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 very poor water transparency related to high levels of inorganic turbidity and algal blooms that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. Violations of the Class A1 criterion for pH also suggest an impairment at this lake. An additional impairment to these uses is suggested by the very large population of cyanobacteria at this lake (=nuisance aquatic life). Excessive nutrient loading to the water column, organic enrichment, nuisance blooms of algae, and re-suspension of sediment also contribute to the impairments at this lake. The Class B(LW) aquatic life uses are assessed (monitored) as “partially supported” due to violations of the Class B(LW) criterion for pH, violations of the Class B(LW) criterion for ammonia also suggest a possible impairment at this lake. Fish consumption uses remain “not assessed” due to a lack of fish tissue data upon which to base an assessment. 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 algae and turbidity at Tuttle Lake was prepared by IDNR and approved by EPA in 2005, and these impairments for this lake were thus moved to IR Category 4a for the 2006 cycle. This lake remains in Category 4a of the 2010 cycle.
EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Tuttle Lake are “not supported.” Using the median values from these surveys from 2004 through 2008 (approximately 25 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 81, 75, and 76 respectively for Tuttle Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Tuttle Lake in the hypereutrophic category. These values suggest very high levels of chlorophyll a and suspended algae in the water, extremely poor water transparency, and extremely high levels of phosphorus in the water column.
The level of inorganic suspended solids is extremely high at Tuttle Lake and suggests that non-algal turbidity contributes to the impairment. The median inorganic suspended solids concentration at Tuttle Lake was 20.0 mg/L, which was the 3rd highest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a very large population of cyanobacteria exists at Tuttle Lake, which contributes to an impairment due to nuisance aquatic life. These data show that cyanobacteria comprised 87% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (90.0 mg/L) was the 2nd 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 (evaluated) as “partially supported” due to violations of the Class B(LW) criterion for ammonia. The ISU and UHL lake surveys results show poor chemical water quality at Tuttle Lake due to excessive nutrient loading to the water column, organic enrichment, nuisance blooms of algae, and re-suspension of sediment. Information from the IDNR Fisheries Bureau also suggests poor water quality at Tuttle Lake. There were no violations for dissolved oxygen in 25 samples. There were 5 violations of the Class A1,B(LW) criterion for pH in 25 samples (20%). Based on IDNR’s assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest an impairment of the Class A1,B(LW) uses at Tuttle Lake. During 2004-2008 there were 2 violations of the Class B(LW) criterion for ammonia in 24 samples. Based on IDNR’s 2010 Section 303(d) listing methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s chronic criterion for ammonia and thus do not suggest an impairment of the Class B(LW) aquatic life uses. Because, however, more than one violation of the chronic criterion for ammonia occurred, this potential impairment will be added to Iowa's list of waters in need of further investigation. Note: Based on consultation with other Region 7 states and U.S. EPA Region 7 staff in 2007, IDNR's methodology for assessing impairments due to violations of chronic criteria for toxic parameters (e.g., ammonia and toxic metals) was changed. Prior to the 2008 listing cycle, IDNR followed a U.S. EPA recommendation (U.S. EPA 1997b, page 3-18) that more than one violation of a water quality criterion for a toxic pollutant in an abundant data set indicates an impairment of aquatic life uses. Because no state in Region 7 collects ambient data with the frequency necessary to accurately identify compliance with a chronic criterion (i.e., to allow calculation of a short-term (4-day or 30-day averages)), Region 7 states concluded that another assessment approach (e.g., the 10% rule) was more appropriate.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
