Green Castle Lake IA 02-IOW-643

Assessment Comments

Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 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 “fully supported.”  The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” Fish consumption uses remain “not assessed.” Note: For the 2010 assessment/listing cycle, Green Castle Lake was listed as "partially supporting" due to high levels of pH that violated the state criterion.  Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.

EXPLANATION: For the 2014 reporting cycle, the Class A1 (primary contact recreation) uses for Green Castle Lake are assessed (monitored) as “fully supported.”   Results from the ISU and UHL surveys suggest good water quality at Green Castle Lake.   Using the median values from these surveys from 2009-2012 (approximately 14 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 64, 59, and 60 respectively for Green Castle Lake.   According to Carlson (1977) the chlorophyll a value places Green Castle Lake in the eutrophic category, while the value for total phosphorus and Secchi depth values place Green Castle Lake in between the eutrophic and hypereutrophic categories.   These values suggest relatively low levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and moderately high levels of phosphorus in the water column.  

The levels of inorganic suspended solids at this lake were moderately high but do not suggest potential problems related to non-algal turbidity.   The median level of inorganic suspended solids in Green Castle Lake (6.1 mg/L) and ranked 89th of the 134 lakes sampled by the ISU and UHL programs.

Data from the 2008-2012 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 61% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (10.9 mg/L) and ranked 29th of the 134 lakes sampled.  

The Class B(LW) (aquatic life) uses are assessed as “fully supported.”  The ISU and UHL lake surveys data from 2008-2012 show no violations of the Class B(LW) criteria for ammonia in 14 samples, no violations for dissolved oxygen in 14 samples and no violations for pH in 14 samples.  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 violations of the pH criteria were not significantly greater than 10% for the 2012 assessment/listing cycle or the current (2014) assessment/listing cycle the pH impairment for Green Castle Lake is suggested for delisting.  
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.