Three Mile Lake IA 05-GRA-1371

Assessment Comments

Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake plankton communities in 2000, and (3) U.S. EPA / IDNR fish tissue (RAFT) monitoring in 2000.

Basis for Assessment

SUMMARY:  The Class B(LW) aquatic life uses are assessed (monitored) as "fully supporting."  This lake has very good water quality relative to other Iowa lakes.   [Note:  this lake is not designated for Class A (primary contact recreation) uses in the Iowa Water Quality Standards.]  Fish consumption uses are assessed as "fully supporting" based on results of fish contaminant monitoring in 2000.   The Class C (drinking water) uses remain “not assessed.”  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), (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 from 2000 through 2002 (approximately nine samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 60, 53, and 50, 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 boundary between mesotrophic and eutrophic lakes.   These index values suggest somewhat elevated levels of phosphorus, very low (and less than expected) levels of chlorophyll-a, and very good  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.   Results of ISU monitoring, however, do not immediately suggest non-phosphorus limitations on algal production at this lake.   Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Threemile Lake is 24.   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 very 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 2002 was 4.8 mg/l; the median level at Threemile Lake was 1.1 mg/l; this is the 7th lowest median level of the 131 lakes sampled.   Data from Downing et al.  (2002) show only moderately large populations of zooplankton species (Cladocerans) 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 summer 2000 average mass of Cladocerans (3.2 mg/l) was the 25th lowest of the 131 lakes and does not suggest the potential for zooplankton grazing to limit 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 but do not impair designated uses.   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.   The summer 2000 average mass of bluegreen algae at this lake (5.8 mg/l), however, is low relative to other Iowa lakes and is not believed to either threaten or impair the designated uses of this lake.  

The Class C (drinking water) uses remain "not assessed" due to lack of water quality information upon which to base such an assessment.   The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate.   While the results of the ISU survey from 2000-02 show that nitrate levels are very low at this lake (maximum value = 1.2 mg/l; median = 0.2 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.   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.