Lake Manawa IA 06-WEM-1711

Results of IDNR beach monitoring from 2012 through 2014 suggest that the Class A1 uses are "Fully Supported." Levels of indicator bacteria at Lake Manawa Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2012 (15 samples), 2013 (15 samples) and 2014 (14 samples), as part of the IDNR beach monitoring program.   According to IDNR’s assessment methodology two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses: (1) the geometric mean of the samples from each recreation season of the three-year assessment period are less than the state's geometric mean criterion of 126 E.  coli orgs/100 ml and (2) not more than 10% of the samples during any one recreation season exceeds the state's single-sample maximum value of 235 E.  coli orgs/100 ml. If a sampling season geometric mean exceeds the state criterion of 1000 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, if a sampling season geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period and/or if significantly more than 10% of the samples in any one of the three recreation seasons exceed Iowa's single-sample maximum value of 235 E.  coli orgs/100 ml, the Class A1 uses should be assessed as “partially supported.” This assessment approach is based on U.S.   EPA guidelines (see pgs 3-33 to 3-35 of U.S.   EPA 1997b).

At Lake Manawa Beach, the geometric means from 2012, 2013 and 2014 were all below the Iowa water quality standard of 126 E.  coli orgs/100 ml.   The geometric mean was 32 E.  coli orgs/100 ml in 2012, 20 E.  coli orgs/100 ml in 2013 and 15 E.  coli orgs/100 ml in 2014.   The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E.  coli orgs/100 ml) was 13% in 2012, 7% in 2013 and 0% in 2014.   None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses.   According to IDNR's assessment methodology and U.S.  EPA guidelines, these results suggest "Fully Supported" of the Class A1 uses.

For the 2016 assessment/listing cycle, however, the Class A1 (primary contact recreation) uses of Lake Manawa are assessed (monitored) as "not supported" due to poor water transparency and aesthetically objectionable conditions caused by algae blooms based on information from the ISU lake survey. Using the median values from these surveys from 2010-2014 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 73, 72, and 72 respectively for Lake Manawa. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Lake Manawa 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. The data show no violations of the Class A1 criterion for pH in 15 samples.

The level of inorganic suspended solids was high at Lake Manawa, and does suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Lake Manawa (9 mg/L) was ranked 112th among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a large population of cyanobacteria exists at Lake Manawa, which suggests the potential for an impairment due to nuisance aquatic life These data show that cyanobacteria comprised 94% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (155.8 mg/L) was ranked 135th of the 138 lakes sampled. This median is in the worst 25% of the 138 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa's narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values from 2010-2014. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. Assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence.

The Class B(LW) (aquatic life) uses are assessed (monitored) as "fully supported." Results of the ISU lake survey from 2010-2014 show there were no violations of the criterion for ammonia in 15 samples(0%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and no violations of the criterion for pH in 15 samples(0%). Based on IDNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of Lake Manawa.

The occurrence of a fish kill in this lake in May 2004 suggests that the Class B(LW) aquatic life uses should be assessed (evaluated) as "partially supported.”  This kill occurred on or before May 10, 2004.   The cause of the kill was identified as disease (bacterial).   An estimated 50 fish were killed; no estimated value of the fish killed was provided.   The species killed were not identified.   The fish kill portion of this assessment is the same as that developed for the 2008 assessment/listing cycle.   The continuance of the IR Category 3b 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.   Thus, this impairment will remain in Category 3b of Iowa’s 2010 Integrated Report.

Fish consumption uses were “not assessed” due to a lack of recent fish contaminant monitoring at this lake.    The most recent fish tissue monitoring was conducted in 2005. Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S.  EPA/IDNR fish contaminant (RAFT) monitoring at Lake Manawa in 2005.   The composite samples of fillets from channel catfish and largemouth bass had low levels of contaminants.   Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.026 ppm; total PCBs: <0.09 ppm; and technical chlordane: <0.03 ppm.   Levels of primary contaminants in the composite sample of largemouth bass fillets were as follows: mercury: 0.0314 ppm; total PCBs: <0.09 ppm; and technical chlordane: <0.03 ppm.   Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).   The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses.   The fish contaminant data generated from the 2005 RAFT sampling conducted in this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory for this waterbody.