Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 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 aesthetically objectionable conditions caused by poor water transparency and algae blooms. 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 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
EXPLANATION: For the 2010 assessment/listing cycle, results from the ISU and UHL lake surveys suggest that the Class A1 uses at Meadow Lake are “partially supported.” Using the median values from these surveys from 2004 through 2008 (approximately 19 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 70, 68, and 73 respectively for Meadow Lake. According to Carlson (1977) the Secchi depth value places Meadow Lake in between the eutrophic and hypereutrophic categories, while the chlorophyll a and total phosphorus values place Meadow Lake in the hypereutrophic category. These values suggest very 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 only moderately high at this lake and suggests that non-algal turbidity contributes somewhat to poor water clarity at this lake. The median inorganic suspended solids concentration at Meadow Lake was 5.0 mg/L, which was the 52nd highest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a large population of cyanobacteria exists at Meadow Lake, which causes an impairment due to nuisance aquatic life. These data show that cyanobacteria comprised 95% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (57.6 mg/L) was also the 10th highest of the 132 lakes sampled. This median is in the worst 25% of the 132 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa’s narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values from 2004-2008. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. The assessment category for assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence.
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. Nutrient loading to the water column, moderately high levels of algal turbidity, and siltation in the lake, however, remain water quality concerns at this lake that may affect support of the Class B(LW) uses. The ISU and UHL lake survey results show generally good chemical water quality at Meadow Lake. During 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 19 samples and no violations of the Class A1,B(LW) criterion for pH in 19 samples. There were two violations of the Class B(LW) criterion for dissolved oxygen in 19 samples (11%). 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.
NOTE: Meadow Lake was drawn down in the winter of 2008-2009 and sediment and nutrient retention structures are scheduled to be built in the watershed in 2010.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
