Assessment Comments
Assessment remains based on: (1) results of a statewide assessment of freshwater mussels in Iowa (Arbuckle et al. 2000). Potential causes & sources of impairment were applied to all sites in study and (2) IDNR/UHL 1999 biological sampling.
Basis for Assessment
SUMMARY: The Class B(WW) aquatic life uses remain are assessed (evaluated) as "partially supported" based on results of a 1998-99 statewide assessment of freshwater mussels in Iowa streams and 1999 IDNR/UHL biological monitoring. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.
EXPLANATION: This segment of Lytle Creek was sampled as part of the 1998-99 statewide study of freshwater mussels in Iowa streams and rivers (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). On a statewide basis, this comparison showed sharp declines in the numbers of mussel species ("species richness") in Iowa streams and rivers from the mid-1980s to the late 1990s. The results of this sampling on this segment of Lytle Creek, however, do not meet IDNR guidelines for developing a monitored assessment of support for the aquatic life uses that is appropriate to support addition to Iowa's Section 303(d) list. That is, 303(d)-level assessment decisions are developed only for those stream segments having an average of four or more species reported in the 1984-85 (Frest) survey due to the difficulty of interpreting status of mussel communities showing relatively low species richness during the both the historical (1984-85) and current (1998-99) surveys. Species richness of freshwater mussels at the one sample site in this segment of Lytle Creek was 3 in the 1984-85 period and was 0 in the 1998-99 period, respectively, for a percent change of minus 100%. Based on these results, the aquatic life uses are assessed (evaluated) as “not supported." For reasons stated above, the confidence level of this assessment is relatively low; thus the assessment type is considered “evaluated” in the context of Section 305(b) reporting. According to Iowa DNR’s assessment methodology, waterbodies identified as “impaired” based on an “evaluated” assessment are not candidates for Section 303(d) listing. Such waters will, however, be placed in either Category 2b or 3b of the IR and thus will be added to the state’s list of “waters in need of further investigation”. Additional monitoring is needed to better define the biological status of this stream segment as well as the site-specific causes and sources of impairment of these uses that may exist.
However, IDNR/UHL biological sampling in 1999 suggest that this segment is fully supporting its aquatic life uses. The assessment was based on data collected in 1999 as part of the DNR/UHL stream biocriteria project. 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 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 1999 FIBI score was 38 (fair) and the BMIBI score was 46 (fair). The aquatic life use support was assessed as fully supporting (=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 FIBI BIC for this ecoregion is 36 and the BMIBI BIC for this ecoregion is 51. This segment passed both the FIBI BIC and BMIBI BIC (with the aid of the UAV +8).
