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 (monitored) as "fully supporting / threatened" due to the potential for aesthetically objectionable blooms of algae and due to presence of nuisance aquatic life (bluegreen algae). The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to nutrient loading to the water column and to turbidity. Fish consumption uses are 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 somewhat elevated levels of algal turbidity may threaten full support of the Class A and Class B(LW) uses of Center 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 73, 61, and 62, 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, however, are in the lower range between eutrophic and hyper-eutrophic lakes. These index values suggest (1) very high levels of phosphorus in the water column, (2) somewhat elevated (and less than expected) levels of chlorophyll-a, and (3) 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. The ISU lake data suggest that algal production at Center Lake is likely limited by a combination of zooplankton grazing and light limitation due to inorganic turbidity. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Center Lake is 17. This TN:TP ratio does not suggest a strong possibility that nitrogen limits the production of suspended algae at this lake. Data from Downing et al. (2002) show relatively large populations of zooplankton species at Center Lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 50% of the dry mass of the zooplankton community in both summer samples. The 2000 average summer mass of Cladocerans at this lake (26.7 mg/l) was the 46th highest of the 131 lakes sampled. The levels of inorganic suspended solids at this lake are somewhat elevated, however, and suggest at least the potential for contributing to in-lake turbidity or limiting production of suspended algae. 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 Center Lake was 3.8 mg/l. The moderately high TSI values for chlorophyll-a and secchi depth suggest potential threats to support of the Class A (primary contact) uses through the potential for aesthetically objectionable conditions due to blooms of algae and to high levels of inorganic turbidity.
In addition, data from Downing et al. (2002) suggest that presence of nuisance algal species (i.e., bluegreen algae) may threaten support of the Class A uses. Sampling in 2000 showed the percent wet mass of bluegreens ranged from just less than 90% in the mid-June sampling, to near 100% in the mid-July and early August samples. The 2000 average summer mass of bluegreen algae at this lake (25.1 mg/l) was the 39th highest of the 131 lakes sampled.
The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should remain assessed as "fully supported / threatened" due to excessive nutrient loading to the water column and the moderately high levels of algal and non-algal turbidity. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
