Pleasant Lake IA 06-LSR-1649
Dickinson County S7T99NR35W 5 mi. SE of Orleans.
The Class B(LW) aquatic life uses are assessed (monitored) as "partially supported" due to high levels of algae and turbidity at this wetland that suggest impairment due to adverse impacts on plant/animal communities. 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 chemistry, aquatic vegetation, and fish monitoring conducted in 2014-2016 by the IDNR shallow lakes and wetland monitoring program and information from the IDNR Wildlife Bureau.
Results of water quality monitoring conducted at Pleasant Lake indicate that the aquatic life uses should be assessed (monitored) as "partiallysupported" due to high levels of algae and turbidity at this lake. Using the overall median values from the DNR shallow lakes and wetlands monitoring program from 2014 through 2015 (approximately 10 samples), the median growing season total suspended solids concentration was 58.5 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, however, suggests "partial support" of the aquatic life uses at Pleasant Lake. The Carlson’s (1977) trophic state index value for chlorophyll a was 75. According to Carlson (1977), the index value for chlorophyll a places Pleasant Lake in the hypereutrophic category. This value suggests high levels of chlorophyll a and suspended algae in the water and "partial 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.
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.