Assessment Comments
Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in summers of 2006, 2007, and 2008, (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, (5) results from EPA/DNR fish tissue (RAFT) monitoring in 2005, and (6) an IDNR investigation of a fish kill in 2004.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to the occurrence of aesthetically objectionable conditions caused by poor water transparency related to algal and non-algal turbidity. Nuisance cyanobacteria blooms also contribute to impairment at this lake. The Class B(LW) uses are assessed (evaluated) as “partially supported” due to siltation impacts and a fish kill. Fish consumption uses remain assessed (monitored) as “fully supported.” Sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2006, 2007, and 2008, (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, (5) results from EPA/DNR fish tissue (RAFT) monitoring in 2005, and (6) an IDNR investigation of a fish kill in 2004.
EXPLANATION: Results of IDNR beach monitoring at Lake Manawa from 2006 through 2008 suggest that the Class A1 uses should be assessed (monitored) as “fully supported.” Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2006 (16 samples), 2007 (15 samples), and 2008 (27 samples) as part of the IDNR beach monitoring program. According to IDNR’s assessment methodology, all thirty-day geometric means for the three-year assessment period must be less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses. If a 5-sample, 30-day geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not 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 of all 46 thirty-day periods during the summer recreation seasons of 2006, 2007 and 2008 were below the Iowa water quality standard of 126 E. coli orgs/100 ml. These results suggest generally low levels of indicator bacteria at this lake. The percentage of samples exceeding the one-time maximum criteria of 235 orgs/100 ml was 0% in 2006, 0% in 2007 and 4% in 2008. Thus, these results do not suggest an impairment of the Class A1 uses of Lake Manawa.
Results from the ISU and UHL lake surveys, however, suggest that the Class A1 uses at Lake Manawa are assessed (monitored) as “not 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 73, 71, 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.
Based on data from the ISU and UHL lake surveys, the level of inorganic suspended solids was very high at this lake and suggests impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Lake Manawa was 13.0 mg/L, which was the 12th highest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a large population of cyanobacteria exists at Lake Manawa, which also contributes to impairment at this lake due to nuisance aquatic life. These data show that cyanobacteria comprised 89% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (37.1 mg/L) was also the 25th highest of the 132 lakes sampled. This median is in the worst 25% of the 132 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 2004-2008. 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. The assessment category for 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 (evaluated) as “partially supported” due the impacts of siltation and a fish kill in 2004. Information from the IDNR Fisheries Bureau suggests that common carp and algae blooms remain concerns at this lake. The ISU and UHL surveys show relatively good chemical water quality at Lake Manawa. Based on data from the ISU and UHL lake surveys from 2004 through 2008, there were no violations of the Class B(LW) criterion for ammonia in 24 samples or of the Class B(LW) criterion for dissolved oxygen in 25 samples. There were three violations of the Class A1,B(LW) criterion for pH in 25 samples (12%). Based on IDNR’s assessment methodology, however, these violations were not significantly greater than 10% of the samples and therefore do not constitute an impairment 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 assessed (monitored) 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. 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 at Lake Manawa 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.
