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 "partially supporting." The Class B(LW) aquatic life uses are 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 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 high levels of non-algal turbidity may adversely affect the Class A and Class B(LW) uses of Spring 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 66, 52, and 67, respectively. According to Carlson (1977), the index values for total phosphorus and secchi depth place this lake in the range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a in the lower range of eutrophic lakes. These index values suggest moderately high levels of phosphorus, low (and much less than expected) levels of chlorophyll-a (suspended algae), and moderately poor water transparency. According to Carlson (1991), the occurrence of high and similar TSI values for total phosphorus and secchi depth compared to the TSI for chlorophyll-a suggests that non-algal particles dominate light attenuation. Thus, the poorer than expected water transparency may be due to non-algal turbidity. The levels of inorganic suspended solids at this lake are slightly above the median for Iowa lakes and suggest at least the potential for increasing lake turbidity. 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 Spring Lake was 6.3 mg/l. In addition to potential impairments due to non-algal turbidity, data from Downing et al. (2002) suggest potential impairments to the Class A (primary contact) uses through presence of nuisance algal species (i.e., bluegreen algae). Sampling in 2000 showed that bluegreen algae (Cyanophyta) comprise a significant portion of the summertime phytoplankton community of this lake, especially in mid to late summer when more than 60% of the wet mass of phytoplankton were from this group. Although bluegreen algae appear to dominate the summertime phytoplankton community, the production of chlorophyll is very low at this lake, and the presence of bluegreen algae likely does not present a significant water quality impact. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Spring Lake is 16. he early September sample. 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." No information is available to support the previous assessment that these uses were "partially supported." The natural shallowness of this lake leads to problems with excessive growth of aquatic macrophytes (="noxious" aquatic plants"); however, this problem threatens, but does not impair, the aquatic life uses of this lake. Although results of ISU lake monitoring in 2000 and 2001 suggest that nutrient-related impairments may exist, neither the amount of data available for characterizing water quality nor the magnitude of the TSI values are sufficient for identifying impairments with the level of confidence necessary to justify adding this lake to Iowa's Section 303(d) list. Thus, further investigation and monitoring are needed to (1) better characterize current water quality conditions and (2) verify that such impairments exist. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
