Diamond Lake IA 06-LSR-1672

Assessment Comments

Assessment is based on results of water quality, aquatic vegetation, and fish monitoring conducted by the IDNR shallow lakes and wetlands monitoring program from 2009 through 2010; also, information from the IDNR Wildlife Bureau.

Basis for Assessment

SUMMARY:  The Class B(LW) aquatic life uses are assessed (monitored) as "fully supported."  The fish consumption uses remain "not assessed" due to the lack of water quality information upon which to base an assessment.   Sources of data for this assessment include results of water quality, aquatic vegetation, and fish monitoring conducted by the IDNR shallow lakes and wetlands monitoring program from 2009 through 2010 and information from the IDNR Wildlife Bureau.

EXPLANATION:  Results of water quality monitoring conducted at Diamond Lake indicate that the aquatic life uses should be assessed (monitored) as "fully supported."  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 2009 through 2010 (approximately 10 samples), the median growing season total suspended solids concentration was 6 mg/L; this median value is well below the UMRCC guideline.  

In addition, the level of algae also suggests "full support" of the aquatic life uses at Diamond Lake.   The Carlson’s (1977) trophic state index value for chlorophyll a was 61.    According to Carlson (1977), the index value for chlorophyll a places Diamond Lake in the eutrophic category, which suggests moderately high levels of chlorophyll a and suspended algae in the water.    

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.

NOTE:  The water level at Diamond Lake was lowered in 2007 for a restoration project that renovated fish populations and increased rooted aquatic vegetation at this lake.   The goals of this project were to improve wildlife habitat and water quality at this shallow lake.