Dan Greene Slough IA 06-LSR-1634

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 in 2011-2012 and information from the IDNR Wildlife Bureau.

Basis for Assessment

SUMMARY:  The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supported." Dan Creen Slough was listed as "not supporting" (evaluated) its designated uses in the 2012 assessment/listing cycle.  Monitoring in 2011 and 2012 suggest dramatic improvements at the wetland and suggest that the wetland should be assessed (monitored) as "fully supported" due to overwhelming evidence.  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 in 2011-2012 and information from the IDNR Wildlife Bureau.
Note: Dan Greene Slough was drawn down and rough fish removed between 2008 and 2010.

EXPLANATION:  Results of water quality monitoring conducted at Dan Greene Slough in 2011-2012 (approximately 10 samples) 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 2011-2012  (approximately 10 samples), the median growing season total suspended solids concentration was 3 mg/L and suggests "full supporr" of the Class A1 and B(LW) uses.   This median value is well below the UMRCC guideline.   Thus, based on (1) suppression of SAV growth, (2) the resultant production of undesirable aquatic life, and (3) IDNR’s assessment and listing methodology, the aquatic life uses at Dan Greene Slough are assessed as “fully supported.”

In addition, the level of algae also suggests "full support" of the aquatic life uses at Dan Greene Slough.   The Carlson’s (1977) trophic state index value for chlorophyll a was 57.    According to Carlson (1977), the index value for chlorophyll a places Dan Greene Slough in the eutrophic category.    This value suggests relatively low 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 instances, 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.

Additional monitoring of fish and aquatic vegetation populations at Dan Greene Slough by IDNR staff also suggests that the water quality has improved in this wetland.   These results further indicate that the aquatic life uses of this wetland should be assessed as "fully supported."