Assessment Comments
Assessment based on (1) results from the IDNR beach monitoring program in 2000 & 2001 and (2) results of surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "partially supporting" due to aesthetically objectionable blooms of algae and presence of nuisance algal species (bluegreen algae). The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened." Fish consumption uses are "not assessed." The sources of data for this assessment include (1) results of IDNR beach monitoring in 2000 and 2001, (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002). EXPLANATION: Results of IDNR beach monitoring suggest "full support" of the Class A uses. Levels of indicator bacteria at Viking 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 Viking Lake beach, none of the 27 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 30 orgs/100ml in 2000 and 35 orgs/100 ml in 2001. One of the 35 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 260 orgs/100 ml in 2000 and 3,400 orgs/100 ml in 2001. According to U.S. EPA and IDNR guidelines for Section 305(b) reporting, if the 30-day geometric means based on at least five samples are less than 200 orgs/100ml, the primary contact recreation uses are "fully supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Despite the result of DNR 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 algal turbidity and presence of nuisance algal species may adversely affect the Class A uses of Viking 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 73, 67, and 61, respectively. According to Carlson (1977), the index values for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index values for clorophyll-a and secchi depth are in the upper and lower range, respectively, between eutrophic and hyper-eutrophic lakes. These index values excessive levels of phosphorus in the water column, moderately high levels of chlorophyll-a (and suspended algae), and relatively good water transparency. Carlson (1991) suggests that a lake with a relatively high TSI for chlorophyll-a and a relatively low TSI for secchi depth may have large populations of algae such as Aphanizomenon that form flakes in the water column. 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, Viking Lake had a median level of inorganic suspended solids of 3.7 mg/l, thus suggesting that non-algal turbidity is relatively low and neither limits the production of algae nor contributes to impairments of primary contact recreation or aquatic life uses. The occurrence of high TSI values for total phosphorus and chlorophyll-a at Viking Lake indicates potential impairments to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise approximately half of the summertime phytoplankton community of Viking Lake. Although results of ISU lake monitoring in 2000 and 2001 suggest that nutrient-related impairments may exist, neither the amount of data available for characterizing water quality nor the magnitude of the TSI values are sufficient for identifying impairments with the level of confidence necessary to justify adding this lake to Iowa's Section 303(d) list. Thus, further investigation and monitoring are needed to (1) better characterize current water quality conditions and (2) verify that such impairments exist. The Class B(LW) aquatic life uses of this lake remain assessed (evaluated) as "fully supported / threatened" based on information from the DNR Fisheries Bureau. Fish consumption are "not assessed" due to the lack of fish contaminant monitoring at this lake.
