Assessment Comments
Assessment is based on (1) fish kill on July 20, 1998, (2) 2001 Biocriteria sampling (Fish IBI= 60 (good), BM-IBI= 52 (fair)), and (3) statewide survey of freshwater mussels. See attached document for details.
Basis for Assessment
SUMMARY: The Class B(LR) aquatic life uses were assessed (evaluated) as "partially supported." The assessment was based on (1) data collected in 2001 as part of the DNR/UHL stream biocriteria project and follow-up sampling to assess recovery from the July 1998 fishkill and (2) results of the report "Statewide Assessment of Freshwater Mussels (Bivalva, Unionidae) in Iowa Streams" by Arbuckle et al. (2000). EXPLANATION: 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. Results from the 1998-99 statewide assessment of freshwater mussels in Iowa streams, however, suggest a potential 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 included the following factors: (1) the percent change in the number of species of freshwater mussels found in the 1984-85 survey versus the 1998-99 survey and (2) the number of mussel species found in the 1998-99 survey. Greater than a 50% decline in species richness from the 1984-84 to the 1998-99 period suggests an impairment of the aquatic life uses. In addition, low species richness in the 1998-99 survey suggests potential impairment. For purposes of Section 305(b) assessment only, staff of the IDNR Water Quality Bureau used results from Arbuckle et al. (2000) to define categories of species richness for Iowa's mussel communities: less than three species indicates low species richness and "nonsupport" or "partial support" of aquatic life use; from four to seven species indicates moderate species richness and potential minor impacts (="fully supported / threatened"); more than seven species indicates high species richness and "full support" of 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).
