Assessment Comments
Assessment is based on results of (1) IDNR/UHL biological (biocriteria) monitoring in 2000: FIBI = 78, BMIBI = 79 (both - excellent); (2) ISU statewide survey of freshwater mussels in 1998-99.
Basis for Assessment
SUMMARY: The Class B(WW) aquatic life uses are assessed (monitored) as "partially supported" based on results of the 1998-99 statewide assessment of freshwater mussels in Iowa streams by Arbuckle et al. (2000). However, results of IDNR/UHL biological (biocriteria) monitoring in 2000 suggest "full support" of these aquatic life uses based on sampling of the fish and benthic macroinvertebrate communities. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.
EXPLANATION: Results from the IDNR/UHL biocriteria sampling in 2000 suggest that the Class B(WW) uses are "fully supported." The 2000 FIBI score was 78 (excellent), the BMIBI score was 79 (excellent). The aquatic life use support was assessed as fully supported (=FS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established for previous Section 305(b) reports. The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004. The riffle site FIBI BIC for this ecoregion is 65 and the natural substrate BMIBI BIC for this ecoregion is 70.
Results from the 1998-99 statewide assessment of freshwater mussels in Iowa streams, however, suggest an impairment to the aquatic life uses of this stream segment. 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). 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 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 one sample site in this segment of Lime Creek was 9 in the 1984-85 period and was 0 in the 1998-99 period for a percent change of minus 100%. Based on these results, the aquatic life uses are assessed (monitored) as “not supported.” The confidence level of this assessment is relatively high; thus the assessment type is considered “monitored” in the context of Section 305(b) reporting. According to Iowa DNR’s assessment methodology, waterbodies identified as “impaired” based on a “monitored” assessment are candidates for Section 303(d) listing. 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.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.
