited by the author for grammar, clarity, and accuracy, and the final document was reviewed by the author.

Contact

James Stegen, Pacific Northwest National Laboratory

Funding

This research was supported by the Department of Energy, Office of Science, Biological and Environmental Research program, Environmental System Science program. This contribution originates from the River Corridor Scientific Focus Area project at Pacific Northwest National Laboratory (PNNL). A portion of this research was performed at the Environmental Molecular Sciences Laboratory user facility, located at PNNL. PNNL is operated by Battelle Memorial Institute for the Department of Energy.

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