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. An additional impairment to these uses is suggested by the very large population of bluegreen algae at this lake (=nuisance aquatic life). The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to siltation, excessive nutrient loading to the water column, and high levels of non-algal turbidity. 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 and Class B(LW) uses of Thayer 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 74, 57, and 69, 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 upper range of eutrophic lakes, and the index value for Secchi depth is in the upper range between eutrophic and hyper-eutrophic lakes. These index values suggest very high levels of phosphorus in the water column, relatively low (and much less than expected) levels of chlorophyll-a (suspended algae), and moderately poor water transparency. According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data suggest that non-algal particles (i.e., inorganic suspended solids) do likely limit algal production at Thayer Lake. 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. Of 131 lakes sampled, Thayer Lake had the 21st highest median level of inorganic suspended solids (14.1 mg/l), thus suggesting that non-algal turbidity may limit the production of algae as well as contribute to in-lake turbidity that impairs the primary contact recreation uses. The ISU data also suggest that nitrogen availability and zooplankton grazing may limit algal production at Thayer Lake. Based on median values of ISU lake sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus (TN/TP) at Thayer Lake is 12; this TN/TP ratio is low and suggests that algal production may, at times, be limited by the availability of nitrogen. In addition, data from Downing et al. (2002) show relatively large populations of zooplankton species (Cladocerans) at Thayer Lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa comprised approximately 90% of the dry mass of the zooplankton community in the mid-July sample and 60% in the early August sample. The summer 2000 average mass of Cladocerans (314.8 mg/l) was the second highest of the 131 lakes sampled. Thus, these data suggest that populations of these Cladocerans may tend to suppress expression of chlorophyll as suspended algae These conditions suggest impairments to the Class A (primary contact) uses primarily due to presence of high levels of inorganic turbidity—and the related reductions in water transparency—that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions.
The presence of nuisance (=noxious) algal species (i.e., bluegreen algae) may also contribute to impairment at Thayer Lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of this lake. Sampling in 2000 showed that 100% of the wet mass of the phytoplankton community at Thayer Lake was in bluegreen algae in the mid-July and early August samples. The average mass of bluegreen algae in summer 2000 at this lake (39.5 mg/l) ranked 21st highest of the 131 lakes sampled. The presence of relatively large populations of bluegreen algae suggests an impairment of support of the designated uses of this lake due to the potential for violating Iowa’s narrative water quality standard protecting against presence of nuisance aquatic life. Although results of plankton sampling in 2000 suggest that designated uses are impaired, the amount of data available for characterizing algal populations at this lake (one season) 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.
The eutrophic to hyper-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 "partially supported" due to siltation, excessive nutrient loading to the water column, and high levels of non-algal turbidity. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
