PNNL

The Science

Rivers “breathe” as microorganisms use oxygen to break down organic matter. A key challenge is to learn what controls this oxygen use across integrated river corridor systems. In this study, a multi-institutional team of researchers examined oxygen use in the water itself, or water column respiration (ERwc), as well as basic water properties (e.g., temperature, dissolved nitrogen, carbon, etc.). Variation in ERwc can provide indirect clues as to how important the subsurface environment is to whole-ecosystem respiration and thus provide insights into overall ecosystem functioning. The team found that ERwc spanned 0 to −7.38 grams of oxygen per cubic meter per day, covering—and even exceeding—the full range reported previously across global streams and rivers. Warmer water, more dissolved organic carbon, higher total dissolved nitrogen, and more suspended particles were the best local predictors and together accounted for about half of the variation across sites. ERwc changed little with watershed features such as upstream drainage area, and the fastest rate occurred at an agriculturally influenced mid-order stream, suggesting strong local controls.

The Impact

Understanding what governs oxygen use helps explain how energy and materials move through watersheds. This study used a basin-wide design to show that straightforward measurements and common water-quality indicators can explain much of the spatial variation in ERwc. The work was distinct in demonstrating that local temperature, nitrogen, organic carbon, and suspended solids were strong controllers of ERwc, while simple position in the river network mattered less. Because observed ERwc values sometimes overlapped those typical of entire river corridor ecosystems— which also consider respiration in river sediments—the results highlight circumstances when in-water processes may contribute substantially to integrated reach-scale metabolism. The results also indicate other situations in which water column processes have little influence and sediment-associated respiration processes are likely dominant. More generally, the results indicate a broad continuum of sediment-to-water-column dominated respiration across the study basin. These insights suggest the need for follow-on studies to more directly parse water column and sediment-associated contributions to respiration rates of integrated river corridor ecosystems. This study makes it clear that transformations of material (e.g., organic matter) from headwater streams to mainstem rivers is driven by a dynamic interplay between processes in the water column and processes occurring on and around sediments. Achieving predictive understanding of the relative roles of water column and sediment-associated processes will require knowledge of what governs this interplay across whole stream networks.

Summary

A multi-institutional team of researchers using resources available at the Environmental Molecular Sciences Laboratory, a U.S. Department of Energy Office of Science user facility, assessed ERwc across a hydrologically and ecologically diverse basin by sampling 47 sites within the Yakima River Basin, spanning stream orders 2–7 during late summer 2021. Along with these measurements, a suite of water-quality assays were performed, including solute concentrations and advanced ultrahigh-resolution mass spectrometry, to characterize dissolved organic matter. ERwc was found to range from 0 to −7.38 g O₂ m⁻³ d, with median and mean ERwc values of −0.58 and −0.84 g O₂ m⁻³ d⁻¹, respectively. Using a machine learning approach, the team found that total dissolved nitrogen, temperature, dissolved organic carbon, and suspended solids explained ~49% of ERwc variation, whereas drainage area and organic matter chemistry showed only weak explanatory power. Interestingly, the highest ERwc occurred at a fifth-order, agriculturally influenced site, indicating that local factors can exert significant control over stream functioning. 

Placing these findings in context, ERwc values from the Yakima River Basin encompassed and exceeded literature ranges and on occasion approached reach-scale whole-ecosystem respiration compiled for U.S. rivers, implying that in-water processes can be a notable part of whole-reach oxygen use under certain conditions. The study’s site selection leveraged a basin-wide clustering of landscape attributes to ensure a collection of field sites representative of the much larger Columbia River Basin, which the Yakima River Basin sits within. In turn, the results are likely transferable to at least the northwest region of the contiguous United States and potentially far beyond.

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. 

Research Contact(s)

Vanessa Garayburu-Caruso, Pacific Northwest National Laboratory

James Stegen, River Corridor, 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. 

Related Links

• River Corridor Hydrobiogeochemistry Science Focus Area PNNL webpage
• Data and scripts used to generate the main findings within this manuscript are published on the U.S. Department of Energy’s Environmental System Science Data Infrastructure for a Virtual Ecosystem (ESS-DIVE) repository at https://data.ess-dive.lbl.gov/datasets/doi:10.15485/2283171 (Laan et al., 2024). Other data collected during the field efforts (i.e., sensor data; surface water chemistry data; and geospatial information, metadata, and maps for the 2021 Spatial Study sampling event) can be accessed on ESS-DIVE at https://data.ess-dive.lbl.gov/datasets/doi:10.15485/1898914 and https://data.ess-dive.lbl.gov/datasets/doi:10.15485/1892052 (Fulton et al., 2022; Grieger et al., 2022)