Twelve-mile Lake IA 04-UDM-1231

Results of water quality monitoring conducted at Twelve-mile Lake indicate that the aquatic life uses should be assessed (monitored) as "not supported" due to high levels of algae at this lake.  

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 15 samples), the median growing season total suspended solids concentration was 74 mg/L; this median value is well above the UMRCC guideline and suggests "no support" of the aquatic life uses.      

In addition, the level of algae also suggests "no support" of the aquatic life uses at Twelve Mile Lake. The Carlson’s (1977) trophic state index value for chlorophyll a was 84. According to Carlson (1977), the index value for chlorophyll a places Twelve Mile Lake in the hypereutrophic category.  This value suggests extremely high levels of chlorophyll a and suspended algae in the water and "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.

The Class B(LW) (aquatic life) uses remain assessed (evaluated) as "partially supported" based on the results of a fish kill investigation in March 2007.   This kill occurred on or before March 13, 2007.   The kill was attributed to “unknown” causes with natural causes suspected.   About 1,000 dead bullheads and 100 minnows were found along the southeast side of the shallow lake, not far from where manure runoff was entering the lake.  The fish kill could not be directly tied to the manure runoff as the fish had been dead for some time.   The fish likely died from low oxygen levels over the winter.   According to DNR environmental investigators and fisheries staff, the fish kill cannot be directly attributed to runoff from the feedlot, as the fish appear to have been dead for weeks.  

This is the same assessment as that developed for the 2008, 2010, and 2012 assessment/listing cycles.   The continuance of the IR Category 3b listing is based on IDNR's assessment methodology that states the following:  the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired”.   If a cause of the kill was not identified during the IDNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated” and will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation.   Thus, this assessment segment will remain in Category 3b of Iowa’s Integrated Report.  

Fish consumption uses remain "not assessed.”  In general, Iowa wetlands are not managed for recreational fisheries; thus, fish contaminant monitoring is not typically conducted at these waterbodies.