Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) ISU report on lake phytoplankton communities, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
[NOTE: For the 2002 reporting cycle, the waterbody ID number for Willow lake was changed from IA 06-LSR-01590-L to IA 06-BOY-00405-L. This new ID number correctly places this lake in the Boyer River watershed.]
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting." The Class B(LW) aquatic life uses remain 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). EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that the Class A uses of Willow Lake are "fully supported." 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 56, 40, and 45, 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 range of mesotrophic lakes. These index values suggest relatively low levels of phosphorus, extremely low levels of chlorophyll-a, and water transparency that is exceptional for Iowa lakes. These results suggest that this lake does not have impairments due to aesthetically objectionable blooms of algae. 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 or zooplankton grazing) limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Willow Lake is 23. This ratio suggests only a slight potential that algal production at this lake is limited by nitrogen availability. In addition, data from Downing et al. (2002) show relatively 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. Thus, algal production at this lake is likely 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 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at Willow Lake was 2.3 mg/l. This median value was the 18th lowest of the 130 lakes sampled. This lake does not appear to have problems due to presence of nuisance algal species (i.e., bluegreen algae). 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. Although bluegreen algae comprise a significant portion of the late summertime phytoplankton community, the production of chlorophyll is extremely low at this lake, and the presence of bluegreen algae likely does not present a water quality impact. Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data 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.
