West Okoboji Lake - Millers Bay IA 06-LSR-2068

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 (RAFT) monitoring in 2004.

Basis for Assessment

[Note:  For the 2006, 2008, and 2010 Section 305(b)/303(d) cycle, West Okoboji Lake was subdivided into the following segments for purposes of developing water quality assessments:  main lake basin south, Emersons Bay, Smiths Bay, Millers Bay, and main lake basin north.    These segments were created to better use the water quality data collected as part of the IDNR beach monitoring program which includes monitoring at four of this lake’s beaches.   Developing separate assessments for the main lake basin as well as for the Lake’s bays represents a more accurate characterization of water quality conditions than did past assessments which developed only one assessment for the entire lake.]  

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.”  The Class C (drinking water) uses are "not assessed" due to a lack of recent information upon which to base an assessment.   Fish consumption uses are assessed (monitored) 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 (RAFT) monitoring in 2004.

EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class A1 (primary contact recreation) uses at West Okoboji Lake are “fully supported.”  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 38, 42, and 48 respectively for West Okoboji Lake.    According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place West Okoboji Lake in the mesotrophic category.    These values suggest extremely low levels of chlorophyll a and suspended algae in the water, exceptional water transparency, and extremely low levels of phosphorus in the water column.    

The level of inorganic suspended solids is also extremely low at West Okoboji Lake and suggests that non-algal turbidity does not cause water quality problems at this lake.    The median inorganic suspended solids concentration at West Okoboji Lake was 0.5 mg/L, which was the lowest of the 132 monitored lakes.  

Data from the 2004-2008 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at West Okoboji Lake.    These data show that cyanobacteria comprised 98% of the phytoplankton wet mass at this lake.    The median cyanobacteria wet mass (13.0 mg/L) was the 62nd lowest of the 132 lakes sampled.    However, these results do not suggest impairment of the Class A1 uses at West Okoboji Lake.    

The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported” based on information from the IDNR Fisheries Bureau and results of the ISU and UHL lake surveys.    Data from the ISU and UHL lake surveys show that from 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 25 samples, dissolved oxygen in 24 samples, or pH in 25 samples.    Based on IDNR’s assessment methodology these results suggest full support of the Class B(LW) uses at West Okoboji Lake.  

The Class C (drinking water) uses are "not assessed" based on a lack of recent data upon which to base an assessment.    The most recent atrazine was collected in 1998 as part of the Syngenta Iowa Voluntary Atrazine Monitoring Program.    While these data suggested "full support" of the Class C uses, they are now too old (>10 years) to base an assessment upon.    The only parameter collected as part of the ISU and UHL lake surveys relevant to support of Class C (drinking water) uses is nitrate.    While the results of the ISU and UHL surveys from 2004-08 show that nitrate levels are very low at this lake (maximum value = 0.3 mg/l; median = 0.1 mg/l) relative to the MCL (10 mg/L), these data are not sufficient for developing a valid assessment of support of the Class C uses.    
  
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.   EPA/IDNR fish contaminant (RAFT) monitoring at West Lake Okoboji in 2004.    The composite samples of fillets from common carp and yellow perch had low levels of contaminants.    Levels of primary contaminants in the composite sample of common carp fillets were as follows: mercury: 0.039 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm.    Levels of primary contaminants in the composite sample of yellow perch fillets were as follows: mercury: 0.068 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm.    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 at West Lake Okoboji show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.