Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) ISU reports on lake plankton communities from 2000-05, 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 2004. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting". Siltation impacts, however, remain a concern at this lake. 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 sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, and (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey.
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 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 55, 57, and 52, respectively. According to Carlson (1977), the index value for total phosphorus is in the middle range of eutrophic lakes, the index value for chlorophyll-a is in the upper range of eutrophic lakes; and the index value for secchi depth is in the lower range of eutrophic lakes. These index values suggest (1) very low levels of total phosphorus, (2) relatively low levels of chlorophyll-a, and (3) very good water transparency 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.
The general agreement between the TSIs for phosphorus, chlorophyll, and Secchi depth suggests that non-phosphorus limitations—which appear to occur at most Iowa lakes—do not occur at Pollmiller Park Lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Pollmiller Lake is 29. This ratio does not suggest that algal production is limited by nitrogen availability.
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 60% 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 (70 mg/l) was the 53rd lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Pollmiller Park Lake.
The levels of inorganic suspended solids at this lake are very low. 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 Pollmiller Lake was 3.0 mg/l. This level of ISS suggests little potential for contributing to in-lake turbidity such that light is attenuated and algal production is limited. Nor is this level sufficiently high to either threaten or impair designated uses.
Thus, the very low nutrient levels, combined with relatively low chlorophyll concentrations and very good water clarity suggests that this lake fully supports its Class A uses. Of some concern, however, is the apparent increase (worsening) in the TSI for chlorophyll-a between the 2000-2002 period (49) to the value calculated for the 2000-2004 period (57). Based on ISU lake survey monitoring data, however, annual summer TSI values for chlorophyll-a do not exhibit the adverse trend suggested by the comparison of the TSI values for the 2000-02 and 2000-04 periods. The annual summer TSIs for chlorophyll-a are as follows: 2000: 58; 2001: 47; 2002 = 59; 2003 = 51; and 2004 = 59.
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 75%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Pollmiller Park Lake (14 mg/l) was the 57th highest of the 131 lakes sampled. This levels is in the lowest two-thirds of the 131 Iowa lakes sampled. The presence of a moderately large population of bluegreen algae at this lake does not immediately 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 in the 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 15 samples collected during summers of 2000 through 2004. One of 15 samples exceeded the Class A,B(LW) criterion for pH (maximum = 9.0; minimum = 8.2 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. This violation likely reflects the levels of primary productivity at Pollmiller Park Lake and does not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
