Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02 and (2) ISU report on lake plankton communities in 2000.
Basis for Assessment
[NOTE: This lake is was added to the Iowa Water Quality Standards in 2004 at a Class B(LW) waterbody.]
SUMMARY: The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened" due potential impacts from turbidity due to algal and/or non-algal sources; the presence of moderately high populations of bluegreen algae suggest an additional threat to full support of beneficial uses at this lake. Fish consumption uses are "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) and (2) 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 high levels of turbidity may threaten the Class B(LW) uses of Fogle Lake. 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 61, 57, and 60, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the lower range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-and secchi depth are in the upper range of eutrophic lakes. These index values suggest somewhat elevated levels of phosphorus, relatively low levels of chlorophyll-a, and marginally good water transparency. According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data suggest that non-algal particles do likely limit algal production at Fogle Lake. The generally good agreement between the TSI values for Fogle Lake, however, suggest only a minor non-phosphorus limitation. The results of ISU monitoring suggest that a combination of zooplankton grazing and light attenuation due to inorganic turbidity may serve to limit algal production at Fogle Lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Fogle Lake is 20. This TN:TP ratio suggests that algal production at this lake is limited by availability of phosphorus. Data from Downing et al. (2002), however, do 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 summer 2000 average mass of Cladocerans (34.2 mg/l) was the 36th highest of the 131 lakes sampled, thus suggesting the potential for zooplankton grazers to limit algal production at this lake. The levels of inorganic suspended solids at this lake are somewhat elevated and suggest the potential for attenuating light and contributing to in-lake turbidity. 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 Fogle Lake was 4.1 mg/l. The marginally high TSI values for Secchi depth suggests a potential threat to the Class B(LW) uses due to reduced water transparency and the potential for aesthetically objectionable conditions at this lake.
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. The 2000 average summer mass of bluegreen algae at this lake (14.4 mg/l) was the 48th highest of the 131 lakes sampled and suggests the potential for bluegreen algae to create nuisance conditions and to threaten full support of designated uses. The results of ISU monitoring at this lake suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported / threatened" due to the somewhat elevated levels of algal and/or non-algal turbidity that lead to only marginally acceptable water transparency. Fish consumption uses are "not assessed" due to the lack of fish contaminant monitoring at this lake.
