Prairie Lake IA 06-LSR-1647

Results of water quality monitoring conducted at Prairie Lake indicate that the aquatic life uses should be assessed (evaluated) as "not supported" due to high levels of algae at this lake.  This assessment is considered "evaluated" because only 5 samples were collected in 2014 during the current assessment period (2012-2014).   Based on IDNR's assessment methodology, 9 samples within the most recent three year period are necessary to accurately characterize water quality for the purposes of identifying impaired waters.  

Guidelines for wetland assessment from the Upper Mississippi River Conservation Committee’s Water Quality Technical Section (2003) state that an average total suspended solids concentration during the growing season of less than 30 mg/L is necessary to provide sufficient water clarity to support the growth of submersed aquatic vegetation in wetlands and shallow lakes.   High levels of total suspended solids impede the growth of submersed aquatic vegetation (SAV).   Submersed aquatic vegetation is critical to the health of wetland and shallow lake ecosystems.   The elimination of SAV can degrade habitat quality such that undesirable aquatic species such as cyanobacteria, common carp and fathead minnows dominate.   As such, the suppression of SAV constitutes a violation of Iowa’s narrative water quality criteria protecting against undesirable or nuisance aquatic life.  

Using the overall median values from the IDNR shallow lakes and wetlands monitoring program from 2012 through 2014 (approximately 5 samples), the median growing season total suspended solids concentration was 20 mg/L; this median value is well below the UMRCC guideline and suggests "full support" of the aquatic life uses.     

In addition, the level of algae, however, suggests "no support" of the aquatic life uses at Prairie Lake. The Carlson’s (1977) trophic state index value for chlorophyll a was 70. According to Carlson (1977), the index value for chlorophyll a places Prairie Lake in the eutrophic category.  This value suggests very high levels of chlorophyll a and suspended algae in the water and "evaluated/no support" of the aquatic life uses.

NOTE:  The TSI value for Secchi depth is not used to evaluate the attainment of aquatic life goals in shallow lakes.   Due to the depth of these shallow lakes, TSI values for Secchi depth can be misleading.   In some instances the Secchi disk remains visible at the bottom of the lake and the depth of the lake is recorded as the Secchi depth.   In these instance, water clarity may be sufficient to support the Class B(LW) uses, but the index value is limited by the depth of the lake.   Thus the combination of total suspended solids and chlorophyll a will be used to determine whether or not the Class B(LW) uses are impaired in these shallow systems.