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), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish contaminant monitoring in 2000.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not 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 assessed (evaluated) as “fully supported.” 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), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish contaminant monitoring in 2000.
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 2010 assessment/listing cycle [turbidity (algal and inorganic)] are addressed by the TMDL, this waterbody remains in IR Category 4a.
EXPLANATION: For the 2010 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 “not supported” due to aesthetically objectionable conditions caused by poor water transparency due to non-algal turbidity. Using the median values from these surveys from 2004 through 2008 (approximately 25 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 72, 65, and 71 respectively for Lower Gar Lake. According to Carlson (1977) the Secchi depth and total phosphorus values place Lower Gar Lake in the hypereutrophic category, while the chlorophyll a value places Lower Gar Lake in between the eutrophic and the hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and very 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 12.0 mg/L, which was the 15th highest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Lower Gar Lake, which may contribute to impairment at this lake. These data show that cyanobacteria comprised 87% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (26.5 mg/L) was also the 38th highest of the 132 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. Results of the ISU and UHL lake surveys from 2004 through 2008 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 23 samples, dissolved oxygen in 25 samples, or pH in 25 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 assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Lower Gar Lake in 2000. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). The composite samples of fillets from both the bottom-feeder and predator species had low levels of contaminants. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated from the 2000 RAFT sampling conducted at Lower Gar Lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory for this waterbody.
