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 2010-2012 and information from the IDNR Wildlife Bureau.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses are assessed (monitored) as "not supported" due to high levels of total suspended solids that leads to an impairment of the narrative criteria protecting against undesirable or nuisance aquatic life. High levels of algae at this wetland also suggest impairment due to aesthetically objectionable conditions. 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 2010-2011 and information from the IDNR Wildlife Bureau.
EXPLANATION: Results of water quality monitoring conducted at Virgin Lake indicate that the aquatic life uses should be assessed (monitored) as "not supported" due to high levels of total suspended solids and 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 2010-2011 (approximately 10 samples), the median growing season total suspended solids concentration was 52 mg/L; this median value exceeds 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 Virgin Lake are assessed as “not supported” due to high levels of total suspended solids.
In addition, the level of algae also suggests impairment of the aquatic life uses at Virgin Lake. The Carlson’s (1977) trophic state index value for chlorophyll a was 79. According to Carlson (1977), the index value for chlorophyll a places Virgin Lake in the hypereutrophic category and suggests extremely 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.
Additional monitoring of fish and aquatic vegetation populations at Virgin Lake by IDNR staff also suggests that the high levels of total suspended solids and algae have adverse effects on fish populations and limit the production of rooted aquatic vegetation in this wetland. These results further indicate that the aquatic life uses of this wetland should be assessed as "not supported."
