Worle Creek IA 03-SSK-6286

Assessment Comments

Assessment based on 2007 IDNR/UHL biological monitoring data: FIBI = 19 (poor), BMIBI = 63 (good). Riffle habitat FIBI BIC = 53, BMIBI BIC = 62.

Basis for Assessment

[Note:  Prior to the current (2008) Section 305(b) cycle, this stream segment was classified only for general uses.   Due to changes in Iowa’s surface water classification that were approved by U.S.    EPA in February 2008 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf), this segment is now presumptively designated for Class A1 (primary contact recreation) uses and for Class B(WW1) aquatic life uses.   According to the Iowa Water Quality Standards, all perennial rivers and streams and all intermittent streams with perennial pools that are not specifically listed in the Iowa surface water classification are designated as Class A1 and Class B(WW1) waters.]

SUMMARY:  The presumptive Class A1 (primary contact recreation) uses are "not assessed" due to the lack of information upon which to base an assessment.    The presumptive Class B(WW1) aquatic life uses are assessed (evaluated) as "partially supported" based on results of IDNR/UHL biological monitoring in 2007.

EXPLANATION:  The assessment of the presumptive Class B(WW1) aquatic life uses are based on data collected in 2007 as part of the IDNR/UHL headwater stream sampling 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 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 2007 FIBI score was 19 (poor) and the BMIBI score was 63 (good).   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 from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004.    The riffle-habitat FIBI BIC for this ecoregion is 53 and the BMIBI BIC is 62.    Even though this site failed the FIBI BIC and passed the BMIBI BIC, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it is a small headwater stream and doesn’t fall in the calibrated watershed size.

This assessment is considered “evaluated” (of lower confidence) due to the lack of a biological assessment protocol for general use-only streams.    IDNR’s existing protocol has been calibrated to aquatic life reference conditions in wadeable Class B(LR) and Class B(WW) streams.    The aquatic environment of most general use streams is one of extremes in water quality.    Consequently, general use-only streams tend to have biological diversity that is low relative to more stable aquatic environments of Class B(LR) and Class B(WW) streams.    Thus, the use of biological assessment methods developed for the ecologically more stable and diverse Class B(LR) streams to assess general use reaches will likely overstate the existence of impairment.    For these reasons, general use-only stream reaches that show a failure to meet regional expectations for aquatic biota (fish or aquatic macroinvertebrates) of Class B(LR) streams, will not be added to Iowa’s Section 303(d) list of impaired waters (Category 5 of the Integrated Report).    The assessment type for these waters will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to “monitored" (indicating an assessment with relatively high confidence).    Such waters will 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”.