Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities in 2000, (4) results of U.S. EPA/IDNR fish tissue monitoring in 1996, and (5) a UHL drinking water study in 1995.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened" due to occasional severe blooms of algae in summer. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to relatively high nutrient loading to the water column and due to the potential for siltation impacts. The Class C (drinking water) uses remain assessed (evaluated) as "fully supported" based on results of a 1995 water quality study. Fish consumption uses remain assessed (evaluated) as "fully supported" based on fish contaminant monitoring in 1996. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), (4) results of U.S. EPA/IDNR fish tissue monitoring in 1996, and (5) results of a drinking water study conducted by UHL in January 1995.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes do not suggest any serious threats to full support of the Class A uses at Twelvemile Creek 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 64, 55, and 56, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the middle range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth are in the middle range of eutrophic lakes. These index values suggest somewhat elevated levels of phosphorus but suggest relatively low levels of chlorophyll-a and relatively good to very good water transparency. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively lower 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, however, do not immediately suggest non-phosphorus limitations on algal production at this lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Twelvemile Creek Lake is 21. This ratio suggests that algal production at this lake is likely not limited by nitrogen availability. The levels of inorganic suspended solids at this lake are relatively low and do not suggest a strong potential for impairing designated uses. 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; the median level at Twelvemile Creek Lake was 2.4 mg/l. Data from Downing et al. (2002) show only moderately large populations of zooplankton species at Twelvemile Lake that graze on algae. Sampling in 2000 showed that the summer 2000 average mass of Cladocerans (13.7 mg/l) was the 54th lowest of the 131 lakes and does not suggest a strong potential for zooplankton grazing to limit algal production.
Despite the indications from the ISU data of above average water quality and the lack of any threats to full support of the Class A uses at this lake, information from the IDNR Fisheries Bureau suggests that severe blooms of algae do, in fact, occur during warm periods of summer. This information suggests that the Class A uses should be assessed (evaluated) as “fully supported / threatened.”
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 in 2000, bluegreen algae (Cyanophyta) comprised less than 30% of the phytoplankton community during early and mid-summer but increased to approximately 60% of the wet mass in late summer. The summer 2000 average mass of bluegreen algae at this lake (3.0 mg/l) is very low relative to other Iowa lakes and does appear to either threaten or impair the designated uses as this lake.
The somewhat 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 relatively high nutrient loading to the water column and due to the potential for siltation impacts. The Class C (drinking water) uses remain assessed (evaluated) as "fully supported " based on results of sampling conducted by UHL in January 1995 (see assessment for the 1996 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). The only parameter collected as part of the ongoing 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 very low at this lake (maximum value = 2.0 mg/l; median = 0.5 mg/l), these data are not sufficient for developing an updated assessment of support of the Class C uses for Twelvemile Lake.
Fish consumption uses remain assessed (evaluated) as "fully supported" based on results of the U.S. EPA/IDNR fish tissue (RAFT) monitoring at Twelvemile Creek Lake in 1996 (see assessment developed for the 1998 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).
