Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake plankton communities, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting" based on results of chemical/physical monitoring conducted as part of the statewide lake survey from 2000 through 2002. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to siltation impacts. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. 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, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002).
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that the Class A uses of Pollmiller 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 59, 49, and 50, respectively. According to Carlson (1977), the index value for total phosphorus is in the upper range of eutrophic lakes, the index value for chlorophyll-a is in the upper range of mesotrophic lakes; and the index value for secchi depth is at the boundary between eutrophic and mesotrophic lakes. These index values suggest (1) relatively low levels of total phosphorus, (2) extremely low levels of chlorophyll-a, and (3) water transparency that is very good to exceptional for Iowa lakes. These results suggest that this lake does not have impairments due to aesthetically objectionable conditions due either to blooms of algae or high levels of inorganic turbidity. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low 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. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Pollmiller Lake is 24. This ratio does not suggest that algal production is limited by nitrogen availability. In addition, data from Downing et al. (2002) show relatively large populations of zooplankton species at Pollmiller Lake that graze on algae. Sampling in 2000 showed that while Cladoceran taxa (e.g., Daphnia) comprised only about 10% of the dry mass of the zooplankton community in the late July sample, these taxa comprised approximately 75% of the mid-August sample. The summer 2000 average mass of Cladocerans at this lake (20.2 mg/l) was the 62nd highest of the 131 lakes sampled; this level suggests at least the possibility that zooplankton grazing may limit algal production. The levels of inorganic suspended solids at this lake are somewhat elevated. 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 Pollmiller Lake was 3.6 mg/l. This level of ISS suggests at least the potential for contributing to in-lake turbidity such that light is attenuated and algal production is limited. This level, however, is not sufficiently high to either threaten or impair designated uses. Thus, the very low nutrient levels, combined with extremely low chlorophyll concentrations and good to exceptional water clarity suggests that this lake fully supports its Class A uses.
This lake does not appear to have serious problems due to presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of this lake. Sampling in 2000 showed that bluegreen algae comprised approximately 95% of the wet mass of the phytoplankton community in the late June sample and decreased slightly to approximately 70% in the late July and early August samples. The summer 2000 average mass of bluegreen algae at Pollmiller Lake (10.0 mg/l) was the 64th highest of the 131 lakes sampled. Although somewhat elevated relative to populations of bluegreen algae at other Iowa lakes, the presence of bluegreen algae at levels seen in Pollmiller Lake likely does not present a water quality impact.
Despite the indications of very good water quality at this lake, information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should remain assessed as "fully supported / threatened" due to siltation impacts in the lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
