Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 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). The Class B(LW) aquatic life uses are assessed (monitored) as “partially supported” due to violations of the Class B(LW) criterion for pH. 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 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 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 2012 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 2006 through 2010 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 80, 74, 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 16.5 mg/L, which was the 5th highest of the 134 monitored lakes.
Data from the 2006-2010 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 85% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (93.1 mg/L) was the 5th highest of the 134 lakes sampled. This median is in the worst 25% of the 134 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 2006-2010. 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 (monitored) as “partially supported” due to violations of the Class B(LW) criterion for pH. 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 ammonia in 21 samples or for dissolved oxygen in 22 samples. There were 3 violations of the Class A1,B(LW) criterion for pH in 22 samples (14%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class A1,B(LW) uses at Tuttle Lake. However, Tuttle Lake was assessed as “partially supporting” in the 2010 assessment/listing cycle due to significant violations of the pH criteria. 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. Therefore, Tuttle Lake will remain assessed as "partially supporting" due to violations of the pH criterion for the 2012 assessment/listing cycle.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
