Lake Manawa IA 06-WEM-1711

Results of DNR beach monitoring from 2014 through 2016 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 2014 (14 samples), 2015 (15 samples) and 2016 (15 samples), as part of the DNR beach monitoring program. According to DNR’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 2014, 2015 and 2016 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 15 E. coli orgs/100 ml in 2014, 17 E. coli orgs/100 ml in 2015 and 45 E. coli orgs/100 ml in 2016. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 0% in 2014, 0% in 2015 and 20% in 2016. None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses. According to DNR's assessment methodology and U.S. EPA guidelines, these results suggest "Fully Supported" of the Class A1 uses.

For the 2018 assessment/listing cycle, 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 and DNR TMDL monitoring. Using the median values from these surveys from 2012-2016 (approximately 20 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 77, 74, and 74 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, extremely 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 20 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 (10 mg/L) was ranked 120th among the 138 lakes by the ISU lake survey.

Data from the 2012-2016 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 92% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (150.1 mg/L) was ranked 132nd 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 2012-2016. 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 2012-2016 show there were no violations of the criterion for ammonia in 20 samples(0%), no violations of the criterion for dissolved oxygen in 20 samples(0%), and no violations of the criterion for pH in 20 samples(0%). Based on DNR'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.

Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S. DNR fish contaminant monitoring at Lake Manawain 2015. 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: 0.02 ppm; total PCBs: <0.06 ppm; and technical chlordane: <0.02 ppm. Levels of primary contaminants in the composite sample of largemouth bass were as follows: mercury: 0.050 ppm. 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 2015 sampling conducted at this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.