Brushy Creek Lake IA 04-UDM-1276

Assessment Comments

Assessment is based on results of (1) IDNR beach monitoring program from 2000-02, (2) ISU lake survey from 2000-02, and (3) the ISU report on lake phytoplankton communities in 2000.

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 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 2000-02, (2) results of the Iowa State University (ISU) lake survey in 2000-02, and (3) ISU report on lake plankton communities (Downing et al.  2002).  

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 Brushy Creek Lake beach were monitored once per week during the primary contact recreation seasons (May through September) 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 thirty-day period is compared to the water quality standard of 200 fecal organisms/100ml.   If a thirty-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 thirty-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 thirty-day period (N=5), the use of single-sample maximum values to assess beaches is problematic.   With less than 10 samples collected during any thirty-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 50 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 thirty-day geometric means were 8 orgs/100ml in 2000, 9 orgs/100 ml in 2001, and 12 orgs/100 ml in 2002.   None of the 62 weekly samples collected during 2000, 2001, and 2002 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, 20 orgs/100 ml in 2001, and 50 orgs/100 ml in 2002.   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 from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 51, 47, and 49, respectively.   According to Carlson (1977), the index value for total phosphorus is in the lower range of eutrophic lakes; the values for chlorophyll-a and secchi depth are in the upper range of mesotrophic lakes.   These index values suggest very low levels of phosphorus, extremely low levels of chlorophyll-a, and exceptional water transparency.  The 2000-02 medians for phosphorus, chlorophyll-a, and Secchi depth suggest excellent water quality at this lake and do not suggest either a threat or impairment of designated uses.   Based on median values from ISU sampling from 2000-02, the ratio of total nitrogen to total phosphorus for this lake is 454.   This TN:TP ratio is extremely high and reflects (1) the very low level of total phosphorus (2000-02 median = 26 ug/l) and (2) the very high level of total nitrogen (2000-02 median = 11.8 mg/l (3rd highest of the 131 lakes sampled) at this lake.   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 (grazer) 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.   The summer 2000 average mass of zooplankton grazers at Brushy Creek Lake (23.8 mg/l) was the 52nd highest of the 131 lakes sampled.   The large population of phytoplankton grazers likely explain the slight differences in the TSI values for phosphorus (51) relative to that for chlorophyll and (47).   The levels of inorganic suspended solids at this lake are very low 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 Brushy Creek Lake was 2.2 mg/l; this was the 12th lowest median level of the 131 lakes sampled.  

Data from Downing et al.  (2002) suggest that bluegreen algae (Cyanophyta)--while at times, comprising a significant portion of the summertime phytoplankton community of this lake, does not present a water quality impact to designated uses at this lake.   Sampling in 2000 showed that while bluegreen algae comprised less than 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.   The average summer 2000 mass of bluegreen algae at this lake (0.6 mg/l), however, was the 6th lowest of the 131 lakes sampled.   Thus, the levels of bluegreen algae at Brushy Creek lake extremely low and do not present either a threat or impairment to designated uses.   Fish consumption are "not assessed" due to the lack of fish contaminant monitoring at this lake.