Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities, 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 (evaluated) as "fully supporting / threatened." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened." Fish consumption uses are assessed (evaluated) as "fully supporting." The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) results of U.S. EPA / IDNR fish contaminant monitoring in 1992. Further monitoring and investigation are needed to better characterize water quality conditions at this lake. EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of algal turbidity may adversely affect the Class A and Class B(LW) uses of East Okoboji Lake. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 69, 57, and 54, respectively. According to Carlson (1977), the index value for total phosphorus is in the upper range between eutrophic and hyper-eutrophic lakes; the values for chlorphyll-a and secchi depth are in the middle range of eutrophic lakes. These index values suggest high levels of phosphorus in the water column, relatively low (and less than expected) levels of chlorophyll-a, and 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) limits production of algae (based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for East Lake Okoboji is 12). This TN:TP ratio suggests the possibility that nitrogen limitation 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 levels of inorganic suspended solids at this lake are relatively low and do not suggest potential problems with high levels of inorganic turbidity. The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at East Okoboji Lake was 2.7 mg/l. The relatively high TSI values for total phosphorus and chlorophyll-a suggest potential threats to the Class A (primary contact) uses through presence of high levels of phosphorus, aesthetically objectionable blooms of algae, and presence of nuisance algal species (i.e., bluegreen algae). 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. These values, however, given the relatively low levels of chlorophyll-a (i.e., suspended algae) at this lake, do not suggest significant problems with bluegreen algae. 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 as "fully supported / threatened" due to excessive nutrient loading to the water column and the potential for nuisance blooms of algae. In addition, historical information for this lake (Bachmann et al. 1994) indicates that fish kills in summer occasionally occur, thus suggesting that organic enrichment/low dissolved oxygen also threaten the aquatic life uses at this lake. Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing a more accurate assessment of support of these uses. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments. Fish consumption uses remain assessed 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).
