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 (monitored) as "fully supporting / threatened" due to the somewhat elevated levels of non-algal (inorganic) turbidity and due to moderately large populations of bluegreen algae. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to nutrient loading to the water column, high levels of non-algal turbidity, and siltation. 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 (inorganic) turbidity may adversely affect the Class A and Class B(LW) uses of Meyer 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 72, 53, and 63, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the middle range of eutrophic lakes, and the index value for secchi depth is in the middle range between eutrophic and hyper-eutrophic lakes. These index values suggest (1) very high levels of phosphorus in the water column, (2) relatively low (and much less than expected) levels of chlorophyll-a (suspended algae), 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 a combination of non-algal (inorganic) turbidity and nitrogen limitation may limit algal production at Meyer Lake. The levels of inorganic suspended solids at this lake are moderately high and suggest at least the potential for contributing to in-lake turbidity and 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 Meyer Lake (4.3 mg/l) was the 78th highest level of the 131 lakes sampled. Nitrogen limitation may also limit algal production at this lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Meyer Lake is 12. This TN:TP ratio is low and suggests that nitrogen availability may limit algal production at this lake. Zooplankton grazing, however, likely does not limit algal production. Data from Downing et al. (2002) show a relatively small populations of zooplankton species at this lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised less than 5% of the dry mass of the zooplankton community in the samples from early August and late August. The 2000 average summer mass of zooplankton grazers at this lake (0.7 mg/l) was the ninth lowest of the 131 lakes. Regardless of the source, the somewhat elevated TSI value for secchi depth (63) suggests potential threats to the full support of Class A (primary contact) uses through presence of non-algal turbidity that reduces water transparency.
The presence of nuisance algal species (i.e., bluegreen algae) may also threaten full support of these uses. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of this lake. Sampling in 2000 showed the percent wet mass of bluegreens ranged from approximately 80% in the early July sampling to approximately 95% in the late August sampling. The 2000 average summer mass of bluegreen algae at Meyer Lake (27.3 mg/l) is moderately high relative to that in other Iowa lakes and suggests a threat to full support of designated uses.
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should remain assessed as "fully supported / threatened" due to excessive nutrient loading to the water column, high levels of non-algal turbidity, and siltation in the lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
