Assessment Comments
Assessment is based on the results of (1) IDNR/UHL biological (biocriteria) sampling in 2001 and (2) the statewide survey of freshwater mussels conducted from 1998-99 (Arbuckle et al. 2000).
Basis for Assessment
SUMMARY: The Class B(LR) aquatic life uses are assessed (monitored) as "partially supported" based on information from the report "Statewide Assessment of Freshwater Mussels (Bivalva, Unionidae) in Iowa Streams" by Arbuckle et al. (2000). The assessment was also based on data collected in 2001 as part of the DNR/UHL stream biocriteria project and follow-up sampling to assess recovery from a July 1998 fishkill.
EXPLANATION: Results of IDNR/UHL biological monitoring conducted in 2001 suggest that the aquatic life uses are "fully supported / threatened." A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species that were collected in the stream sampling reach. The biological metrics were combined to make a fish community index of biotic integrity (F-IBI) and a benthic macroinvertebrate index (BM-IBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2001 Fish IBI score was 60(good) and the BM-IBI score was 52(fair). The aquatic life use support was upgraded from partially supporting to fully supporting / threatened (=FST), based on a comparison of the F-IBI and BM-IBI scores with biological assessment criteria established specifically for the 2002 Section 305(b) report. The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001. The results of this biological monitoring suggest that the imacts from the July 1998 fish kill have dissipated.
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). In general, this comparison showed sharp declines in the numbers of mussel species ("species richness") from the mid-1980s to the late 1990s. For purposes of Section 305(b) reporting, results of this comparison were used by staff of the Iowa DNR Water Quality Bureau to assess the degree to which the aquatic life uses of the sampled stream segments are supported. 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 with greater than a 50% decline in species richness from the 1984-84 to the 1998-99 period suggesting an impairment of the aquatic life uses. Species richness of freshwater mussels at the one sample site in this segment of Buck Creek was 6 in the 1984-85 period and was 1 in the 1998-99 period for a percent change of -83%. Based on a combination of these results with results of IDNR/UHL biociteria sampling in 2001, the Class B(LR) aquatic life uses are assessed as "partially 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. Additional monitoring is needed to better define the biological status of this stream segment. The possibility exists that the July 1998 fish kill in this stream segment negatively influenced sampling for freshwater mussels as conducted by Arbuckle et al. (2000) (see the assessment developed for the 2000 report for more information on the July 1998 fish kill).
