Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to aesthetically objectionable conditions caused by nuisance algae blooms that cause poor water transparency, and due to violations of Iowa’s water quality criterion for pH. The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to violations of Iowa’s water quality criterion for pH. Fish consumption uses remain “not assessed” due to a lack of fish contaminant monitoring at this lake. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
Note: A TMDL for organic enrichment and noxious aquatic plants (cyanobacteria) at Indian Lake was prepared by IDNR and approved by EPA in 2005. Because all Section 303(d) impairments identified for the 2008 and 2010 assessment/listing cycle (algal growth, pH, and cyanobacteria) are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).
EXPLANATION: For the 2010 reporting cycle, the Class A1 (primary contact recreation) uses for Indian Lake are assessed (monitored) as “not supported” due to aesthetically objectionable conditions caused by nuisance algal blooms that cause poor water transparency. Using the median values from the ISU and UHL lake surveys from 2004 through 2008 (approximately 19 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 70, 74, and 75 respectively for Indian Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Indian Lake in the hypereutrophic category. These values suggest very high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and very high levels of phosphorus in the water column.
The levels of inorganic suspended solids at this lake were moderately high and do not suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Indian Lake (3.0 mg/L) was the 50th lowest median of the 132 lakes sampled by the ISU and UHL programs.
Data from the 2004-2008 ISU and UHL surveys suggest a large population of cyanobacteria exists at Indian Lake, which contributes to impairment at this lake. These data show that cyanobacteria comprised 87% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (53.8 mg/L) was also the 12th highest of the 132 lakes sampled. This median is in the worst 25% of the 132 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa’s narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values from 2004-2008. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. The assessment category for assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence.
The Class B(LW) (aquatic life) uses are assessed as “partially supported” based on information from IDNR’s Fisheries Bureau and results from the ISU and UHL lake surveys. Information from the IDNR Fisheries Bureau shows that gizzard shad and silver carp were introduced to this lake during the 2008 flooding. Fish kills also remain a concern at this lake. The ISU and UHL lake surveys data from 2004-2008 show no violations of the Class B(LW) criteria for dissolved oxygen in 19 samples, and only one violation of the Class B(LW) criterion for ammonia in 19 samples. Based on IDNR's updated assessment methodology the single violation of the chronic ammonia criterion is not significantly greater than 10% of the samples and therefore does not constitute an impairment of the Class B(LW) uses. The pH data show 3 violations of the Class A1,B(LW) criterion for pH in 19 samples (16%). This is not significantly greater than 10% of the samples, however, Indian Lake was assessed as “partially supporting” in the 2008 assessment/listing cycle due to significant violations of the pH criteria and therefore remains “partially supported” due to the continued violations. Based on IDNR’s assessment methodology 2 consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the criterion are necessary to propose delisting based on pH violations. The pH violations at Indian Lake likely reflect the excessive primary productivity at the lake and do not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
