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Alese Colehour, Department of Biology, Macalester College
Daniel J. Hornbach, Department of Biology, Macalester College
Mark C. Hove, Department of Biology, Macalester College

The St. Croix National Scenic Riverway holds most of the mussel species native to the upper Mississippi River basin. The Riverway habitats are home to many threatened and endangered mussel species including federally listed winged maple leaf (Quadrula fragosa) and Higgins eye (Lampsilis higginsii). The objective of this study is to describe temporal and spatial changes in mussel species richness and substrate composition at four sites in the St. Croix River and suggest possible reasons for these changes.

Sediment "diversity" is important to mussel communities that likely depend on larger rocks as protection from rushing currents while smaller pebbles and sand allow them to burrow for anchoring and as a means to avoid predation. Juvenile mussels attach to small sand grains for the first few years of life, and later take on a burrowing life style. Subsequently, sediment composition and stability are thought to be especially important for young to survive to reproductive age.

One of the sites in this study (Wild River) is located above the hydroelectric dam at St. Croix Falls, WI while the other three sites (Interstate, Franconia and Osceola) are found within an 11 kilometer stretch below the dam. Mussels and sediment were excavated from 0.25 m2 quadrats and passed through a series of 6 sieves. Mussels were identified and the sediment was weighed to assess sediment size. Sampling took place at each of the four sites 5-7 times since 1991. "Sediment diversity" was calculated using the proportions of each sediment size.

The range of sediment diversity found at each site since 1991 is 0.0 (essentially all sand) to –0.78 (equal mass of each sediment size). The average number of mussels found at Interstate and Wild River each sampling year was 739 and 612 respectively, while Franconia averaged only 146 and Osceola 122 adult mussels. Sediment was also the coarsest and most diverse at Interstate and Wild River with fine sediments being found at the most downstream sites, Franconia and Osceola. Sediment diversity positively correlates with changes in mussel density since mussels utilize a range of sediment sizes as described above. Since 1991, sediment diversity has decreased at all four sites and mussel diversity also decreased during this time although the change was not statistically significant. Based on flow data obtained daily from the USGS website at St. Croix Falls there has been a pattern of decreasing discharge since 1991. Decreased flow allows finer sediments to settle in the riverbed, decreasing sediment diversity and appears to be negatively impacting mussels.

Increased development, sediment build-up behind the St. Croix Falls dam, a change in dam operations, and flooding may be causing an increase in fine sediment and a decrease in sediment diversity. If federally listed mussel species populations are to remain healthy in the St. Croix River, conservation and propagation efforts may be required in current and former ranges in the Riverway.