Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau, (3) ISU reports on lake plankton communities from 2000-05, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1996.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting". Occasional severe blooms of algae in summer, however, present a concern for Class A uses at this lake. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting". Relatively high nutrient loading to the water column and due to the potential for siltation impacts, however, remain water quality concerns at this lake. The Class C (drinking water) are “not assessed” due to the lack of recent information upon which to base an assessment. 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 sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (4) results of U.S. EPA/IDNR fish contaminant monitoring in 1996.
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 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 60, 56, and 57, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range of 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 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 38. 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 very 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 2004 was 5.2 mg/l; the median level at Twelvemile Creek Lake was 2.7 mg/l.
In terms of all Iowa lakes sampled, data from the ISU survey show relatively small populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 50% of the dry mass of the zooplankton community of this lake. The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (61 mg/l) was the 46th lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Twelvemile Lake.
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 algal blooms present a water quality concern to support of the Class A uses of this lake.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (approximately 80%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Twelvemile Lake (12.4 mg/l) was the 68th lowest of the 131 lakes sampled. This level is near the median level for the 131 Iowa lakes sampled (=12.2 mg/l). The presence of a relatively small population of bluegreen algae at this lake does not suggest a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes. No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed (evaluated) as "fully supported". Water quality concerns at this lake, however, include relatively high nutrient loading to the water column and the potential for siltation impacts. Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. Results of this monitoring show that only one of the 14 samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that only two of the 15 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 9.3; minimum = 7.6 pH units). Based on IDNR’s assessment methodology, these results do not suggest violation frequencies that are significantly greater than 10% for either parameter and thus do not suggest impairment of either the Class A and Class B(LW) uses of Twelvemile Creek Lake. The violations of the pH criterion likely reflect high levels of primary productivity at this lake and do not reflect the input of pollutants.
The Class C (drinking water) uses are “not assessed” due to the lack of recent information upon which to base an assessment. In previous 305(b) assessment cycles, the Class C uses were assessed as "fully supported" based on results of sampling conducted by UHL in January 1995 (see assessment for the 1996 and 1998 reports). However, these data are now considered too old (greater than five years) to characterize current water quality conditions. 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, N=15), these data are not sufficient for developing an updated assessment of support of the Class C uses for Twelvemile Creek Lake.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Twelvemile Creek Lake in 1996. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes. Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa. In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health, in cooperation with Iowa DNR, developed a risk-based advisory protocol. This protocol went into effect in January 2006 (see http://www.iowadnr.gov/fish/news/consump.html for more information on Iowa’s revised fish consumption advisory protocol). Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest either a threat to, or impairment of, fish consumption uses. This scenario, however, does not apply to the fish contaminant data generated from the 1996 RAFT sampling conducted at Twelvemile Creek Lake: levels of all contaminants from this monitoring were below advisory trigger levels, thus suggesting the continued “full support” of fish consumption uses.
