Brushy Creek Lake IA 04-UDM-1276

Assessment Comments

Assessment is based on (1) results from IDNR beach monitoring program in 2000-01, (2) ISU lake survey in 2000-01, and (3) ISU report on lake phytoplankton communities.

Basis for Assessment

Note:  Not assessed for previous Section 305(b) reports.   Brushy Creek lake was constructed in 1998 and had completely filled by summer 1999.   This 690-acre lake has a mean depth of 29 feet and a maximum depth of 80 feet.

SUMMARY:  The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting."  The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting."  Fish consumption uses are "not assessed."  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), and (3) ISU report on lake phytoplankton communities (Downing et al.  2002).   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 Brushy Creek 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 Brushy Creek Lake beach, none of the 26 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 8 orgs/100ml in 2000 and 9 orgs/100 ml in 2001.   None of the 34 weekly samples collected during 2000 and 2001 exceeded the U.S.  EPA's recommended single-sample maximum value of 400 orgs/100 ml.   The single sample maximum values were extremely low:  10 orgs/100 ml in 2000 and 20 orgs/100 ml in 2001.   These geometric means and seasonal maximum values are some of the lowest reported for the IDNR beach monitoring program.   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.  

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 Brushy 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 57, 51, and 51, respectively.   According to Carlson (1977), the index value for total phosphorus is in the mid to upper range of eutrophic lakes; the values for chlorophyll-a and secchi depth are in the lower range of eutrophic lakes.   These index values suggest relatively low levels of phosphorus, very low levels of chlorophyll-a, and very good water transparency.   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) other than phosphorus limits production of algae.   Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 278.   This TN:TP ratio is extremely high and suggests that (1) algal production is limited by phosphorus and (2) levels of total nitrogen at this lake are extremely high.   Data from Downing et al.  (2002) suggest large populations of zooplankton species at Brushy Creek Lake that graze on algae.   Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised 100% of the dry mass of the zooplankton community in the mid-July sample and approximately 75%  in the early August sample.   These large populations of phytoplankton grazers likely explain the differences in the TSI values for phosphorus relative to those for chlorophyll and secchi depth.   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 Brushy Creek Lake was 1.7 mg/l; this is the 9th lowest median level of the 130 lakes sampled.   Data from Downing et al.  (2002) suggest that bluegreen algae (Cyanophyta) can, at times, comprise a significant portion of the summertime phytoplankton community of this lake.   Sampling in 2000 showed that while bluegreen algae were comprised less that 5% of the wet mass of the phytoplankton community in mid-late June, approximately 65% of the wet mass in the mid-July sample was bluegreen algae.   Approximately 20% of the wet mass of the early August sample was comprised of bluegreen algae.   These values, however, given the very low levels of chlorophyll-a (i.e., suspended algae) at this lake, do not suggest significant problems with bluegreen algae.   Although results of ISU lake monitoring in 2000 and 2001 suggest 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 are "not assessed" due to the lack of fish contaminant monitoring at this lake.