Brushy Creek Lake IA 04-UDM-1276

Assessment Comments

Assessment is based on results of (1) IDNR beach monitoring program from 2002-04, (2) ISU lake survey data from 2000-04, and (3) ISU reports on lake plankton communities from 2000-05.

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 remain "not assessed."  The sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2002, 2003, and 2004, (2) results from ISU lake surveys from 2000 through 2004, and (3) ISU reports on plankton communities at Iowa lakes from 2000 through 2005.

EXPLANATION: Results of IDNR beach monitoring at Brushy Creek Lake from 2002 through 2004 suggest that the Class A uses are "fully supported".   Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2002 (28 samples), 2003 (28 samples), and 2004 (16 samples) as part of the IDNR beach monitoring program.   According to IDNR’s assessment methodology, two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A (primary contact recreation) uses:  (1) all five-sample, thirty-day geometric means for the three-year assessment period are less than the state’s geometric mean criterion of 126 E.  coli orgs/100 ml and (2) not more than 10 % of the samples during any one recreation season exceeds the state’s single-sample maximum value of 235 E.  coli orgs/100 ml.   This assessment approach is based on U.S.  EPA guidelines (see pgs 3-33 to 3-35of U.S.  EPA 1997b).  

At Brushy Creek Lake beach, the geometric means of all 60 thirty-day periods during the summer recreation seasons of 2002, 2003 and 2004 were below the Iowa water quality standard of 126 orgs/100 ml.   Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion did not significantly exceed 10% in any of the three recreation seasons (2002:  0%; 2003:  0%; 2004:  6%).    According to U.S.  EPA guidelines and IDNR’s assessment methodology, these results suggest “full support” of the Class A (primary contact recreation) uses.

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 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 63, 56, and 49, respectively.   According to Carlson (1977), the index value for total phosphorus is in the lower range between eutrophic and hyper-eutrophic lakes; the value for chlorophyll-a is in the middle range of eutrophic lakes, and the value for secchi depth is in the upper range of mesotrophic lakes.   These index values suggest very moderately high levels of phosphorus, relatively low levels of chlorophyll-a, and exceptional water transparency.  The 2000-04 medians for phosphorus, chlorophyll-a, and Secchi depth suggest very good water quality at this lake and do not suggest either a threat or impairment of designated uses.   While the ISU data continue to suggest very good water quality at this lake, TSI index values for total phosphorus and chlorophyll-a showed increases in the 2002-04 versus 2000-02 periods:  the TSI-TP index value increased from 51 to 63, and the TSI-Chlorophyll-a value increased from 47 to 56.   These increases suggest a potential shift from a mesotrophic lake (atypical for Iowa) to an eutrophic lake (typical for Iowa).   This shift may reflect a natural maturing of this relatively new impoundment.   Index values for Secchi depth, however, have remained “very good” to “exceptional” throughout the five-year monitoring period.

Based on median values from ISU sampling from 2000-02, the ratio of total nitrogen to total phosphorus for this lake is 220.   This very high TN:TP ratio reflects the often exceptionally high levels of total nitrogen in impoundments of the Des Moines Lobe region of north-central Iowa.   For example, Brushy Creek Lake had the second highest median summer total nitrogen over the 2000-04 period (13.1 mg/l) of the 131 lakes sampled for the ISU survey.  

Data from the ISU survey show moderately large populations of zooplankton species at this lake that graze on algae.   Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised only about 70% of the dry mass of the zooplankton community.   Brushy Creek Lake had the 59th highest per summer sample mass of zooplankton (Cladoceran) grazers of the 131 lakes sampled.   This level of zooplankton grazers may be able to suppress algal production.  The large population of phytoplankton grazers likely explain the moderate differences in the TSI values for phosphorus (63) relative to that for chlorophyll and (56).  

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 2004 was 5.2 mg/l; the median level at Brushy Creek Lake was 2.3 mg/l; this was the 16th lowest median level of the 131 lakes sampled.  

Nuisance aquatic (algal) species (i.e., bluegreen algae) do not appear to be a problem at Brushy Creek Lake.   While data from the ISU survey from 2000-04 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (approximately 45%) of the summertime phytoplankton community of this lake, the average per summer sample mass (biovolume) of bluegreen algae in summers of 2000 through 2004 at this lake (1.2 mg/l) was the 7th lowest of the 131 lakes sampled.   These conditions do not suggest any impairments or threats to uses at this lake due to presence of bluegreen algae.

The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected during summers of 2000 through 2004.   One of 15 samples (7%), however, violated the Class A,B(LW) criterion for pH (maximum = 9.3; minimum = 8.2 pH units).   This violation likely reflects the primary productivity at Brushy Creek Lake and does not reflect the input of pollutants into this lake.

Fish consumption remain "not assessed" due to the lack of fish contaminant monitoring at this lake