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HABITAT FACTORS AND MUSSEL ASSEMBLAGE OF FEDERALLY ENDANGERED MUSSEL SPECIES
Skadi von Reis Crooks, Department of Biology, Macalester College
Unionid mussels are long-lived filter feeders and useful as indicators of the quality of riverine environments. Overall mussel density is decreasing at several locations in the St. Croix River (Hornbach et al., 2006), with juvenile mussel density decreasing by as much as 96% in some reaches below St. Croix Falls in the last decade (Kushner et al., 2006). These declines are especially concerning since two federally endangered species, Winged Mapleleaf (Quadrula fragosa) and Higgins Eye (Lampsilis higginsii),are found in the St. Croix River.
We sought to determine those habitat factors that differed significantly between mussel communities with and without federally endangered mussels. We also sought to identify mussel species within mussel communities that are significantly associated with the federally endangered mussel species. Mussel community and habitat data collected at three locations (Interstate, Peaslee Lake, and Franconia) during the past three years were analyzed using JMP 3.2.2 (SAS Institute, Cary, NC). A t-test was used to determine significant differences in habitat measures between sites containing only non-endangered species and sites that contained one of the endangered species. Chi-square analysis was used to assess the significance of unionid aggregations.
Mussel density, species richness, and sediment size were significantly greater at locations with federally endangered species. A correspondence analysis revealed correlations among a variety of environmental factors and the distribution of species. We observed significant relationships between mussel assemblage and two of the habitat factors: sediment size and FST (a direct measure of shear stress that combines depth and flow). We used canonical scores to graphically display the relationship between these habitat factors and species assemblage. The presence of the majority of the species, including Winged Mapleleaf, was correlated with small to medium sized sediment and high FST. Fewer species, including Higgins Eye, showed a correlation with lower FST and coarser sediment. Higgins Eye was significantly associated with Three ridge in their assemblage. Winged Mapleleaf was significantly associated with both Wabash Pigtoe and Monkeyface. These factors may be useful in future efforts to identify river reaches suitable for relocating adult Winged Mapleleaf or Higginseye or for planting propagated juveniles. These results demonstrate the need for detailed information on habitat preferences for the detection and preservation of key habitats for endangered mussel populations.