Lacey Keosauqua Lake IA 04-LDM-1008

Assessment Comments

Assessment based on results of (1) IDNR beach monitoring program in 2000-01, (2) ISU lake survey in 2000-01, (3) ISU report on lake phytoplankton communities, and (4) surveys by IDNR Fisheries Bureau. Dam failure occurred in May 2002.

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 are assessed (evaluated) as "fully supporting / threatened" (minor impacts).   The Class C (drinking water) uses remain "not assessed."  Fish consumption uses remain "not assessed."  The sources of data for this assessment include (1) results of the IDNR-UHL beach monitoring program in summers of 2000 and 2001, (2) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, and (4) information on plankton communities at Iowa lakes in 2000 from Downing et al.  (2002).  

EXPLANATION:  Results of IDNR beach monitoring in 2000 and 2001 suggest "full support" of the Class A uses.   Levels of indicator bacteria at Lacey Keosauqua Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2000 and 2001 as part of the IDNR beach monitoring program.   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.   At Lacey Keosauqua Lake beach, none of the 27 thirty-day periods during summers of 2000 and 2001 had geometric means (N = 5 samples per period) greater than 200 orgs/100ml.   The maximum 30-day geometric means were 14 orgs/100ml in 2000 and 13 orgs/100 ml in 2001.   None of the weekly samples collected during 2000 and 2001 exceeded the U.S.  EPA's recommended single-sample maximum value of 400 orgs/100 ml (maximum sample values were 50 orgs/100 ml in 2000 and 240 orgs/100 ml in 2001).   These results suggest (1) full support of primary contact recreation uses at this beach and (2) that levels of bacteria at this beach 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 uses of Lacey Keosauqua 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 66, 43, and 50, respectively.   According to Carlson (1977), the index value for total phosphorus places this lake in the middle range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is in the middle to lower range of mesotrophic lakes, and the index value for secchi depth is at the boundary between eutrophic and mesotrophic lakes.   These index values suggest relatively high levels of phosphorus, extremely low (and much lower than expected) levels of chlorophyll-a, and very good water transparency for Iowa lakes.   These results suggest that this lake does not have impairments due to aesthetically objectionable blooms of algae.   The relatively high levels of phosphorus in the water column, however, present a potential 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 Lacey Keosauqua Park Lake is 10.   This TN:TP ratio suggests the possibility that nitrogen limitation suppresses the production of suspended algae at this lake.   The relatively high proportion of Daphnia in the zooplankton population of this lake suggest the potential for grazing to reduce populations of suspended algae and thus levels of chlorophyll-a.   Sampling by Downing et al.  (2002) in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 45% of the dry mass of the zooplankton community in the late July sample and approximately 90% of the mid-August sample.   The levels of inorganic suspended solids at this lake are relatively low 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 Lacey Keosauqua Park Lake was 3.8 mg/l.   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 small portion of the summertime phytoplankton community of this lake.   Sampling in 2000 showed that bluegreen algae comprised approximately 15% of the wet mass of the phytoplankton in the late June and mid August samples but increased to nearly 50% of the mass weight in the late July sample.   Considering the relatively low levels of chlorophyll-a in this lake and the relatively small contribution of bluegreen algae to the phytoplankton community, 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.   The Class C (drinking water) uses remained "not assessed" due to a lack of water quality information upon which to base an assessment.   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.   Previous Section 305(b) assessments contain conflicting information:  previous assessment suggest that the lake has problems with non-algal turbidity and that the fishery is relatively poor.   Data from the ISU lake survey in 2000 and 2001, however, suggest water quality at this lake--in terms of nutrient levels and water transparency--that is well above average for Iowa lakes.   Follow-up monitoring is needed to better determine the status of the aquatic life uses at this lake.    

[NOTE:  A dam failure occurred at this lake in May 2002.]