Thayer Lake IA 05-GRA-1369

For the 2016 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Thayer Lake are assessed (monitored) as “not supported” (IR 5a) due to very poor water transparency related mainly to high levels of inorganic turbidity based on information from the ISU lake survey. Using the median values from these surveys from 2010-2014 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 59, 54, and 64 respectively for Thayer Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Thayer Lake in the Eutrophic category. These values suggest low levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and moderately high levels of phosphorus in the water column. The data show one violation of the Class A1 criterion for pH in 15 samples.

IDNR's assessment methodology indicates that at least two assessment/listing cycles with a TSI value less than or equal to 63 are necessary to suggest de-listing of an impairment.   The chlorophyll a TSI value in Thayer Lake for the 2008 assessment listing cycle was 67, which based on IDNR's assessment/listing methodology indicated an impairment based on the narrative criteria protecting against aesthetically objectionable conditions.   The TSI value for the 2014 assessment/listing cycle was less than the trigger of 65 (2014 listing for chlorophyll a was 62), and the TSI value for the current listing cycle is also 63. Based on IDNR's methodology, the impairment (5a) for chlorophyll a is suggested for delisting for the 2016 assessment/listing cycle.

While the TSI value for Secchi depth in Thayer Lake is below the impairment trigger of 65 for the 2016 reporting cycle, Thayer Lake was listed as "not supported" the Class A1 uses for the 2014 reporting cycle due to aesthetically objectionable conditions caused by poor water clarity.   Based on IDNR's assessment methodology, median-based TSI values for Secchi depth must be 63 or less for two consecutive assessment/listing cycles before a lake can be removed from the state’s Section 303(d) list (IR Category 5).    Therefore, Thayer Lake remains assessed as "not supported" due to poor water clarity for the 2016 assessment cycle.

The level of inorganic suspended solids was moderately high at Thayer Lake, and does suggest that non-algal turbidity may contribute to the impairment at this lake. The median level of inorganic suspended solids in Thayer Lake (3 mg/L) was ranked 71st among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a small population of cyanobacteria exists at Thayer Lake. These data show that cyanobacteria comprised 58% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (5.6 mg/L) was ranked 15th of the 138 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. 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. Results of the ISU lake survey from 2010-2014 show there were no violations of the criterion for ammonia in 15 samples(0%), 3 violations of the criterion for dissolved oxygen in 15 samples(20%), and one violation of the criterion for pH in 15 samples(7%). Based on IDNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of Thayer Lake.

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.

Note:This lake was renovated in 2006.