Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) ISU report on lake plankton communities, (3) 1995 survey of Iowa water supply reservoirs (Miller and Kennedy 1995).
Basis for Assessment
[NOTE: This lake is not in the current (September 2001) version of the Iowa Water Quality Standards but is proposed for inclusion to the standards for Class B(LW),C uses.]
SUMMARY: The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened." The Class C (drinking water) uses remain assessed (evaluated) as "fully supporting / threatened." Fish consumption uses are "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 phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002). EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of phosphorus may threaten full support of the Class B uses of Greenfield Lake. 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 68, 56, and 58, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth are in the middle and upper range, respectively, of eutrophic lakes. These index values suggest relatively high levels of phosphorus in the water column, relatively low (and less than expected) levels of chlorophyll-a, and relatively good water transparency. 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) other than phosphorus limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Greenfield Lake is 15. This TN:TP ratio suggests the possibility that nitrogen limitation suppresses the production of suspended algae at this lake. In addition, data from Downing et al. (2002) show relatively large populations of zooplankton species at this lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 50% of the dry mass of the zooplankton community in the mid-July and earlyl August samples. The levels of inorganic suspended solids at this lake are moderately high and suggest the potential for contributing to in-lake turbidity. 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 Greenfield Lake was 6.9 mg/l. Of the 130 lakes sampled, Greenfield lake has the 48th highest median value to inorganic suspended solids. Despite the indications of relatively good water quality, the high TSI value for total phosphorus, and the moderately high value for chlorophyll-a, suggest threats to the full support of the Class B (aquatic life) uses through presence of aesthetically objectionable blooms of algae. The presence of nuisance algal species (i.e., bluegreen algae), however, does not appear to be a significant problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a very low portion of the summertime phytoplankton community of this lake. In 2000, less than 5% of the wet mass of the phytoplankton community was bluegreen algae in samples collected in mid-June, mid-July, and early August. 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 Class C (drinking water) uses remain assessed as "fully supported / threatened" based on results of sampling conducted by UHL in January 1995 (see assessment for the 1996 report above. However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively low confidence). Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
