Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as “partially supported” due to the extremely high populations of nuisance (noxious) aquatic (algal) species (bluegreen algae) at this lake; levels of chlorophyll-a suggest that designated uses are "fully supporting / threatened." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to moderately high levels of nutrients in the water column, algal turbidity, and siltation in the 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 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). 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 extremely large populations of bluegreen algae impair the Class A uses and that high levels of algal turbidity may threaten full support of the Class A uses of Mill Creek Lake. 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 69, 63, and 61, respectively. According to Carlson (1977), the index values for all three parameters place this lake in the range between eutrophic and hyper-eutrophic lakes. These index values suggest moderately high levels of phosphorus in the water column, moderately high (but somewhat less than expected) levels of chlorophyll-a (suspended algae), and marginally 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, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. Results of ISU monitoring suggest that both nitrogen limitation and the somewhat elevated levels of inorganic solids may account for the apparent non-phosphorus limitations at this lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Mill Creek Lake is 12. This TN:TP ratio does not indicate a strongly nitrogen limited lake but does suggest at least the potential for nitrogen limitation on algal production. 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. The summer 2000 average mass of Cladocerans (9 mg/l) was the 45th lowest of the 131 lakes sampled. 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 somewhat elevated but do not suggest the potential for impairing designated uses. These levels are sufficiently high, however, to contribute to in-lake turbidity. 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 Mill Creek Lake was 3.9 mg/l. 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 2000 average summer mass of bluegreen algae at this lake (165.2 mg/l) was the 7th highest of the 131 lakes sampled, thus strongly suggesting a violation of Iowa’s narrative water quality standard protecting against nuisance aquatic life. The amount of data available for characterizing algal populations at this lake (one season), however, is not sufficient for developing a more accurate assessment of support of these uses. Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to improve the accuracy of future water quality assessments for this lake.
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. Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake.
