Silver Lake IA 01-MAQ-17

Assessment Comments

Assessment is based on results of (1) results of WQ monitoring as part of University of Northern Iowa lake project, (2) results of fish tissue (RAFT) monitoring in 1999, and (3) surveys by DNR Fisheries Bureau.

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation) uses were assessed as "partially supported."  The Class B(LW) aquatic life uses remain assessed as "not supported."  Fish consumption uses are assessed as "fully supported.   EXPLANATION:  The Class A uses were assessed as "partially supported" based on results of monitoring conducted in 1999 as part of the University of Northern Iowa Summer Lakes Study (report available).   This monitoring showed high levels of chlorophyll in the lake, as well as populations of bluegreen algae, thus suggesting an impairment to the Class A uses.   Levels of indicator bacteria, however, did not suggest an impairment of the Class A uses.   Ninety samples from selected sites on Silver Lake were analyzed for indicator bacteria (fecal coliforms) during a six-week period.   Results of this monitoring show that weekly levels were generally below the Iowa water quality criterion of 200 orgs/100 ml.   The Class B(LW) uses remain assessed as "not supported" based on results of the University of Northern Iowa Summer Lakes Study that show a phytoplankton community (cyanobacteria dominance), levels of soil erosion, nutrient delivery, and biomass production that suggest impairment of the lake's aquatic life.   The only fish species observed during the 1999 study was black bullhead.   This study concluded that levels of dissolved oxygen are maintained above state criteria only through year-round use of a lake aeration system.   Sources of nutrients delivered to the lake included farmland and the high volume of land-applied liquid manure in the watershed.   In addition, the DNR Fisheries Bureau reviewed and approved the previous (1998) assessment of the Class B(LW) aquatic life uses ("not supported").   Results of EPA/DNR fish tissue monitoring (RAFT) in 1999 showed very low levels of very few contaminants in the two composite samples of fillets from black bullhead.   Of the 23 contaminants analyzed for, only two (mercury and selenium) were found above analytical levels of detection.   Thus, because levels of all contaminants were less than ½ of the respective FDA action levels and DNR levels of concern, the fish consumption uses were assessed as "fully supported."