Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake plankton communities in 2000, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened" due to the presence of moderately large populations of nuisance aquatic life (bluegreen algae). The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to siltation impacts at this lake. 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 the Class A uses of Willow Lake are "fully supported"; this lake has some of the best trophic state index numbers of any lake in the state. 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 53, 38, and 47, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the middle range of eutrophic lakes; the index value for chlorophyll-a is at the oligotrophic/mesotrophic boundary, and the index value for secchi depth is in the middle to upper range of mesotrophic lakes. These index values suggest very low levels of phosphorus, extremely low levels of chlorophyll-a (the lowest of the 131 lakes sampled), and water transparency that is exceptional for Iowa lakes. These results suggest that this lake does not have impairments due to aesthetically objectionable conditions related either to blooms of algae or to high levels of inorganic turbidity. 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. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Willow Lake is 27. This ratio suggests little potential that algal production at this lake is limited by nitrogen availability. In addition, data from Downing et al. (2002) show moderately large populations of zooplankton species at Willow Lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 40% of the dry mass of the zooplankton community in the mid-July sample and approximately 50% in the early August sample. The summer 2000 average mass of Cladocerans at this lake (32 mg/l) was the 39th highest of the 131 lakes sampled. Thus, algal production at this lake may be limited by the presence of these zooplankton grazers. The levels of inorganic suspended solids at this lake are very low and do not suggest the potential for impairing designated uses. 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 Willow Lake was 2.0 mg/l. This median value was the 11th lowest of the 131 lakes sampled.
Despite the TSI numbers that suggest “full support” of the Class A uses of Willow Lake, levels of nuisance aquatic life (i.e., bluegreen algae) at this lake suggest a threat to full support of the Class A uses. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a relatively small portion of the summertime phytoplankton community of this lake, with the exception of late summer. Sampling in 2000 showed that bluegreen algae comprise approximately 10% or less of the wet mass of phytoplankton in the late June and late July samples and increased to comprise over 90% of the wet mass of phytoplankton in the early August sample. The summer 2000 average mass of bluegreen algae at this lake (28.4 mgl) was the 34th highest of the 131 lakes sampled and suggests a potential threat to full support of the Class A uses. Although results of plankton sampling in 2000 suggest a threat to support of designated uses, the amount of data available for characterizing algal populations at this lake 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 water quality 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" due to siltation impacts in the lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
