Swan Lake IA 04-UDM-1232

Data Sources:

Class BLW - Lake TSI's

The TSI value for Secchi depth is not used to evaluate the attainment of the aquatic life use 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 BLW use, but the index value is limited by the depth of the lake. Thus, the combination of total suspended solids and chlorophyll a was used to determine whether or not the Class BLW use was impaired in these shallow systems.

DNR's assessment methodology indicates that a TSI value less than or equal to 63 are necessary to suggest de-listing of an impairment. The chlorophyll a TSI value for the 2020 assessment listing cycle was 67, which based on DNR's assessment/listing methodology indicated an impairment based on the narrative criteria protecting against adverse conditions that impacts on plant/animal communities. Because the median TSI value for the 2026 assessment/listing cycle was 60 which was less than or equal to the delisting trigger of 63, based on DNR's methodology, these values suggest "full support" of the Class BLW uses protecting against adverse conditions that impacts on plant/animal communities and the impairment (5a) for chlorophyll a is suggested for delisting for the 2026 assessment/listing cycle.

Class BLW - Binomial Parameters:

Class BLW - Protection of Submerged Aquatic Vegetation:

Guidelines for wetland assessment from the Upper Mississippi River Conservation Committee’s Water Quality Technical Section (2003) state that a median 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 (SAV) in wetlands and shallow lakes. High levels of total suspended solids impede the growth of SAV. SAV 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.

DNR's assessment methodology indicates that a median growing season total suspended solids concentration of 30 mg/L with at least 10 samples collected over 3 years is needed to suggest de-listing of the SAV impairment. The total suspended solids concentration for the 2020 assessment listing cycle was 48, which based on DNR's assessment/listing methodology indicated an impairment. Because the total suspended solids concentration for the current listing cycle was 6, based on DNR's methodology, this value suggest "full support" of the Class BLW uses protecting against adverse conditions that impacts on plant/animal communities and the impairment (5a) for SAV is suggested for delisting for the 2026 assessment/listing cycle.

Class BLW - Toxic Parameters:

Class HH - No Data