Assessment Comments
Assessment remains based on results of the statewide assessment of freshwater mussels in Iowa (Arbuckle et al. 2000). Potential causes & sources of impairment were applied to all sites in study.
Basis for Assessment
SUMMARY: The Class B(LR) uses are assessed (evaluated) as "not supported" based on information from the report "Statewide Assessment of Freshwater Mussels (Bivalva, Unionidae) in Iowa Streams" by Arbuckle et al. (2000). As part of this study, sampling results from 1998 and 1999 (Arbuckle et al. 2000) were compared to results from stream sites surveyed in 1984 and 1985 by Frest (1987). On a statewide basis, this comparison showed sharp declines in the numbers of mussel species ("species richness") in Iowa streams and rivers from the mid-1980s to the late 1990s.
EXPLANATION: For purposes of Section 303(d) listing, this assessment was based on the percent change in the number of species of freshwater mussels found in the 1984-85 survey versus the 1998-99 survey. For stream segments having four or more species reported for the 1984-95 survey, results of this comparison were used by staff of the Iowa DNR TMDL & Water Quality Assessment Section to assess the degree to which the aquatic life uses of the sampled stream segments are supported. Greater than a 50% decline in species richness from the 1984-85 to the 1998-99 period suggests an impairment of the aquatic life uses. Species richness of freshwater mussels at the three sample sites in this segment of Buffalo Creek was 4, 6, and 5, respectively, in the 1984-85 period (average of approximately 7 species) and was 0, 2, and 0, respectively, in the 1998-99 period (average of less than one species) for an average percent change of minus 89%. Based on these results, the aquatic life uses are assessed (monitored) as “not supported.” As presented by Arbuckle et al. (2000), the potential causes of declines in species richness of Iowa's freshwater mussels include siltation, destabilization of stream substrate, stream flow instability, and high instream levels of nutrients (phosphorus and nitrogen). Their study also suggested the importance of stream shading provided by riparian vegetation to mussel species richness. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). Despite this change in assessment type, this waterbody remains in IR Category 5b.
