Pleasant Creek Lake IA 02-CED-459

Assessment Comments

Assessment is based on results of (1) IDNR beach monitoring program from 2000-02, (2) surveys of DNR Fisheries Bureau, (3) ISU lake survey from 2000-02, (4) ISU report on lake plankton communities, and (5) fish tissue monitoring in 1999.

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" based on results of fish contaminant monitoring in 1999.   The sources of data for this assessment include (1) results of the IDNR-UHL beach monitoring program in summers of 2000-02, (2) results of the Iowa State University (ISU) lake survey from 2000-2002, (3) ISU report on lake plankton communities (Downing et al.  2002), (4) information from the IDNR Fisheries Bureau, and (5) results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring in 1999.  

EXPLANATION:  Results of IDNR beach monitoring from 2000 through 2002 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, 2001, and 2002 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 52 thirty-day periods during summers of 2000, 2001, and 2002 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, 31 orgs/100 ml in 2001, and 17 orgs/100 ml in 2002.   None of the 64 weekly samples collected from 2000 through 2002 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 and that levels of indicator bacteria at this beach are extremely low.  

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 from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 55, 49, and 46, respectively.   According to Carlson (1977), the index value for total phosphorus places this lake in the middle range of eutrophic lakes; the value for chlorophyll-a is in the upper range of mesotrophic lakes, and the index value for secchi depth is in the middle range of mesotrophic lakes.   These index values suggest very low levels of phosphorus, extremely low levels of chlorophyll-a, and exceptional water transparency for an Iowa lake.   These results suggest that this lake does not have threats or impairments due to aesthetically objectionable conditions.  

Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Pleasant Creek Lake is 26.   This ratio suggests 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.   The summer 2000 average mass of Cladocerans at Pleasant Creek Lake was very low (0.4 mg/l) and was the fourth lowest of the 131 lakes sampled.   These results suggest that zooplankton grazers do not influence algal production.   The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential attenuating light penetration or for threatening or 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 Pleasant Creek Lake was 2.2 mg/l.   This median level is the 13th lowest of the 131 lakes sampled.   Thus, the very low nutrient levels, combined with extremely low chlorophyll concentrations and exceptional water clarity suggests that this lake not only fully supports its Class A uses but has some of the best water quality of any impoundment in Iowa.  

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.   However, the 2000 average summer mass of bluegreen algae (9.2 mg/l) was the 63rd lowest of the 131 lakes sampled.   Although this median is somewhat elevated relative to that from other Iowa lakes, this level does not likely represent an impact to water quality at this lake.  Fish consumption remain assessed (monitored) as "fully supported" based on results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring in August 1999 (see assessment for the 2000 report).