Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened." Fish consumption uses remain "not assessed." The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002). Further monitoring and investigation are needed to better characterize water quality conditions at this lake. EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of algal turbidity may threaten full support of the the Class A uses of Mill Creek Lake. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 72, 61, and 60, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth place this lake in the lower range between eutrophic and hyper-eutrophic lakes. These index values suggest very high levels of phosphorus in the water column, moderately high (but much less than expected) levels of chlorophyll-a (suspended algae), and relatively good water transparency. 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 or zooplankton grazing) other than phosphorus limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Mill Creek Lake is 14. This TN:TP ratio suggest the potential for nitrogen limitation on algal production at this lake. Data from Downing et al. (2002) show relatively small populations of zooplankton species at Mill Creek Lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised less than 20% of the dry mass of the zooplankton community in the two summer samplings. This relatively small population of phytoplankton grazers would not appear capable of lowering algal populations to the levels suggested by the TSI values. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for impairing designated uses. The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at Mill Creek Lake was 3.9 mg/l. The high TSI value for total phosphorus, and the moderately high TSI for chlorophyll-a, suggest potential threats to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae. Also, data from Downing et al. (2002) suggest that the presence of nuisance algal species (i.e., bluegreen algae) threatens the Class A uses. Sampling in 2000 showed the percent wet mass of bluegreens ranged from approximately 75% in the mid-June sampling, to approximately 90% in the mid-July sampling, and up to nearly 100% in the early August sampling. The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported / threatened" due to moderately high levels of nutrients in the water column, algal turbidity, and siltation in the lake. Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments. Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake.
