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 “partially supported” due to aesthetically objectionable conditions caused by poor water transparency due primarily to high levels of non-algal turbidity. High levels of algae also contribute to the impairment of the Class A1 uses. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to high levels of (inorganic) turbidity related primarily to sediment re-suspension at this shallow lake. Fish consumption uses are "not assessed" based on a lack of recent 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 turbidity at Lower Gar Lake was prepared by IDNR and approved by EPA in 2002; thus, this waterbody was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Because all Section 303(d) impairments identified for the 2012 assessment/listing cycle [turbidity (algal and inorganic)] are addressed by the TMDL, this waterbody remains in IR Category 4a.
EXPLANATION: For the 2012 assessment/listing cycle, results from the ISU and UHL lake surveys indicate that the Class A1 (primary contact recreation) uses of Lower Gar Lake are assessed as “partially supported” due to aesthetically objectionable conditions caused by poor water transparency due to non-algal turbidity. 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 69, 63, and 69 respectively for Lower Gar Lake. According to Carlson (1977) the Secchi depth, chlorophyll a and total phosphorus values all place Lower Gar Lake in between the eutrophic and the hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, poor water transparency, and high levels of phosphorus in the water column.
Based on data from the ISU and UHL lake surveys, the level of inorganic suspended solids was very high at this lake and suggests impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Lower Gar Lake was 10.7 mg/L, which was the 17th highest of the 134 monitored lakes.
Data from the 2006-2010 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Lower Gar Lake, which could contribute to aesthetically objectionable conditions at this lake. These data show that cyanobacteria comprised 69% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (30.5 mg/L) was also the 48th highest of the 134 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to high levels of (inorganic) turbidity related primarily to sediment re-suspension at this shallow lake. This potential impairment is not appropriate for Iowa’s Section 303(d) list of impaired waters (Integrated Report Category 5) but is appropriate for IR Category 3b of Iowa’s Integrated Report and Iowa’s list of waters in need of further investigation. Results of the ISU and UHL lake surveys from 2006 through 2010 suggest good chemical water quality at Lower Gar Lake. Data from these surveys show no violations of the Class B(LW) criterion for ammonia in 21 samples, dissolved oxygen in 21 samples, or pH in 22 samples. Based on IDNR’s assessment methodology these results do not suggest impairment of the Class B(LW) uses at Lower Gar Lake.
Fish consumption uses were “not assessed” due to a lack of recent fish contaminant monitoring at this lake. The most recent fish tissue monitoring was conducted in 2000. While these results suggest that levels of contaminants were low at Lower Gar Lake, they are now too old (greater than 10 years) to be used for an assessment.
