Dams have been shown to impact freshwater mussels. We examined how mussels respond to differently sized dams (18 vs. 4 m) on the St. Croix River and its tributary, the Sunrise River. We hypothesized that: mussel density and growth rate would be greater downstream of the smaller dam due to the relatively greater food subsidies and temperature effects of the reservoir above it; and the effects of the small dam would moderate downstream as the localized impacts of the dam were reduced. We quantitatively sampled mussels upstream and downstream of the dams. For a common species, Actinonaias ligamentina, we ascertained growth rates by measuring successive growth rings. The highest mussel richness and diversity were upstream and downstream of the large dam. Higher mussel density was found immediately below the small dam but declined downstream. A. ligamentina downstream of the small dam grew faster and were larger than individuals in other reaches. Food availability and temperature appeared to influence mussel density and growth rate for A. ligamentina downstream of the small dam. This study provides information that may help managers decide whether to remove small dams or to maintain them because of the unique mussel habitats below these structures.
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Acknowledgments We thank a number of Macalester College students who have participated in the collection of mussels over the years, Dan MacNalley and Guy Donnelly for providing access to the Sunrise River through their property, Konrad Schmidt of the MN Department of Natural Resources for discussions that improved this manuscript and Macalester College, the National Park Service, and the National Science Foundation for funding.
- Actinonaias ligamentina
- Community density
- Freshwater mussel
- Species diversity