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, 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 / threatened" due to siltation impacts at this lake. Fish consumption uses remain 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 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), 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 2002 (approximately nine 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 2002 was 4.8 mg/l; the median level at Mormon Trail Lake was 3.2 mg/l. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Mormon Trail Lake is 30. This ratio suggest that algal production at this lake is likely not limited by the availability of nitrogen. In addition, populations of zooplankton species (Cladocerans) that graze on algae appear to be very low at this lake. The summer 2000 average mass of Cladocerans at Mormon Trail Lake (1.9 mg/l) was the 18th lowest of the 131 lakes sampled, thus suggesting little if any non-phosphorus limitation on algal production due to algal grazers.
This lake does not appear to have problems due to presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a relatively small portion of the phytoplankton community in early and mid-summer but tend to dominate in late summer. Sampling in 2000 showed that bluegreen algae comprised less than 10% of the wet mass of the phytoplankton community in the mid-June and mid-July samples but increased to approximately 75% in the early August sample. The summer 2000 average mass of bluegreen algae (2.0 mg/l) was the 19th lowest of the 131 lakes sampled, thus suggesting little if any impact from these nuisance algae.
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "fully supported / threatened" due to siltation impacts in the lake. Fish consumption uses remain assessed as "fully supported" based on results of the U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1994. 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).
