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. 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. High levels of algae also contribute to aesthetically objectionable conditions at this lake. 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 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: 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 2006 through 2010 (approximately 16 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 75, 63, and 73 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 moderately high levels of chlorophyll a and suspended algae in the water, extremely poor water transparency, and very 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 8.4 mg/L, which was the 24th highest of the 134 monitored lakes.
Data from the 2006-2010 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Thayer Lake. These data show that cyanobacteria comprised 92% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (34.1 mg/L) was also the 42nd highest of the 134 lakes sampled.
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 2006-2010 there were no violations of the Class B(LW) criterion for ammonia in 16 samples. There were two violations of the Class B(LW) criterion for dissolved oxygen in 16 samples (12%) and one violation of the Class A1,B(LW) criteria for pH in 16 samples (6%). 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.
