Assessment Comments
Assessment based on results of (1) IDNR beach monitoring program from 2000-02, (2) ISU lake survey from 2000-02, (3) ISU study of lake plankton communities in 2000, and (4) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supporting" due to presence of aesthetically objectionable conditions related primarily to blooms of algae and secondarily to the somewhat elevated levels of inorganic turbidity. The presence of nuisance algal species (bluegreen algae) presents an additional threat to full support of the Class A uses. Results of IDNR beach monitoring, however, show full support of the Class A uses. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened." Fish consumption uses are "not assessed" due to the lack of fish contaminant monitoring at this lake. The sources of data for this assessment include (1) results of IDNR beach monitoring from 2000 through 2002, (2) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, and (4) information on plankton 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) from 2000-02 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 52 thirty-day periods during summers from 2000-02 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, 35 orgs/100 ml in 2001, and 70 orgs/100 ml in 2002. Two of the 64 weekly samples collected from 2000-02 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml. 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 from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 66, 68, and 62, respectively. According to Carlson (1977), all three index values are in the range between eutrophic and hyper-eutrophic lakes. These index values indicate moderately high levels of phosphorus in the water column, moderately high levels of chlorophyll-a (and suspended algae), and marginally 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 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l. Of 131 lakes sampled, Viking Lake had a median level of inorganic suspended solids of 3.8 mg/l, thus suggesting that non-algal turbidity (1) is somewhat elevated and may thus limit the production of algae to some degree and (2) possibly contributes to impairments of primary contact recreation or aquatic life uses.
Thus, the occurrence of a moderately high TSI value for chlorophyll-a and the occurrence of only marginally good water transparency at Viking Lake indicates impairments to the Class A (primary contact) uses due primarily to blooms of algae that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The IDNR Fisheries Bureau concurs that Viking Lake is impaired by excess algae.
In addition, the presence of moderately large populations of nuisance algal species (i.e., bluegreen algae) at this lake suggests a threat to full support of the Class A uses. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise approximately half of the summertime phytoplankton community of Viking Lake. The summer 2000 average mass of bluegreens at this lake (27.9 mg/l) was the 35th highest of the 131 lakes sampled, and suggests the potential for impacts due to nuisance aquatic life. Although results of plankton sampling in 2000 suggest that designated uses are threatened by presence of bluegreen algae, the amount of data available for characterizing algal populations at this lake (one season) is not sufficient for developing a more accurate assessment of support of these uses. Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to improve the accuracy of future water quality assessments.
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.
