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), (3) information from the IDNR Fisheries Bureau, and (4) results of a fish kill investigation on August 16, 2007.
Basis for Assessment
SUMMARY: The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to high levels of pH that exceed Iowa’s Class B(LW) criterion. A fish kill on August 16, 2007 also suggests impairment of the Class B(LW) uses. However, in terms of nutrient concentrations, water clarity, and algae populations monitoring data suggest excellent water quality at this lake. Fish consumption uses remain “not assessed” due to a lack of information on which to base an assessment. 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), (3) information from the IDNR Fisheries Bureau, and (4) results of a fish kill investigation on August 16, 2007.
EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class B(LW) uses at Moorehead Lake are “partially supported.” Using the median values from these surveys from 2004 through 2008 (approximately 25 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 54, 60, and 62 respectively for Moorehead Lake. According to Carlson (1977) the chlorophyll a and Secchi depth values place Moorehead Lake in the eutrophic category, while the total phosphorus value places Moorehead Lake in between the eutrophic and hypereutrophic categories. These values suggest low levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and moderately high 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 Moorehead Lake was 2.0 mg/L, which was the 30th lowest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a small population of cyanobacteria exists at Moorehead Lake, which does not contribute to impairment at this lake. These data show that cyanobacteria comprised 43% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (6.2 mg/L) was the 33rd lowest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to violations of the Class B(LW) criterion for pH. Results of the ISU and UHL lake surveys from 2004-2008 show no violations of the Class B(LW) criterion for ammonia in 25 samples and no violations of the Class B(LW) criterion for dissolved oxygen in 25 samples. However, there were 7 violations of the Class B(LW) criterion for pH in 25 samples (28%). Based on IDNR’s assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest an impairment (partial support/monitored) of the Class B(LW) uses of Moorehead Lake.
Results of a fish kill investigation that occurred on August 16, 2007 also suggest “partial support” of the Class B(LW) uses of Moorehead Lake. The fish kill was attributed to low oxygen conditions caused by an algae die-off (natural conditions/non-pollutant). The number and type of fish killed was not reported. This is the same assessment as that developed for the 2008 assessment/listing cycle. The continuance of the IR Category __b listing is based on IDNR's 2010 assessment methodology that states the following: 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 (2006-2009) 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 IDNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated” and will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation.
According to the IDNR's Fisheries Bureau, the fish population lake was renovated in 2009.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
