Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01 and (2) ISU report on lake plankton communities.
Basis for Assessment
Not assessed as part of Section 305(b) reporting prior to the 2002 cycle.
[NOTE: This lake is not in the current (2001) version of the Iowa Water Quality Standards but has been proposed for inclusion to the standards for Class B(LW) uses.]
SUMMARY: The Class B(LW) aquatic life uses are 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) and (2) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002). Further monitoring and investigation are needed to better characterize water quality conditions at this lake. EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of turbidity may threaten and Class B(LW) uses of Fogle 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 66, 57, and 60, 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 values for chlorophyll-and secchi depth are in the middle to upper range of eutrophic lakes. These index values suggest relatively high levels of phosphorus, relatively low (and less than expected) levels of chlorphyll-a, and moderately poor 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 Fogle Lake is 20. This TN:TP ratio suggests a slight 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 Fogle Lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 95% of the dry mass of the zooplankton community in the mid-July sample but only 20% in the early August sample. The levels of inorganic suspended solids at this lake are moderately low and do not suggest a strong 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 Fogle Lake was 4.7 mg/l. The relatively high TSI values for total phosphorus suggests a potential threat to the Class B(LW) uses presence of aesthetically objectionable blooms of algae and presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of this lake in mid and late summer. Sampling in 2000 showed the percent wet mass of bluegreens ranged from less than 10% in the late June sampling, to approximately 95% in the mid-July and early August samplings. These values, however, given the relatively low levels of chlorophyll-a (i.e., suspended algae) at this lake, do not suggest significant problems with bluegreen algae. The eutrophic conditions at this lake suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported / threatened" due to relatively high nutrient loading to the water column and moderately high levels of algal and/or non-algal turbidity. Fish consumption uses are "not assessed" due to the lack of fish contaminant monitoring at this lake.
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 a more 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.
