Assessment Comments

Assessment is based on results of (1) IDNR/UHL biological (REMAP) monitoring in 2002 and (2) IDNR/UHL monthly ambient monitoring 4 mi N of Indianola from 2002-04.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life are assessed (monitored) as "partially supported" based on results of IDNR/UHL biological monitoring in 2002.   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.   The sources of data for this assessment include (1) the results of IDNR/UHL ambient monthly monitoring during the 2002-2004 assessment period at STORET station 10910001 (formerly station 200551) at the USGS gaging station approximately 1.5 miles west of Highway 65-69 near Indianola and (2) results of IDNR/UHL biological monitoring conducted in 2002 as part of the REMAP project.  

EXPLANATION:  Results of IDNR/UHL biological monitoring in 2002 conducted as part of the REMAP project suggest an impairment.   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 2002 FIBIs = 21 (poor), 38 (fair); BMIBIs = 40 (fair), 24 (poor).   The FIBI average was 29.5 and the BMIBI average was 32.   The aquatic life use support was assessed 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 1994-2004.   The FIBI BIC for this ecoregion is 36 and the BMIBI BIC for this ecoregion is 51.

Previous assessments of this stream segment have indicated potential impacts due to alteration of aquatic habitats (see assessments for previous 305(b) reports).   Thus, problems with habitat quality may explain the relatively poor IBI values for fish and aquatic macroinvertebrates.

As part of the REMAP project, diurnal dissolved oxygen/temperature monitoring conducted during July, September, and October 2002 found substantial fluctuation of dissolved oxygen levels and temperature, including nighttime sags in dissolved oxygen (<5 mg/L) and daytime temperatures exceeding 35 C (95 F).   These conditions were associated with very high estimated levels of community respiration and primary production.   Levels of BMIBI metrics that are sensitive to organic enrichment suggest a nutrient enrichment/algal growth-related water quality problem.   Habitat modification has resulted in a wide and shallow stream channel subjected to high solar input.   These physical factors probably contribute to excessive levels of algal growth, and extreme dissolved oxygen and temperature fluctuations.   Additional sampling data are needed to evaluate the extent, causes and sources of this water quality concern.

Despite results of biological monitoring which suggest impairment, results of IDNR/UHL ambient chemical/physical monitoring do not suggest an impairment.   None of the 36 samples collected during the 2002-2004 assessment period at the IDNR monthly station violated Class B(WW) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen; no violations occurred in the approximately 10 samples analyzed for toxic metals, pesticides and other toxic organic compounds.  

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.