South Twin Lake IA 04-RAC-1168

Assessment Comments

Assessment remains based on information from the IDNR Wildlife Bureau.

Basis for Assessment

SUMMARY:  The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supported" based on information from the IDNR Wildlife Bureau.   The fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this wetland.   This is the same assessment as that developed for the 2002 and 2004 assessment/listing cycles.

EXPLANATION:  The previous (2000) assessment of support of the Class B(LW) uses ("partially supported") was reviewed by the DNR Wildlife Bureau in 2002.   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.   The assessment category for this wetland of "evaluated" indicates that the assessment is based entirely on "best professional judgement."  In terms of Section 305(b) reporting, "monitored assessments" are based primarily on recent, site-specific ambient monitoring data and thus have relatively high confidence.   IDNR considers waterbodies identified as "impaired" based on "monitored assessments" as candidates for the state's Section 303(d) list.   "Evaluated assessments" are those based on data older than five years or other than site-specific ambient monitoring data (e.g., questionnaire surveys of fish and game biologists [=best professional judgement]) and thus have relatively lower confidence.   IDNR does not consider waterbodies identified as "impaired" based on "evaluated assessments" as candidates for the state's Section 303(d) list.  

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.