Pleasant Creek Lake IA 02-CED-459

Assessment Comments

Assessment is based on results of (1) IDNR beach monitoring program, (2) surveys of DNR Fisheries Bureau, (3) ISU lake survey in 2000-01, (4) ISU report on lake plankton communities, and (5) fish tissue monitoring.

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation) uses were assessed as "fully supporting."  The Class B(LW) aquatic life uses remained assessed as fully supporting."  Fish consumption uses remained assessed as "fully supporting."  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, and (5) results of fish tissue (RAFT) monitoring in 1999.   EXPLANATION:  Results of IDNR beach monitoring in 2000 and 2001 suggest "full support" of the Class A uses at this lake.   Levels of indicator bacteria at Pleasant Creek Lake Beach were monitored once per week during the primary contact recreation seasons (April through October) 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 Pleasant Creek 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 13 orgs/100ml in 2000 and 31 orgs/100 ml in 2001.   None of the 35 weekly samples collected during 2000 and 2001 exceeded the U.S.  EPA's recommended single-sample maximum value of 400 orgs/100 ml.   These results suggest full support of primary contact recreation uses at this beach.  

Results from the ISU statewide survey of Iowa lakes also suggest "full support" of the Class A uses, as well as the Class B(LW) uses, at Pleasant Creek 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 53, 51, and 48, respectively.   According to Carlson (1977), the index values for total phosphorus and chlorophyll-a places this lake in the lower range of eutrophic lakes; the index value for secchi depth is in the upper range of mesotrophic lakes.   These index values suggest very low levels of phosphorus and chlorophyll-a and exceptional water transparency for an Iowa lake.   These results suggest that this lake does not have impairments due to aesthetically objectionable blooms of algae.   Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Pleasant Creek Lake is 26.8).   This ratio and the agreement between the TSI values suggest that algal production at this lake is limited by phosphorus.   Data from Downing et al.  (2002) show very small populations of zooplankton species at this lake that graze on algae.   Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately less than 5% of the dry mass of the zooplankton community in the early August and mid-September samples.   These results suggest that zooplankton grazers do not influence water transparency.   The levels of inorganic suspended solids at this lake are 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 Pleasant Creek Lake was 2.1 mg/l.   This median level is the 13th lowest 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) tend to dominate the summertime phytoplankton community of this lake, especially in mid and late summer.   Sampling in 2000 showed that bluegreen algae comprised less than 10% of the wet mass of the phytoplankton community in early June but increased to over 70% of the community wet mass in the early August and early September samplings.   Although bluegreen algae appear to dominate the late summertime phytoplankton community, the production of chlorophyll is very 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.   Fish consumption remain assessed as "fully supported" based on results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring in August 1999 (see assessment for the 2000 report above).