East Okoboji Lake IA 06-LSR-1652

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), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 2004.

Basis for Assessment

SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “fully supported.”  The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.”  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 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), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring in 2004.

EXPLANATION: For the 2012 reporting cycle, the Class A1 (primary contact recreation) uses for East Okoboji Lake are assessed (monitored) as “fully supported” based on results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program.   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 58, 55, and 67 respectively for East Okoboji Lake.   According to Carlson (1977) the Secchi depth and chlorophyll a values place East Okoboji Lake in the eutrophic category, while the total phosphorus value places East Okoboji Lake in between the eutrophic and hypereutrophic categories.   These values suggest relatively low levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and high levels of phosphorus in the water column.  

The levels of inorganic suspended solids at this lake were relatively low and do not suggest problems related to non-algal turbidity.   The median level of inorganic suspended solids in East Okoboji Lake (3.0 mg/L) was the 52nd lowest median of the 134 lakes sampled by the ISU and UHL programs.

Data from the 2006-2010 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at East Okoboji Lake, which does not suggest impairment of the Class A1 uses.   These data show that cyanobacteria comprised 67% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (31.2 mg/L) was the 64th lowest of the 134 lakes sampled.  

The Class B(LW) (aquatic life) uses are assessed as “fully supported.”  Results from the ISU and UHL lake surveys from 2006-2010 show no violations of the Class B(LW) criteria for ammonia in 22 samples, dissolved oxygen in 21 samples, or pH in 22 samples.   Based on IDNR’s assessment methodology, these results suggest “full support” of the Class B(LW) uses of East Okoboji Lake.

Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S.  EPA/IDNR fish contaminant (RAFT) monitoring at East Okoboji Lake in 2004.   The composite samples of fillets from channel catfish and yellow perch had low levels of contaminants.   Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.032 ppm; total PCBs: 0.092 ppm; and technical chlordane: <0.03 ppm.   Levels of primary contaminants in the composite sample of yellow perch fillets were as follows: mercury: 0.0.037 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm.   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 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 2004 RAFT sampling conducted in this 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.