Blue Lake IA 06-WEM-1728

Assessment Comments

Assessment based on results of (1) IDNR beach monitoring program, (2) ISU lake survey in 2000-01, (3) ISU report on lake phytoplankton communities, and (4) information from IDNR Fisheries Bureau and Wildlife Bureau.

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation) uses are assessed (monitored) as "partially supporting" due to aesthetically objectionable algal blooms and aquatic nuisance species.   The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened."  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 2000 and 2001, (2) results from ISU lake surveys in 2000-01, (3) ISU report on lake phytoplankton communities, and (4) information from the IDNR Fisheries Bureau.   EXPLANATION:  Results of IDNR beach monitoring suggest full support of the Class A (primary contact recreation) uses.   Levels of indicator bacteria at Blue 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 Blue Lake beach, none of the 22 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 26 orgs/100ml in 2000 and 14 orgs/100 ml in 2001.   None of the 32 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 very low:  80 orgs/100 ml in 2000 and 40 orgs/100 ml in 2001.   These are some of the lower seasonal levels of indicator bacteria 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 very low.   Despite the results of IDNR baech monitoring that suggest full support of the Class A uses, results from the ISU statewide survey of Iowa lakes suggest that high levels of algal turbidity may adversely affect the Class A uses of Blue 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 71, 67, and 71, respectively.   According to Carlson (1977), the index values for total phosphorus and secchi depth place this lake in the lower range of hyper-eutrophic lakes; the index value for chlorophyll-a places this lake in the middle range between eutrophic and hyper-eutrophic lakes.   These index values suggests high levels of phosphorus in the water column, moderately high levels chlorophyll-a leading to nuisance blooms of algae, and poor water transparency.   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.   Of 130 lakes sampled, Blue Lake had the 12th highest median level of inorganic suspended solids (19.9 mg/l), thus suggesting that non-algal turbidity limits the production of algae as well as contributes to impairments of both the primary contact recreation and aquatic life uses.   These conditions indicate potential impairments to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae.   Nuisance algal species (i.e., bluegreen algae) do not appear to be a problem at Blue Lake.   Data from Downing et al.  (2002) suggest that bluegreen algae (Cyanophyta) comprise a relatively small portion (approximately 10%) of the summertime phytoplankton community of this lake.   Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 11.   The Class B(LW) aquatic life uses of this lake are assessed (evaluated) as "fully supported / threatened " based on information from the DNR Fisheries Bureau and DNR Wildlife Bureau.   Fish consumption remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake (see results for the 2000 report).