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 "partially supporting" due to high levels of inorganic turbidity that reduce water transparency. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened" due to excessive growth of aquatic macrophytes. 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).
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of non-algal turbidity may adversely affect the Class A uses of Spring 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 64, 53, and 69, 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 somewhat elevated levels of phosphorus in the water column, low (and 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 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 Spring Lake was 6.6 mg/l. This moderately high level of inorganic suspended solids likely accounts for the reductions in water transparency observed at this lake.
Neither nitrogen limitation nor zooplankton grazing appear to limit algal production at Spring Lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 18, thus suggesting that phosphorus (versus nitrogen) is the nutrient that most often limits production of suspended algae at this lake. Data from Downing et al. (2002) show relatively small populations of zooplankton species at this lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) were absent in the mid-July sample but increased to approximately 50% in the early August sample. The average 2000 summer mass of Cladoceran grazers (0.8 mg/l) was the 11th lowest of the 131 lakes sampled. At these population levels, grazing of algae by zooplankters likely does not affect algal production. These water quality conditions suggest that the levels of inorganic suspended solids reduce water transparency such that an aesthetically objectionable condition exists, thus indicating only “partial support” of the Class A uses.
Data from Downing et al. (2002) suggest a potential threat to full support of the Class A (primary contact) uses due to the somewhat elevated levels 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. The average summer 2000 mass of bluegreen algae was 10.1 mg/l; this was the 63rd highest level of the 131 lakes sampled. This level is of concern but does not likely either threaten or impair designated uses at 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." 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. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
