Assessment Comments
Assessment based on results of (1) surveys by IDNR Fisheries Bureau and (2) IDNR beach monitoring program in 2001 and 2002, (3) ISU statewide lake survey from 2000-02, (4) ISU plankton study in 2000, and (5) EPA/DNR fish tissue (RAFT) monitoring in 1995. Note: the assessment originally entered for the 2002 cycle was in error; the corrected assessment was entered in August 2004.]
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supporting" primarily due to moderately high levels of algal turbidity, and secondarily to non-algal turbidity, that create an aesthetically objectionable condition. Results of beach monitoring for indicator bacteria suggest “full support” of the Class A uses. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to nutrient loading to the water column, siltation, and due to algal blooms. The Class C (drinking water) uses remain “not assessed” due to the lack of water quality information upon which to base an assessment. Fish consumption uses remain assessed (evaluated) as “fully supported” based on fish contaminant monitoring in 1995. The sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summer of 2001 and 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, (4) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (5) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1995.
EXPLANATION: EXPLANATION: Levels of indicator bacteria at Green Valley Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2001 and 2002 as part of the IDNR beach monitoring program (this lake was drained for lake restoration work, and the beach area was not open during summer 2000). 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 Green Valley beach, none of the 35 thirty-day periods during summers of 2001 and 2002 had geometric means (N = 5 samples per period) greater than 200 orgs/100ml. The maximum thirty-day geometric mean was 5 orgs/100ml in 2001 and 16 orgs/100ml in 2002. None of the 44 weekly samples collected during summer 2001 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml. The single sample maximum value was 5 orgs/100 ml in 2001 and 60 orgs/100ml in 2002. 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 extremely low.
Despite results from IDNR beach monitoring that suggest “full support” of primary contact uses, results from the ISU statewide survey of Iowa lakes suggest that high levels of algal turbidity impair the Class A uses of Green Valley 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 75, 67, and 63, respectively. According to Carlson (1977), the index value for total phosphorus is in the range of hyper-eutrophic lakes, the index value for chlorophyll-a is in the mid to upper range between eutrophic and hyper-eutrophic lakes, and the index value for Secchi depth is in the lower range between eutrophic and hyper-eutrophic lakes. These index values suggest very high levels of phosphorus the water column, moderately high levels of chlorophyll-a (suspended algae), and marginally good water transparency for Iowa lakes. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low values for chlorophyll-a and secchi depth indicate that some factor (e.g., nitrogen limitation, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. Results of ISU monitoring from 2000-02 suggest that either large populations of zooplankton grazers or somewhat elevated levels of inorganic suspended solids may limit algal production at this lake. Based on median values from ISU sampling in 2000-02, the ratio of total nitrogen to total phosphorus for Green Valley Lake is 15, thus suggesting the potential for some nitrogen limitation at this lake. In addition, data from Downing et al. (2002) show that the zooplankton community of Green Valley Lake has an extremely large population of species known as algal grazers. Sampling at Green Castle Lake in 2000 showed that Cladocerans comprised nearly approximately 70% of the dry mass of the zooplankton community in both the mid-July and early August samples. The median summer mass of zooplankton grazers at this lake in 2000 (557 mg/l) was the highest, by far, of the 131 lakes sampled. This abnormally high level of Cladocerans likely reflects the pre-2000 lake renovation by the IDNR Fisheries Bureau and the lack of zooplankton predators. At these high levels, the Cladoceran grazers could be expected to limit algal production. The levels of inorganic suspended solids at this lake are moderately high relative to other Iowa lakes and suggest at the potential for contributing to problems with high levels of inorganic turbidity. Of 131 lakes sampled as part of the ISU survey from 2000-02, Green Valley Lake had the 56th highest median level of inorganic suspended solids (5.1 mg/l). Thus, based on results of ISU lake monitoring, the water quality conditions at Green Valley Lake indicate an impairment to the Class A (primary contact) uses through presence of algal populations and algal-related turbidity that violate Iowa’s narrative water quality standard protecting against “aesthetically objectionable conditions.” In addition, the levels of inorganic suspended solids are sufficiently high at this lake to threaten full support of these uses. The IDNR Fisheries Bureau concurs with this assessment.
The presence of nuisance algal species (i.e., bluegreen algae) does not appear to be a significant problem at Green Valley Lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise small portion the summertime phytoplankton community of this lake. Sampling in summer 2000 showed that less than 5% of the wet mass of the phytoplankton of this lake was in bluegreen algae. The 2000 average summer mass of bluegreen algae at this lake was less than 1 mg/l and ranked as the 4th lowest of the 131 lakes sampled.
The water quality conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses are assessed (evaluated) as "fully supported / threatened" due to excessive nutrient loading to the water column and due to large populations of algae. The Class C (drinking water) uses remained "not assessed" due to a lack of water quality information upon which to base an assessment. The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU survey from 2000-02 show that nitrate levels are low at this lake (maximum value = 4.7 mg/l; median = 0.6 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses. Fish consumption uses remain assessed as "fully supported" based on results of the U.S. EPA/IDNR fish tissue (RAFT) monitoring near in 1995 (see assessment developed for the 2000 report). However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively lower confidence).
