PNNL

The Science

Large rivers support complex food webs and provide ecosystem services. Despite their importance, metabolism in large rivers is not well-understood because existing estimation/determination methods apply only to smaller streams. Researchers at the Pacific Northwest National Laboratory modified existing methods to estimate metabolism for the Hanford Reach of the Columbia River in Washington State, USA. They found that Columbia River metabolism rates, seasonal patterns, location of metabolism, and the coupling of photosynthesis and respiration all differed from that typically observed in smaller rivers.

The Impact

Metabolism patterns for the Hanford Reach section of the Columbia River differ from those observed in most rivers. Peak photosynthesis occurred in late summer, as opposed to spring or mid-summer as is expected for most other rivers. Photosynthesis rates were primarily influenced by temperature and secondarily influenced by light. Photosynthesis and respiration rates were among the highest measured and the two were strongly connected, indicating little accumulation of algae. Finally, most metabolism occurs in the water column by plankton rather than in the sediments. Conducting more metabolism studies in other large rivers will help determine whether these patterns are typical for large rivers.

Summary

Rivers are a major component of the Earth system. The study of river metabolism is key for understanding nutrient dynamics, ecosystem health, and food webs in river ecosystems. This study focuses on understanding ecosystem metabolism in large rivers, an area that has received limited attention compared to small and medium rivers. Large rivers present unique challenges for measurements of depth and gas exchange because of their size and the presence of large dams. Here, the research team estimated reach-scale metabolism for the Hanford Reach of the Columbia River in Washington State, a free-flowing stretch with substantial discharge. The team used existing, reach-specific hydrologic models to estimate depth and a combination of semi-empirical models and tracer tests to estimate gas exchange. They found that metabolism metrics were comparatively high in the Columbia River, with peak values occurring in late summer or early fall, and strong coupling between photosynthesis and respiration. The river exhibited plankton-dominated metabolism driven primarily by temperature and secondarily by light. These patterns deviate from those typically observed in small and medium rivers, and demonstrate that metabolism patterns from smaller rivers may not be accurately scaled to large rivers.

PNNL Contact

James Stegen, Pacific Northwest National Laboratory, james.stegen@pnnl.gov

Funding

This work was partially funded by the River Corridor Science Focus Area at the Pacific Northwest National Laboratory. PNNL is operated by Battelle Memorial Institute for the Department of Energy (DOE). The science focus area is supported by the DOE’s, Biological and Environmental Research program, Environmental System Science program.