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. Note: The fish population of this lake was renovated in fall 2006; a very large population of common carp was eliminated. The IDNR Fisheries Bureau anticipates that, with the removal of the common carp, water clarity of this lake will improve.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to very poor water transparency related mainly to high levels of inorganic turbidity, but also to algae blooms. An additional impairment to the Class A1 uses is suggested by the very large population of cyanobacteria at this lake (=nuisance aquatic life). The Class B(LW) aquatic life uses remain assessed (evaluated) as “partially supported” due to siltation, excessive nutrient loading to the water column, and high levels of non-algal turbidity. Fish consumption uses remain “not assessed” due to the 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: The fish population of this lake was renovated in fall 2006; a very large population of common carp was eliminated. The IDNR Fisheries Bureau anticipates that, with the removal of the common carp, water clarity of this lake will improve.
EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Thayer Lake are “not supported.” Using the median values from these surveys from 2004 through 2008 (approximately 18 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 79, 69, and 75 respectively for Thayer Lake. According to Carlson (1977) the Secchi depth and total phosphorus values place Thayer Lake in the hypereutrophic category, while the chlorophyll a value places Thayer Lake in between the eutrophic and hypereutrophic categories. These values suggest 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 very high at Thayer Lake and suggests that non-algal turbidity causes an impairment at this lake. The median inorganic suspended solids concentration at Thayer Lake was 15.3 mg/L, which was the 10th highest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a large population of cyanobacteria exists at Thayer Lake, which contributes to impairment at this lake. These data show that cyanobacteria comprised 81% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (42.0 mg/L) was also the 20th 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 siltation, excessive nutrient loading to the water column, and high levels of non-algal turbidity. Information from the IDNR Fisheries Bureau suggests that water quality has improved after a fishery renovation. No common carp have been sampled in the lake and small areas of rooted aquatic vegetation have appeared. Algae blooms, however, remain a concern at this lake. The ISU and UHL lake surveys results, however, show relatively good chemical water quality at Thayer Lake. During 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 18 samples, and no violations for dissolved oxygen in 18 samples. There were 2 violations of the pH criterion in 18 samples (11%). 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 B(LW) uses at Thayer Lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
