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 2006 through 2008 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 poor water clarity caused by high levels of turbidity and algae in the water column. The fish consumption uses remain "not assessed" due to the lack of water quality information upon which to base an assessment. In general, Iowa wetlands are not managed for recreational fisheries; thus, fish contaminant monitoring is not typically conducted at these waterbodies.
EXPLANATION: Results of water quality monitoring conducted at South Twin Lake indicate that the aquatic life uses should be assessed (monitored) as "not supported" due to high levels of algae and turbidity at this lake. Using the median values from this survey from 2006 through 2008 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 75, 74, and 74 respectively for South Twin Lake. According to Carlson (1977) the index values for Secchi depth, chlorophyll a, and total phosphorus all place South Twin Lake in the hypereutrophic category. These values suggest very high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and very high levels of phosphorus in the water column.
Additional monitoring of fish and aquatic vegetation populations at South Twin Lake suggest that the high levels of algae and turbidity have adverse effects on fish populations and limit the production of rooted aquatic vegetation in this wetland and therefore cause the aquatic life uses to 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.
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.
