Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “fully supported.” The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” Fish consumption uses remain “not assessed” due to a lack of information on which to base an assessment. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Mill Creek Lake are “fully supported.” Using the median values from these surveys from 2002 through 2006 (approximately 20 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 57, 59, and 63 respectively for Mill Creek Lake. According to Carlson (1977) the Secchi depth and chlorophyll a values place Mill Creek Lake in the eutrophic category, while the total phosphorus value places Mill Creek Lake in between the eutrophic and the hypereutrophic categories. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and moderately high levels of phosphorus in the water column.
The level of inorganic suspended solids is relatively low at this lake and does not suggest impairment due to non-algal turbidity. The median inorganic suspended solids concentration at Mill Creek Lake was 3.0 mg/L, which was the 46th lowest of the 132 monitored lakes.
Data from the 2002-2006 ISU and UHL surveys suggest a relatively small population of cyanobacteria exists at Mill Creek Lake, which does not suggest impairment at this lake. These data show that cyanobacteria comprised only 36% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (6.0 mg/L) was also the 23rd lowest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed as “fully supported” based on information from the IDNR Fisheries Bureau and results from the ISU and UHL lake surveys. Moderately high levels of nutrients in the water column, algal turbidity, and siltation, however, remain water quality concerns at this lake according to the IDNR Fisheries Bureau. The ISU and UHL lake survey results show generally good chemical water quality at Mill Creek Lake. During 2002-2006 there were no violations of the Class B(LW) criterion for ammonia in 14 samples or the Class B(LW) criterion for dissolved oxygen in 20 samples. Data from the ISU and UHL surveys show 3 violations of the Class A1,B(LW) criterion for pH in 20 samples (15%). Based on IDNR’s assessment methodology, however, these violations are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class B(LW) uses of Mill Creek Lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
