West Okoboji Lake - Main Basin South IA 06-LSR-1653

Assessment Comments

Assessment is based on results of (1) IDNR beach monitoring from 2000-02, (2) ISU lake survey from 2000-02, (3) surveys by IDNR Fisheries Bureau, (4) ISU report on lake plankton communities in 2000, (5) results of U.S. EPA/IDNR fish tissue monitoring in 1995, (6) UHL drinking water study in 1995, and (7) results of atrazine monitoring in 1998.

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting."  The Class B(LW) aquatic life uses are also assessed (evaluated) as "fully supporting.”  The Class C drinking water uses remain assessed (monitored) as "fully supported."  Fish consumption uses remain assessed (evaluated) as "fully supported" based on results of fish contaminant monitoring in 1995.   The sources of data for this assessment include (1) results of the IDNR-UHL beach monitoring program in summers of 2000 through 2002, (2) results of the Iowa State University (ISU) lake survey from 2000 through 2002, (3) ISU report on lake plankton communities (Downing et al.  2002), (4) information from the IDNR Fisheries Bureau, (5) results of the 1998 "Iowa Voluntary Atrazine Monitoring Program," (6) results of a UHL drinking water study in 1995, and (7) results of U.S.  EPA/IDNR 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) from 2000-02; Emerson Bay beach and Triboji Beach were monitored weekly during the primary contact recreation seasons of 2001 and 2002.   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 51 thirty-day periods during summers from 2000-02 at Gull Point Beach, and none of the 52 thirty day periods at Pikes Point Beach, 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, 9 orgs/100ml in 2001, and 78 orgs/100 ml in 2002.   At Gull Point beach, the maximum 30-day geometric means were 11 orgs/100 ml in 2000, 7 orgs/100ml in 2001, and 43 orgs/100 ml in 2002.   None of the 63 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 three of the 64 weekly samples collected at Pikes Point beach during 2000, 2001, and 2002 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); two samples collected on August 5, 2002 exceeded this single sample maximum value (410 and 600 orgs/100 ml).   None of the approximately 37 thirty-day periods during summers of 2001 and 2002 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 in 2001 and 102 orgs/100 ml in 2002; Triboji:  24 orgs/100ml in 2001 and 72 orgs/100 ml.  in 2002.   None of the 45 weekly samples collected at Triboji Beach during this period exceeded the U.S.  EPA’s single-sample maximum value of 400 orgs/100 ml.   Two of the 44 weekly samples collected at Emerson Bay Beach during this period exceeded the U.S.  EPA's recommended single-sample maximum value of 400 orgs/100 ml:  the sample collected on June 11, 2002 contained 1,000 orgs/100 ml; the sample collected on August 5, 2002 contained 500 orgs/100 ml.    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 very low.  

Results from the ISU statewide survey of Iowa lakes also suggest “full support” of the Class A uses designated for West 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 53, 42, and 34, respectively.   According to Carlson (1977), the index value for total phosphorus places this lake in the lower range of eutrophic lakes; the index value for chlorophyll-a is in the lower range of mesotrophic lakes, and the index value for secchi depth is in the middle range of oligotrophic lakes.   These index values suggest very low levels of phosphorus, extremely low (and less than expected) levels of chlorophyll-a, and exceptionally good water transparency (the 2000-02 median secchi depth for this lake (5.6 meters) was the best of the 131 lakes sampled as part of the ISU survey).   These results suggest that this lake does not have threats or impairments to the Class A uses due to either high levels of chlorophyll or to inorganic turbidity.   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, zooplankton grazing, or some other factor) other than phosphorus limits production of algae.   The results of ISU monitoring from 2000-02 do not immediately suggest a non-phosphorus limitation that would account for the low chlorophyll-a concentration relative to that for total phosphorus.   Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Lake West Okoboji is 25; this ratio suggests that nitrogen availability does not limit algal production at this lake (note:  the TN:TP ratio for the 2000-01 period was 9; this suggests that, at times, algal production at Lake West Okoboji may be nitrogen-limited).   Data from Downing et al.  (2002) show moderately large populations of zooplankton species (Cladocerans) 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 summer 2000 mass of Cladocerans (0.18 mg/l), however, was the fourth lowest of the 131 lakes sampled and thus does not suggest the potential for zooplankton grazers to limit algal production.   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 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 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 131 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.   However, the summer 2000 average mass of bluegreen algae (2.7 mg/l) was the 26th lowest of the 131 lakes sampled and thus does not suggest either a threat or impairment to the lake’s designated uses due to nuisance aquatic life.  

Drinking water uses are remain assessed (monitored) 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).   Results of drinking water study conducted by UHL in January 1995 also suggest full support of the drinking water uses (see assessment for the 1996 and 1998 reports).  

Fish consumption uses remain assessed (evaluated) 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).   This lake has been scheduled for fish contaminant monitoring as part of the 2004 U.S.  EPA/IDNR RAFT monitoring program.