Watershed scale predictors of freshwater mussel distribution in Austin, Texas

Background/Question/Methods

Freshwater mussels are a poorly understood, rapidly declining class of invertebrate. Of the 53 species in Texas, 15 are listed as state threatened, one is a candidate for federal protection, and 11 are currently petitioned for listing under the Endangered Species Act. Major threats include urbanization and its impacts to hydrology and sediment dynamics, nutrient loading, water withdrawal and diversion, and invasive species introduction. Understanding patterns of distribution and abundance of mussel populations in relation to water quality characteristics and stream habitat is needed for conservation planning.

The City of Austin sampled 28 creeks over three years within our 1,600 sq. km jurisdictional area, primarily Travis County, to quantify how mussel distribution at the sub-watershed level is impacted by land-use and water quality constituents (TSS, turbidity, pH, DO, conductivity, NH₃, NO₃, SO₄, phosphorus, and E. coli). Mussels were located using a range of survey methods: visual, tactile, snorkeling, shoreline collection and SCUBA in streams and reservoirs with drainage areas ranging from 5 to 547 sq. km.

Results/Conclusions

A total of 15 species consisting of 633 live mussels and 1,032 spent valves were collected from 15 Austin-area watersheds. Four species Lampsilis teres, Pyganodon grandis, Uniomerus tetralasmus and Utterbackia imbecillis made up 96% of live specimens and 72% of spent valves. Changes in number of taxa, species diversity, dominance and evenness of live specimens, when compared to spent valves, suggests a temporal shift in composition to more tolerant, generalist species.

Watersheds containing mussels had lower impervious cover (P < 0.001) and E. coli (P < 0.01) levels and higher levels of total suspended solids (P < 0.05) and turbidity (P < 0.05) than watersheds without live mussels or spent valves. DO, conductivity, pH, and nutrient concentrations were not significantly related to mussel distribution at the sub-watershed scale.

These results suggest that development, and resulting changes in hydrology and sediment regime are primary factors for mussel distribution in Austin, TX. The linkage of spatial patterns in mussel distribution to land-use development at the catchment scale is an important tool for use in mussel conservation efforts.