Assessment Comments
Assessment remains based on the results of (1) IDNR/SHL biological ssmpling in 2001 and (2) the statewide survey of freshwater mussels conducted from 1998-99 (Arbuckle et al. 2000).
Basis for Assessment
[Note: Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(LR) aquatic life uses. Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008, this segment is now presumptively designated for Class A1 (primary contact recreation) uses. The stream remains designated for aquatic life uses (now termed Class B(WW2) aquatic life uses). Thus, for the current assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses remain "not assessed" (IR 3a) due to the lack of data for indicator bacteria upon which to base an assessment. The Class B(WW-2) aquatic life uses remain assessed (evaluated) as "partially supported" (5b-t) 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 IDNR/SHL stream biocriteria project and follow-up sampling to assess recovery from a July 1998 fishkill. Although the data from both the assessment of freshwater mussels and the fish/macroinvertebrates are now greater than 10 years old, these data were used to identify a Section 303(d) impairment for the 2004 IR cycle, and thus both datasets remain relevant to the assessment of this stream segment.
EXPLANATION: Results of IDNR/SHL biological sampling conducted in 2001 suggest that the aquatic life uses are "partially supported" based on an evaluated assessment. The 2001 sampling data are now greater than five years old; therefore, this assessment is now considered "evaluated" instead of "monitored". 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 (FIBI) and a benthic macroinvertebrate index (BMIBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2001 FIBI score was 60 (good) and the BMIBI score was 54 (fair). The aquatic life use support was assessed (evaluated) as partially supporting (=PS), 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 1994-2008. This segment passed the FIBI BIC (1/1) and failed the BMIBI BIC in 2001. The non-riffle habitat FIBI BIC for this ecoregion is 44 and the natural substrate BMIBI BIC for this ecoregion is 70.
This aquatic life assessment is considered "evaluated" because there were not two or more samples collected from this segment in multiple years from within a five-year period. Additionally, 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). According to IDNR’s assessment/listing methodology, impairments based on “evaluated” assessments are of lesser confidence and are thus not appropriate for Section 303(d) listing (Category 5 of the Integrated Report). IDNR does, however, consider these impairments as appropriate for listing under either Category 2b or 3b of the Integrated Report (waters potentially impaired and in need of further investigation). However, despite this change in assessment methodology and type, this waterbody remains in IR Category 5b-t and remains on Iowa’s Section 303(d) list of impaired waters.
Results from the 1998-99 statewide assessment of freshwater mussels in Iowa streams also 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 biocriteria sampling in 2001, the Class B(WW2) 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). (Note: because the data from Arbuckle et al. (2000) are now older than five years, the assessment category is changed from a “monitored” (i.e., a higher confidence assessment) to “evaluated” (i.e., lower confidence assessment). Despite this change in assessment category, the impairment indicated by these data remains in IR Category 5 (i.e., Section 303(d) list) until more recent data suggest a good cause for de-listing.)
