Swan Lake IA 04-UDM-1232

Results of water quality monitoring conducted at Little Swan Lakes indicate that the aquatic life uses should be assessed (evaluated) as "not supported" due to high levels of total suspended solids and algae at this lake.This assessment is considered "evaluated" because only 6samples were collected in2014during the current assessment period (2014-2016).Based on DNR's assessment methodology, 9 samples within the most recent three year period are necessary to accurately characterize water quality for the purposes of identifying impaired waters.

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 DNR shallow lakes and wetlands monitoring program from 2014 through 2016 (approximately 6 samples), the median growing season total suspended solids concentration was 68 mg/L; this median value is well above the UMRCC guideline and suggests "evaluated/no support" of the aquatic life uses.

In addition, the level of algae also suggests "no support" of the aquatic life uses at Little Swan Lake. The Carlson’s (1977) trophic state index value for chlorophyll a was 69. According to Carlson (1977), the index value for chlorophyll a places Little Swan Lakes in the eutrophic category. This value suggests very high levels of chlorophyll a and suspended algae in the water and "evaluated/no 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.

According to the DNR Wildlife Bureau, a fish kill in April 2004 in Little Swan Lakes suggests that the aquatic life uses of this wetland should be assessed (evaluated) as only "partially supported". This kill occurred on April 29, 2004; the cause of the kill was identified as natural/winterkill. An estimated 30 fish were killed including common carp, channel catfish and yellow perch. No estimate of the value of the fish killed was provided. Little Swan Lake is part of the “Swan Lake and Christopherson Slough State Wildlife Management Area in northeastern Dickinson County. According to the DNR investigation, a fish survey following the kill showed live fish in Little Swan Lake (i.e., bullheads, carp, buffalo, crappie and northern pike).

According to DNR’s assessment/listing methodology, the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired”. If a cause of the kill was not identified during the DNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated.”Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5). Waterbodies affected by such fish kills will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation. This type of impairment, however, may not be appropriate for publicly-owned wetlands that are not managed as recreational fisheries. Regardless, this assessment segment will remain in Category 3b of Iowa’s Integrated Report.

Fish consumption uses remain "not assessed.”In general, Iowa wetlands are not managed for recreational fisheries; thus, fish contaminant monitoring is not typically conducted at these waterbodies.