Document Type

Thesis

Date of Degree Completion

Spring 2025

Degree Name

Master of Science (MS)

Department

Biology

Committee Chair

Clay Arango

Second Committee Member

Jason Irwin

Third Committee Member

Alexa Maine

Abstract

Freshwater mussels are critical components of freshwater systems through their contributions to ecosystem function via water filtration, nutrient deposition and release, and food web stimulation. However, they remain under-studied in the Pacific Northwest, which precludes their consideration during stream restoration activities. For example, although mussel species richness has declined 35% across western watersheds, and today, the most historically diverse watersheds support far fewer species, stream restoration projects that commonly aim to restore salmonid habitat do not consider the conservation needs of freshwater mussels. Large knowledge gaps about the ecology of western mussel species limit our ability to understand their local and broader effects on stream food web productivity and hinder the development of effective restoration and climate change mitigation strategies. I used a three-part approach to analyze how western freshwater mussels contribute to stream food webs. In part one, I conducted lab experiments to quantify how mussel size and increasing water temperature affect clearance, nutrient excretion, and biodeposition (feces and pseudofeces) rates using three mussel species (Margaritifera falcata, Gonidea angulata, and Anodonta californiensis) extant in the Yakima River Basin, Washington. Individual clearance rates ranged from 0.04-4.11 L h-1 I found species-specific differences in clearance rates across temperatures whereby Anodonta californiensis had significantly higher clearance rates than G. angulata and M. falcata. At elevated temperatures (20-25 degrees C), while G. angulata maintained low, consistent clearance rates across temperatures. Biodeposition per gram dry mass decreased with temperature for A. californiensis, however, their biodeposition was higher than G. angulata at 15 degrees C. All species excreted more ammonium with increasing temperature. In part two, I performed field experiments to quantify stream nutrient demand in six streams in the Upper Yakima River Basin and in the mainstem Yakima River, WA. These uptake rates served to put mussel nutrient release rates into an ecosystem context. Finally, I synthesized lab and field experiments into a numeric model to estimate mussels contribution to whole-stream nutrient demand and stream food webs. The compiled model revealed western mussels can filter large volumes of water (up to 65% of daily river flow), deposit substantial amounts of biodeposition and alleviate N-limitation in high abundance. This model suggests that restoring mussel beds could have significant, positive ecosystem effects, and it also suggests that actual ecosystem impacts with mussel densities observed in the field are likely reduced due to population declines. Species-specific thermal sensitivities suggest climate change could shift the magnitude and direction of ecosystem function by freshwater mussels.

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