Assessment Comments
Assessment is based on results of (1) IDNR beach monitoring in 2000-02, (2) ISU lake survey in 2000-01, (3) surveys by IDNR Fisheries Bureau, (4) ISU report on lake plankton communities, and (5) results of U.S. EPA/IDNR fish tissue monitoring in 1995.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened" (minor impacts). The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened" (minor impacts). Drinking water uses remain assessed (monitored) as "fully supported." Fish consumption uses remain assessed (evaluated) as "fully supported." The sources of data for this assessment include (1) results of the IDNR-UHL beach monitoring program in summers of 2000-01, (2) results of the Iowa State University (ISU) lake survey in 2000-01 (Downing and Ramstack 2001, 2002), (3) ISU report on lake phytoplankton communities (Downing et al. 2002), (4) information from the IDNR Fisheries Bureau, (5) results of the 1998 "Iowa Voluntary Atrazine Monitoring Program," and (6) results of fish tissue (RAFT) monitoring in 1995. EXPLANATION: Results of IDNR beach monitoring suggest "full support" of the Class A uses. Four beaches at West Lake Okoboji are monitored as part of the IDNR Beach monitoring program: Emerson Bay, Triboji, Gull Point, and Pikes Point beaches. Levels of indicator bacteria at Gull Point and Pikes Point beaches were monitored once per week during the primary contact recreation seasons (April through October) of 2000 and 2001; Emerson Bay beach and Triboji Beach were monitored weekly during the primary contact recreation season of 2001. According to U.S. EPA guidelines for determining support of primary contact recreation uses (U.S. EPA 1997b, page 3-35), the geometric mean of fecal coliform bacteria level from at least five samples collected over a 30-day period is compared to the water quality standard of 200 fecal organisms/100ml. If a 30-day geometric mean exceeds 200 orgs/100 ml, the primary contact recreation uses are assessed as "not supported." In addition, the U.S. EPA guidelines state that if more than 10% of the total samples taken during any 30-day period has a bacterial density that exceeds 400 fecal coliform organsims/100 ml, the primary contact recreation uses are assessed as "partially supported." Due to the relatively low numbers of samples collected during any 30-day period (N=5), the use of single-sample maximum values to assess beaches is problematic. With less than 10 samples collected during any 30-day period at Iowa beaches, the occurrence of a single level of bacteria above the single-sample maximum value will result in more than 10% violation of the single-sample maximum value and thus suggest impairment of the primary contact recreation uses. The use of less than 10 samples in an assessment based on a critical value of 10% results in large probabilities (approximately 60%) of incorrectly concluding that an impairment exists. For this reason, the single-sample maximum value is not used to assess support of primary contact recreation uses with data from the IDNR beach monitoring program. None of the 27 thirty-day periods during summers of 2000 and 2001 at either Gull Point or Pikes Point beaches had geometric means (N = 5 samples per period) greater than 200 orgs/100ml. At Pikes Point beach, the maximum 30-day geometric means were 13 orgs/100 ml in 2000 and 9 orgs/100ml in 2001. At Gull Point beach, the maximum 30-day geometric means were 11 orgs/100 ml in 2000 and 7 orgs/100ml in 2001. None of the 35 weekly samples collected at Gull Point beach during 2000 and 2001 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml. Only one of the 35 weekly samples collected at Pikes Point beach during 2000 and 2001 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml: the sample collected on May 22, 2000 contained 710 orgs/100 ml (the next highest level at this beach during the 2000-2001 period was 30 orgs/100 ml). None of the 13 thirty-day periods during summer 2001 at Emerson Bay or Triboji beaches had geometric means (N = 5 samples per period) greater than 200 orgs/100ml. Maximum 30-day geometric means were as follows: Emerson Bay: 16 orgs/100 ml; Triboji: 24 orgs/100ml. None of the 17 weekly samples collected at these three beaches in 2001 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml. Maximum weekly sample values are as follows: Emerson Bay: 110 orgs/100 ml; Triboji: 64 orgs/100ml. These results suggest "full support" of primary contact recreation uses at all four beaches. These results also suggest that levels of indicator bacteria at West Lake Okoboji are extremely low.
Despite the results of IDNR beach monitoring that suggest "full support" of the Class A uses, results from the ISU statewide survey of Iowa lakes suggest that levels of phosphorus may threaten full support of the Class A and Class B(LW) uses of West Lake Okoboji. 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 67, 44, and 34, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is in the middle range of mesotrophic lakes, and the index value for secchi depth is in the middle range of oligotrophic lakes. These index values suggest relatively high levels of phosphorus, extremely low (and much less than expected) levels of chlorophyll-a, and exceptionally good water transparency (the 2000-01 median secchi depth for this lake (5.9 meters) was the best of the 130 lakes sampled as part of the ISU survey). These results suggest that this lake does not have impairments to either the Class A or Class B(LW) uses due to aesthetically objectionable blooms of algae. These results do, however, suggest that levels of phosphorus are relatively high and thus present a threat to the continued full support of the Class A and Class B(LW) uses. 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 Lake West Okoboji is 9). This TN:TP ratio suggests the possibility that nitrogen limitation suppresses the production of suspended algae at this lake. Data from Downing et al. (2002) show moderately large populations of zooplankton species at this lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 10% of the dry mass of the zooplankton community in the mid-July sample and 30% of the early August sample. The levels of inorganic suspended solids at this lake are extremely low for Iowa lakes and do not suggest the potential for impairing designated uses. 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 West Lake Okoboji was 0.6 mg/l. This was the lowest median level of inorganic suspended solids of the 130 lakes sampled. This lake does not appear to have problems due to presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a relatively large portion of the summertime phytoplankton community of this lake. Sampling in 2000 showed that bluegreen algae comprised approximately 45% of wet mass of the phytoplankton community in the mid-June sample, over 95% in the mid-July sample, and comprised approximately 75% of the phytoplankton community in the early August sample. Although bluegreen algae comprise a significant portion of the summertime phytoplankton community, the production of chlorophyll is extremely low at this lake, and the presence of bluegreen algae likely does not present a water quality impact. 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 an 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. Drinking water uses are remain assessed as "fully supported" based on results of monitoring conducted in 1998 for the "Iowa Voluntary Atrazine Monitoring Program" (see above). Fish consumption uses remain assessed as "fully supported" based on results of the U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1995 (see assessment for the 1996 report). 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).
