Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities in 2000, and (4) results of U.S. EPA/IDNR fish tissue monitoring in September 1992.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting / threatened" due to slightly elevated levels of chlorophyll and due to slight reductions in water transparency. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to nutrient loading to the water column and to algal blooms. Fish consumption uses are assessed (evaluated) as "fully supporting" based on results of fish contaminant monitoring in 1992. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) results of U.S. EPA / IDNR fish contaminant monitoring in 1992.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of turbidity due to algal and non-algal sources may threaten the full support of the designated Class A and Class B(LW) uses of East Okoboji Lake. Using the median values from this survey from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 65, 57, and 59, respectively. According to Carlson (1977), the index value for total phosphorus is in the middle range between eutrophic and hyper-eutrophic lakes; the values for chlorophyll-a and Secchi depth are in the middle to upper range of eutrophic lakes. These index values suggest moderately high levels of phosphorus in the water column, but relatively low (and somewhat less than expected) levels of chlorophyll-a, and suggest relatively good (and somewhat better than expected) water transparency. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low values for chlorophyll-a and secchi depth indicate that some factor (e.g., nitrogen limitation or zooplankton grazing) other than phosphorus limits production of algae. Results of ISU monitoring suggest that the somewhat elevated levels of inorganic suspended solids and the large population of zooplankton grazers may serve to limit algal production at this lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for East Lake Okoboji is 15). This TN:TP ratio suggests a slight possibility that nitrogen limitation, at times, suppresses the production of suspended algae at this lake. In addition, data from Downing et al. (2002) show relatively large populations of zooplankton species at this lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 30% of the dry mass of the zooplankton community in the mid-July and early August samples. The 2000 average summer mass of Cladocerans at East Okoboji Lake (23.0 mg/l) was the 57th highest of the 131 lakes sampled, thus suggesting a potential non-phosphorus limitation due to algal grazing. The levels of inorganic suspended solids at this lake are slightly elevated and suggest at least the potential for limiting algal production. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l; the median level at East Okoboji Lake was 3.0 mg/l, thus suggesting the potential for limiting algal production and contributing to the slight reductions in water transparency at this lake. Although indicating relatively good water quality, the TSI values for chlorophyll-a and Secchi depth do suggest potential threats to full support of the Class A (primary contact) uses through occurrence of aesthetically objectionable conditions due to blooms of algae and due to inorganic turbidity.
Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of this lake, especially in mid to late summer. Sampling in 2000 showed the percent wet mass of bluegreens ranged from less than 40% in the mid-June sampling to 100% in the mid-July and early August samplings. The summer 2000 average summer mass of bluegreen algae at East Okoboji Lake (10.3 mg/l) was moderately high but not at the level believed to threaten full support of designated uses. The amount of data available for characterizing algal populations at this lake (one season), however, is not sufficient for developing a more accurate assessment of support of these uses. Thus, the assessment category is considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to improve the accuracy of future water quality assessments for this lake.
The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed (evaluated) as "fully supported / threatened" due to excessive nutrient loading to the water column and the potential for nuisance blooms of algae. Fish consumption uses remain assessed (evaluated) as "fully supported" based on results of the U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1992. 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).
