Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake phytoplankton communities in 2000, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to siltation impacts. 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 Hickory Grove Lake are "fully supported." 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 56, 52, and 52, respectively. According to Carlson (1977), the index value for total phosphorus places this lake middle range of eutrophic lakes; the index values for chlorophyll-a and Secchi depth are in the lower range of eutrophic lakes. These index values suggest very good water quality at this lake with relatively low levels of phosphorus, very low levels of chlorophyll, and very good water transparency. These results suggest that this lake does not have impairments due to aesthetically objectionable conditions. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Hickory Grove Lake is 244. This ratio suggests that algal production at this lake is not limited by nitrogen availability but by phosphorus and that levels of nitrogen in the lake are very high. The median level of total nitrogen during the 2000-02 monitoring period (8.7 mg/l) was the 8th highest of the 131 lakes sampled. 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 algal 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 summer 2000 average mass of Cladocerans (1.5 mg/l) was the 16th lowest of the 131 lakes sampled. The levels of inorganic suspended solids at this lake are somewhat elevated but 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 Hickory Grove Lake was 3.2 mg/l. This median value is the 38th lowest reported for the 131 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. The summer 2000 average mass of bluegreen algae at this lake (10.6 mg/l) was the 60th highest of the 131 lakes. Although moderately high compared to other Iowa lakes, this level of bluegreen algae is not believed to either threaten or impair the Class A uses.
Despite indications of very 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.