Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 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 “partially supported” due to high levels of algae (chlorophyll a) and poor water clarity. The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” Fish consumption uses are “not assessed” due to a lack of information upon which to base an assessment. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
Note: A TMDL for noxious aquatic plants at Lake Smith was prepared by IDNR and approved by EPA in 2005. The basis of the impairment at this lake was high populations of cyanobacteria (=noxious aquatic plants). Based on recent monitoring, a fish kill in the winter of 2000-01 and a subsequent lake renovation has significantly decreased the amount of cyanobacteria in this lake. Recent ISU and UHL lake monitoring data strongly suggest that the impairment due to noxious aquatic plants (cyanobacteria) no longer exists. Non-cyanobacteria algal populations, however, remain elevated at this lake and suggest "partial support" of the Class A1 uses. Because algae populations are addressed in the TMDL this lake is placed in IR Category 4a (impaired; TMDL approved) for the 2012 cycle.
EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Lake Smith are “partially supported” due to high levels of algae and poor water clarity. Using the median values from these surveys from 2006 through 2010 (approximately 16 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 67, 65, and 72 respectively for Lake Smith. According to Carlson (1977) the Secchi depth and chlorophyll a values place Lake Smith in between the eutrophic and hypereutrophic categories, while the total phosphorus value places Lake Smith in the hypereutrophic category. These values suggest high levels of chlorophyll a and suspended algae in the water, poor water transparency, and very high levels of phosphorus in the water column.
The level of inorganic suspended solids is moderately high at this lake but does not suggest impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Lake Smith was 4.5 mg/L, which was the 59th highest of the 134 monitored lakes.
Data from the 2006-2010 ISU and UHL surveys suggest a very small population of cyanobacteria exists at Lake Smith, which does not suggest impairment at this lake due to noxious aquatic plants. These data show that cyanobacteria comprised only 24% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (7.2 mg/L) was the 17th lowest of the 134 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported” based on information from the IDNR Fisheries Bureau and results from the ISU and UHL lake surveys. The ISU and UHL lake surveys show that during 2006-2010 there were no violations of the Class B(LW) criterion for ammonia in 16 samples or dissolved oxygen in 15 samples. There were 3 violations of the Class A1,B(LW) criterion for pH in 16 samples (19%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class B(LW) uses at Lake Smith. Information from the IDNR Fisheries Bureau also suggests that the Class B(LW) aquatic life uses should, due to recent improvements in water quality at this lake, be assessed as "fully supported."
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
