Assessment Comments

Assessment is based on statewide assessment of freshwater mussels in Iowa (Arbuckle et al. 2000). Potential causes & sources of impairment were applied to all sites in study. See attached document for details.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses are assessed (evaluated) as "not supported" based on results of a 1998-99 statewide assessment of freshwater mussels in Iowa streams.   EXPLANATION:  The data upon which the previous assessment was based were collected in 1995 (see above).   These data are now considered too old (greater than five years) for characterizing current water quality conditions.   For the 2002 reporting cycle, The Class B(WW) 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).   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 support / threatened"); more than seven species indicates high species richness and "full support" of aquatic life uses.   Species richness of freshwater mussels at the three sample sites in this stream segment was 4, 6, and 5 in the 1984-85 period and were 0, 2, and 0 at the three sites in the 1998-99 period for an average percent change in this segment of -89%.   Based on these results, the aquatic life uses are assessed 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.   Despite the potential impairment of the aquatic life uses suggested by this assessment, historical monitoring of fish population in this segment of Buffalo Creek suggest a relatively diverse fish community (see assessment for the 1996 and 1998 reports above).   Additional monitoring is needed to better define both the biological status of this stream segment as well as the site-specific causes and sources of impairment of these uses that may exist.