Blue Lake IA 06-WEM-1728

Assessment Comments

Assessment is based on results of (1) IDNR beach monitoring program from 2000-02, (2) ISU lake survey from 2000-02, (3) ISU report on lake phytoplankton communities in 2000, 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 "not supporting" due to aesthetically objectionable conditions related primarily to high levels of inorganic turbidity and secondarily to blooms of suspended algae.   Results of IDNR beach monitoring show full support of the primary contact recreation uses during the 2000-02 period.   The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened."  Fish consumption uses remain "not assessed" (no cause/source given).   The sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2000, 2001, and 2002, (2) results from ISU lake surveys in 2000-02, (3) ISU report on plankton communities at Iowa lakes in 2000 (Downing et al.  2002), 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, 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 Blue Lake beach, none of the 47 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 26 orgs/100ml in 2000, 14 orgs/100 ml in 2001, and 40 orgs/100 ml in 2002.   None of the 63 weekly samples collected during 2000-02 period exceeded the U.S.  EPA's recommended single-sample maximum value of 400 orgs/100 ml.  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 beach monitoring that suggest full support of the Class A uses, results from the ISU statewide survey of Iowa lakes suggest that high levels of turbidity related to algal production and inorganic suspended solids impair the Class A uses of Blue 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 69, 65, and 71, respectively.   According to Carlson (1977), the index values for total phosphorus and chlorophyll-a place this lake in the range between eutrophic and hyper-eutrophic lakes; the index value for Secchi depth places this lake in the lower range of hyper-eutrophic lakes.   These index values suggests moderately high levels of phosphorus in the water column, moderately high levels chlorophyll-a, and very poor water transparency.   According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation.   The ISU lake data suggest that non-algal particles do likely limit algal production at Blue Lake.   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.   Of 131 lakes sampled, Blue Lake had the ninth highest median level of inorganic suspended solids (21.0 mg/l), thus suggesting that non-algal turbidity limits the production of algae.   These conditions suggest an suggest impairments to the Class A (primary contact) uses primarily due to presence high levels of inorganic turbidity that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions.   Blue Lake also had the 29th highest TSI value for chlorophyll-a, thus suggesting that suspended algae also contribute to aesthetically objectionable conditions at Blue Lake.   The IDNR Fisheries Bureau concurs that turbidity-related impairments exist at this lake.   Other factors may limit algal production at Blue lake.   Based on median values from ISU sampling from 2000-02, the ratio of total nitrogen to total phosphorus for this lake is 12; this ratio suggests the possibility that algal production at this lake is limited by nitrogen availability.   In addition, data from Downing et al.  (2002) show very large zooplankton populations at this lake, including a large population (approximately 78% dry mass) of species known as algal grazers; the median summer mass of zooplankton grazers at this lake in 2000 (132 mg/l) was the 8th highest of the 131 lakes sampled.

Nuisance aquatic (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.   The average mass of bluegreen algae in summer 2000 at this lake (2.4 mg/l) was the 24th lowest of the 131 lakes sampled.   These conditions do not suggest any impairments due to presence of nuisance aquatic (e.g., algal) species.   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).