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 1994.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting". Siltation impacts at this lake, however, remain a concern. Fish consumption uses are assessed (evaluated) as "fully supported" based on results of fish contaminant monitoring in 1994. 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 1994.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that the Class A uses of Mormon Trail Lake are "fully supported." 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 54, 46, and 52, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the middle range of eutrophic lakes; the index value for chlorophyll-a is in the middle range of mesotrophic lakes, and the index value for secchi depth is in the lower range of eutrophic lakes. These index values suggest excellent water quality for an Iowa lake: very low levels of phosphorus, extremely low levels of chlorophyll, and very good water transparency. These results suggest that this lake does not have either threats or impairments due to aesthetically conditions caused by objectionable blooms of algae or by high levels of inorganic turbidity.
According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data suggest that non-algal particles may limit algal production at Mormon Trail Lake; however, neither N-limitation nor zooplankton grazing appear to limit algal production. The levels of inorganic suspended solids at this lake are relatively low and, while not sufficiently high to either threaten or impair designated uses, may attenuate light and limit algal production to some degree. 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 Mormon Trail Lake was 3.1 mg/l. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Mormon Trail Lake is 32. This ratio suggests that algal production at this lake is likely not limited by the availability of nitrogen.
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 about 55% 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 (58 mg/l) was the 38th lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Mormon Trail 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 60%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Mormon Trail Lake (5 mg/l) was the 26th lowest of the 131 lakes sampled. This levels is in the lowest 25% 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).
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "fully supported". Siltation impacts at this lake, however, remain a concern. Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected during summers of 2000 through 2004. Three of 15 samples exceeded the Class A,B(LW) criterion for pH (maximum = 9.4; minimum = 7.7 pH units). Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A and Class B(LW) uses of this lake. These violations likely reflect primary productivity at Mormon Trail 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 Mormon Trail Lake in 1994 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.
Although this scenario does not apply to the fish contaminant data generated from the 1994 RAFT sampling conducted in this assessment segment, levels of contaminant are sufficiently high for concern and justify follow-up monitoring. The results from the 1994 sampling show that levels of chlordane and PCBs are well below levels of concern. The level of mercury in the sample of largemouth bass (0.34 ppm), however, exceeds the new (2006) IDNR/IDPH trigger level of 0.20 ppm for a one meal per week consumption advisory. According to IDNR’s assessment methodology, the single occurrence of contaminant above an advisory trigger level neither warrants issuance of an advisory nor indicates impairment of the fish consumption uses: two consecutive samplings that show contaminant levels are above the trigger level in fillet samples are needed to justify issuance of an advisory. But, this elevated level does indicate a concern and the need to conduct additional monitoring to better define contaminant levels in fish from this river segment. Thus, follow-up monitoring will be conducted in 2006 to better determine whether a one-meal-per-week consumption advisory needs to be issued.
