South Twin Lake IA 04-RAC-1168

Sources of Data:

Class BLW - TSI's

• 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.
• Although the index value for Chlorophyll a is below the impairment trigger of 65 for this assessment cycle, South Twin Lake was listed as partially supporting its Class BLW uses due to aesthetically objectionable conditions. Based on DNR's methodology, the median TSI value for Chlorophyll a must be 63 or less for two consecutive assessment/listing cycles before a lake can be removed from the state's Section 303(d) list (IR Category 5). Therefore, South Twin Lake will remain listed as "not supported" for the 2020 assessment/listing cycle.

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 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.

Class HH - No Data

Additional Notes:

• Additional monitoring of fish and aquatic vegetation populations at South Twin Lake by DNR 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."
• According to the local DNR Wildlife Biologist (Mahn), siltation at South Twin Lake is a problem but probably not a serious one because (1) the wetland has a rather small watershed and (2) most of the contributing watershed comes from the outlet of North Twin Lake and contains relatively little silt. The majority of the land immediately surrounding South Twin Lake, however, is intensively farmed and thus contributes to a siltation problem. Some progress has been made on the north side of the lake on private property with the establishment of grass that reduces silt. DNR has purchased a small tract on the southwest side of the lake that includes a small wetland and some adjacent uplands which have been seeded to native grasses. The primary water quality problems at South Twin Lake are increased turbidity, reduced light penetration, suppressed aquatic vegetation, and an accelerated eutrophication of this wetland. A large populations of common carp population exists in the wetland and no doubt plays a significant role in the turbidity problem and the suppression of rooted aquatic vegetation.