Assessment Comments
Assessment based on (1) surveys by IDNR Fisheries Bureau and (2) results from the IDNR beach monitoring program in 2001. Note: the assessment originally entered for the 2002 cycle was in error; the corrected assessment was entered in August 2004.]
Basis for Assessment
[Note: In developing the 2002 assessment for Green Valley Lake, the TSI values for total phosphorus and chlorophyll-a from Green Castle Lake (59 and 53, respectively), were inadvertently used. This resulted an erroneous assessment that the Class A uses of Green Valley Lake were "fully supported / threatened." The correct TSI values for the 2000-01 period were 78 for total phosphorus and 67 for chlorophyll-a. The corrected assessment was entered into ADB+ in August 2004 and is presented below.]
For the 2002 report: SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "partially supporting." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened." Fish consumption uses are assessed as “fully supported.” The sources of data for this assessment include (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, (3) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002) and (4) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1995.
EXPLANATION: Levels of indicator bacteria at Green Valley Lake beach were monitored once per week during the primary contact recreation season (May through September) of 2001 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 10 thirty-day periods during summer of 2001 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. None of the 14 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. 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 may adversely affect the Class A uses of Green Valley 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 78, 67, and 65, respectively. According to Carlson (1977), the index value for total phosphorus is in the range of hyper-eutrophic lakes, the index values for chlorophyll-a and Secchi depth are in the mid to upper range between eutrophic and hyper-eutrophic lakes. These index values suggest excessive levels of phosphorus the water column, moderately high levels of chlorophyll-a (suspended algae), and only 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 or zooplankton grazing) limits production of algae
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 suggest that moderately high levels of inorganic turbidity and extremely large populations of zooplankton that graze of algae serve to limit algal production at Green Valley Lake. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Green Valley Lake is 14, thus suggesting a slight possibility for nitrogen limitation at this lake. The levels of inorganic suspended solids at this lake, however, are moderately high and do suggest the potential for both limiting algal production and contributing to non-algal turbidity problems at this lake. 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; the median level at Green Valley Lake was 7.0 mg/l; this is the 46th highest median value of the 130 lakes sampled. In addition, data from Downing et al. (2002) show relatively large populations of zooplankton species at Green Valley Lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 70% of the dry mass of the zooplankton community in the mid-July sample and 65% of the early August sample. These large populations of zooplankton grazers may limit, to some degree, production of algae at this lake.
The water quality conditions at Green Valley Lake indicate an impairment of the Class A (primary contact recreation) uses at Green Valley Lake. The high TSI values for chlorophyll-a and Secchi depth suggests that impairments due to high levels of turbidity related primarily to algae and secondarily to inorganic suspended solids (turbidity) that lead to aesthetically objectionable conditions.
The presence of nuisance algal species (i.e., bluegreen algae), however, does not appear to be a significant problem at this 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 eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported / threatened" due to high levels of non-algal turbidity and siltation in the lake. Fish consumption uses remain assessed as "fully supported" based on results of the U.S. EPA/IDNR fish tissue (RAFT) monitoring at Green Valley Lake in 1995. 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 low confidence).
