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 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results of the 1998 "Iowa Voluntary Atrazine Monitoring Program", and (5) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2004.

Basis for Assessment

[Note:  For the 2006 and 2008 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 assessed (evaluated) as “fully supported.”  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 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results of the 1998 "Iowa Voluntary Atrazine Monitoring Program", and (5) 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 2002 through 2006 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 36, 40, and 47 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 1.0 mg/L, which was the 4th lowest of the 132 monitored lakes.

Data from the 2002-2006 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at West Okoboji Lake.   These data show that cyanobacteria comprised 99% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (21.7 mg/L) was the 59th highest 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 2002-2006 there were no violations of the Class B(LW) criterion for ammonia in 17 samples, dissolved oxygen in 23 samples, or pH in 23 samples.   Based on IDNR’s assessment methodology these results suggest full support of the Class B(LW) uses at West Okoboji Lake.

Drinking water uses are remain assessed (evaluated) as "fully supported" based on results of monitoring conducted in 1998 for the "Iowa Voluntary Atrazine Monitoring Program” (VMP) (this lake has not been part of the Iowa VMP since 1998).   However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively low confidence).   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 2002-06 show that nitrate levels are very low at this lake (maximum value = 0.2 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.