ADBNet

Assessment Comments

Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.

Basis for Assessment

SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “fully supported.”  The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.”  Fish consumption uses are “not assessed” due to a lack of information to base an assessment on.   Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.

EXPLANATION: Results from the ISU and UHL lake surveys also suggest that the Class A1 uses at Nelson Lake are “fully supported.”  Using the median values from these surveys from 2004 through 2008 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 50, 56, and 55 respectively for Nelson Lake.   According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Nelson Lake in the eutrophic category.   These values suggest relatively low levels of chlorophyll a and suspended algae in the water, very good water transparency, and relatively low levels of phosphorus in the water column.  

The level of inorganic suspended solids is low at this lake and does not suggest impairment due to high non-algal turbidity.   The median inorganic suspended solids concentration at Nelson Lake was 1.9 mg/L, which was the 14th lowest of the 132 monitored lakes.

Data from the 2004-2008 ISU and UHL surveys suggest a small population of cyanobacteria exists at Nelson Lake, which does not suggest impairment at this lake.   These data show that cyanobacteria comprised only 28% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (2.2 mg/L) was also the 11th lowest of the 132 lakes sampled.  

The Class B(LW) (aquatic life) uses are assessed as “fully supported” based on information from the IDNR Fisheries Bureau and results from the ISU and UHL lake surveys.   The ISU and UHL lake surveys show that during 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 22 samples or dissolved oxygen in 23 samples.   There were 2 violations of the Class A1,B(LW) criterion for pH in 23 samples (9%).   Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class B(LW) uses of Nelson Lake.

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.