Three Mile Lake IA 05-GRA-1371

Assessment Comments

Assessment is based on results of (1) ISU lake survey in 2000-01, (2) ISU report on lake plankton communities, and (3) U.S. EPA / IDNR fish tissue (RAFT) monitoring in 2000. Documentation for the 2002 assessment was updated and corrected in August 2004.

Basis for Assessment

SUMMARY:  The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting."  Fish consumption uses are assessed as "fully supporting."  The Class C (drinking water) uses are “not assessed.”  (This lake is not designated for Class A (primary contact recreation) uses in the Iowa Water Quality Standards.)  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, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al.  (2002), and (4) results of U.S.  EPA / IDNR fish contaminant monitoring in 2000.  

EXPLANATION:  The water quality conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported."  Using the median values from Iowa State University’s survey in 2000 and 2001 (approximately six samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 60, 53, and 49, respectively.   According to Carlson (1977), the index value for total phosphorus places this lake at the upper range of eutrophic lakes; the index value for chlorophyll-a is in the lower range of eutrophic lakes, and the index value for secchi depth is at the upper range of mesotrophic lakes.   These index values suggest relatively low levels of phosphorus, low (and less than expected) levels of chlorophyll-a, and exceptional (and much better than expected) water transparency.   According to Carlson (1991), the occurrence of a low TSI value for chlorophyll-a and secchi depth relative to that for total phosphorus indicates that algal production is limited by zooplankton grazing, nitrogen limitation, or some factor other than phosphorus.   Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Threemile Lake is 28.2.   This ratio does not suggest a strong likelihood that algal production at this lake is limited by nitrogen availability.   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 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at Threemile Lake was 1.6 mg/l; this is the 7th lowest median level of the 130 lakes sampled.   Data from Downing et al.  (2002) show only moderately large populations of zooplankton species at Three Mile Lake that graze on algae.   Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised only 5% of the dry mass of the zooplankton community in the mid-July sample but increased to almost 40% of the early August sample.   The presence of a moderately large populations of zooplankton grazers, at least in late summer, suggests the potential for zooplankton grazers to limit, at least to some degree, algal production.   Data from Downing et al.  (2002) suggest that bluegreen algae (Cyanophyta) comprise a significant portion of the summertime phytoplankton community of this lake.   Sampling in 2000 showed less than 10% of the wet mass of the phytoplankton community in late summer was bluegreens, but from 35 to nearly 70% was bluegreens in the mid-June and mid-July samplings, respectively.   Although bluegreen algae comprise approximately half of the summertime phytoplankton community, the production of chlorophyll is low at Threemile Lake, and the presence of bluegreen algae likely does not present a significant water quality impact.  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 are "not assessed" due to lack of water quality information upon which to base such an assessment.   Fish consumption are assessed as "fully supported" based on results of U.S.  EPA / IDNR fish tissue monitoring in August 2000.   Levels of all contaminants in the composite samples of fillets from channel catfish and walleye were well-below the respective FDA action levels and IDNR levels of concern.   The levels of the primary contaminants of Iowa fish (i.e., chlordane, DDE, dieldrin, mercury, and PCBs) were all very low.