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 pH that violate the state criterion. The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to high levels of pH that violate the state criterion. Fish consumption uses remain “not assessed.” 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.
EXPLANATION: For the 2012 reporting cycle, the Class A1 (primary contact recreation) uses for Green Castle Lake are assessed (monitored) as “partially supported” due to pH values that violate the Class A1 criterion. Additional results from the ISU and UHL surveys, however, suggest good water quality at Green Castle Lake. Using the median values from these surveys from 2006 through 2010 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 48, 48, and 54 respectively for Green Castle Lake. According to Carlson (1977) the Secchi depth and chlorophyll a values place Green Castle Lake in the mesotrophic category, while the value for total phosphorus places Green Castle Lake in the lower end of the eutrophic category. These values suggest extremely low levels of chlorophyll a and suspended algae in the water, exceptional water transparency, and low levels of phosphorus in the water column.
The levels of inorganic suspended solids at this lake were very low and do not suggest potential problems related to non-algal turbidity. The median level of inorganic suspended solids in Green Castle Lake (2.0 mg/L) was the 16th lowest median of the 134 lakes sampled by the ISU and UHL programs.
Data from the 2006-2010 ISU and UHL surveys suggest small populations of cyanobacteria exist at Green Castle Lake that do not suggest problems due to nuisance algae blooms. These data show that cyanobacteria comprised 75% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (4.4 mg/L) was the 7th lowest of the 134 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed as “partially supported” due to violations of the Class B(LW) criterion for pH. The ISU and UHL lake surveys data from 2006-2010 show no violations of the Class B(LW) criteria for ammonia in 22 samples. There was one violation of the Class B(LW) criterion for dissolved oxygen in 22 samples (5%). Based on IDNR's assessment methodology this violation is not significantly greater than 10% of the samples and therefore does not suggest impairment of the Class B(LW) uses. The pH data show 3 violations of the Class A1,B(LW) criterion for pH in 22 samples (14%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore does not indicate an impairment of the Class A1,B(LW) uses of Green Castle Lake. Green Castle Lake was assessed as “partially supporting” in the 2010 assessment/listing cycle due to significant violations of the pH criteria. Based on IDNR’s assessment methodology 2 consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the criterion are necessary to propose delisting based on pH violations. Because the current (2012) assessment/listing cycle is the first cycle where there were not significantly greater than 10% of the samples violating the criteria, this lake will remain assessed as "partially supported."
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
