Assessment is based on results of (1) ISU lake survey from 2000-04, (2) ISU reports on lake phytoplankton communities from 2000-05, 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 are assessed (evaluated) as "fully supporting". Siltation impacts, however, remain a concern 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 sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, and (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey.
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 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 57, 53, and 53, 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 very 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 2004, the ratio of total nitrogen to total phosphorus for Hickory Grove Lake is 210. 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. This very high TN:TP ratio reflects the often exceptionally high levels of total nitrogen in impoundments of the Des Moines Lobe region of north-central Iowa. The median level of total nitrogen during the 2000-02 monitoring period (8.2 mg/l) was the 11th highest of the 131 lakes sampled.
Data from Downing et al. (2002) show . In terms of all Iowa lakes sampled, data from the ISU survey show moderately to very large populations of zooplankton species at Hickory Grove Lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 70% of the dry mass of the zooplankton community of this lake. The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (131 mg/l) was the 40th highest of the 131 lakes sampled. , 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 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 2004 was 5.2 mg/l; the median level at Hickory Grove Lake was 3.3 mg/l. This median value is the 35th lowest of the 131 lakes sampled.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (approximately 60%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Hickory Grove Lake (8.2 mg/l) was the 43rd lowest of the 131 lakes sampled. This levels is in the lowest one-third of the 131 Iowa lakes sampled. The presence of a relatively small population of bluegreen algae at this lake does not suggest a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes. No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "fully supported". Siltation impacts in the lake, however, remain a concern. The ISU lake survey data also show good chemical water quality at this lake. These data show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected, or for pH in the 13 samples collected, at Hickory Grove Lake during summers of 2000 through 2004.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.