The Yakima River basin in Washington State is exceptionally vulnerable to climate change impacts. Increasingly hotter and drier summers are leading to lower flows and warmer water, which threaten the Pacific salmon and steelhead trout that migrate and spawn in the basin. In the lower river, these fish face the additional barrier of low nighttime dissolved oxygen levels in the water. As cold-water fish, salmon and trout are unable to migrate when daytime temperatures are high, but low oxygen levels interfere with their migration at night and in the early morning when temperatures are cooler.

Low dissolved oxygen levels in the lower Yakima are likely caused by the respiration of water stargrass, a native aquatic plant that dominates this section of the river. Plants produce oxygen through photosynthesis, but also consume it through cellular respiration. At night, photosynthesis shuts off, and aquatic plant respiration can quickly deplete the oxygen in a water body. Since water stargrass is most abundant in low-water years, and warm water holds less oxygen, as low-water years and warmer stream temperatures become more common under climate change, this problem will likely become worse.

In this project, the research team will collaborate with the Benton Conservation District and the Confederated Tribes and Bands of the Yakama Nation, which are both working to conserve salmonid (i.e., salmon and trout) populations in the lower Yakima River. The Benton Conservation District is working to protect salmonid habitat in a free-flowing stretch of the Yakima, while Yakama Nation Fisheries is investigating salmonid mortality upstream of a small dam. Together, the team will investigate whether removing medium-size sections of water stargrass can improve dissolved oxygen levels in the river, creating better habitat conditions for fish. For the Conservation District, understanding how plant removal affects dissolved oxygen levels in the river will inform their future plans to remove larger sections of water stargrass. For the Yakama Nation, understanding whether water stargrass may be contributing to fish mortality will aid them as they plan how best to protect salmon as they pass by the dam. More broadly, results from this study will help scientists better understand how aquatic plants impact river ecosystems in a changing climate.