East Okoboji Lake IA 06-LSR-1652

For the 2016 assessment/listing cycle, the Class A1 (primary contact recreation) uses of East Okoboji Lake are assessed (monitored) as "fully supported" based on information from the ISU lake survey. Using the median values from these surveys from 2010-2014 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 60, 58, and 66 respectively for East Okoboji Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place East Okoboji Lake in the Eutrophic category. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and high levels of phosphorus in the water column. The data show no violations of the Class A1 criterion for pH in 15 samples.

The level of inorganic suspended solids was moderately high at East Okoboji Lake, and does suggest that non-algal turbidity may contribute to the impairment at this lake. The median level of inorganic suspended solids in East Okoboji Lake (2.85 mg/L) was ranked 70th among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a moderate population of cyanobacteria exists at East Okoboji Lake. These data show that cyanobacteria comprised 73% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (17.2 mg/L) was ranked 52nd of the 138 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” based on information from the IDNR Fisheries Bureau and results from the ISU lake surveys.   Based on information from the IDNR Fisheries Bureau, this lake was infested with Zebra mussels in 2012 and the population exploded in 2015. This invasive species will likely cause water quality to degrade in the future. 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 lake survey from 2010-2014, however, show there were no violations of the criterion for ammonia in 15 samples(0%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and no violations of the criterion for pH in 15 samples(0%). Based on IDNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of East Okoboji Lake.

The level of support of the Class HH-fish consumption uses is changed from “fully supporting” to “not assessed” due to the age of the data upon which the previous assessment was based.  The previous assessment was 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.