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 1997.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supported" due to moderately poor water transparency caused primarily by inorganic turbidity that indicates an aesthetically objectionable condition. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to siltation impacts at this lake. Fish consumption uses remain assessed (evaluated) as "fully supported" based on U.S. EPA/IDNR fish contaminant monitoring in 1997. 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 tissue monitoring in 1997.
Note: A TMDL for nutrients and siltation at Easter Lake was prepared by IDNR and approved by EPA in 2005. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle (siltation and turbidity) are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that moderately poor water transparency impairs the Class A uses of Easter Lake; the somewhat elevated levels of chlorophyll-a are also of concern at this lake but do not appear to impair the Class A uses. 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 62, 62, and 66, respectively. According to Carlson (1977), the index value for total phosphorus places this lake at the lower range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth place this lake in the range between eutrophic and hyper-eutrophic lakes. These index values suggest moderately high levels of phosphorus in the water column, relatively low levels of chlorophyll-a, and moderately poor water transparency.
Results of ISU monitoring do not suggest that nitrogen-availability limits algal production. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Easter Lake is 20. This TN:TP ratio suggests that phosphorus is the nutrient that likely limits algal production at this lake.
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. Although sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised over 40% 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 (107 mg/l) was only the 54th highest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Easter Lake.
The levels of inorganic suspended solids at this lake are typical for Iowa lakes and suggest at least some potential for contributing to in-lake turbidity. 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 Easter Lake was 6.5 mg/l. This level of ISS, the 49th highest of the 131 lakes sampled, is moderately high and may lead to increases in-lake turbidity that may contribute to reductions in water transparency.
Thus, the moderately high TSI for Secchi depth (66) suggests an impairment of the Class A (primary contact) uses through reductions in water transparency caused primarily by inorganic suspended solids and secondarily by suspended algae.
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 (over 70%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Easter Lake (10.8 mg/l) was the 59th lowest of the 131 lakes sampled. This levels is in the lower half of the 131 Iowa lakes sampled. 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).
The eutrophic conditions at this lake, including high nutrient levels and moderate levels of inorganic turbidity, suggest that the Class B(LW) aquatic life uses should be assessed (evaluated) as "partially supported." Similarly, information from the IDNR Fisheries Bureau, suggests that siltation remains a significant problem at this lake and that the Class B(LW) uses should thus be assessed (evaluated) as "partially supported." A comparison of annual TSI values for total phosphorus, chlorophyll-a, and Secchi depth does not suggest any trends, improving or adverse, at Easter Lake.
The ISU lake survey data show generally good chemical water quality at Easter Lake. Results of this monitoring show that only 1 of the 14 samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that only 2 of the 15 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 9.1; minimum = 8.1 pH units). Based on IDNR’s assessment methodology, these results do not suggest violation frequencies 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 Easter Lake. The violations of the pH criterion likely reflect primary productivity at Easter Lake and do not reflect the input of pollutants into this lake.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Easter Lake in 1997. 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 1997 RAFT sampling conducted at this lake: levels of all contaminants from this monitoring were below advisory trigger levels, thus suggesting the continued “full support” of fish consumption uses.
