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
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 Hickory Grove 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 63, 49, and 53, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range between eutrophic and 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 lower range of eutrophic lakes. These index values suggest that, despite the moderately high levels of phosphorus, the production of suspended algae is extremely low and water transparency is very good 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 Hickory Grove Lake is 126.3. This ratio suggests that algal production at this lake is not limited by nitrogen availability and that levels of nitrogen in the lake are relatively high. Data from Downing et al. (2002) show only moderately large populations of zooplankton species at Hickory Grove Lake that graze on algae, thus suggesting that zooplankton grazing is not a strong limiter of agal production. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 20% of the dry mass of the zooplankton community in both the late July and late August samples. 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 Hickory Grove Lake was 1.8 mg/l. This median value is the 10th lowest reported for the 130 lakes sampled. This lake does not appear to have serious problems due to presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of this lake, especially in late summer. Sampling in 2000 showed that bluegreen algae comprised approximately 30% of the wet mass of the phytoplankton community in the late June and late July samples but increased to approximately 85% in the late August sample. Although bluegreen algae comprise a significant portion of the summertime phytoplankton community, the production of chlorophyll is very 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.
Despite indications of good water quality at this lake, information from the IDNR Fisheries Bureau suggests 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.