Swan Lake IA 04-RAC-1199
Carroll County S31T84NR34W 3 mi. SE of Carroll.
- Cycle
- 2018
- Release Status
- Final
- Overall IR
- 5 - Water is impaired or threatened and a TMDL is needed.
- Trophic
- Eutrophic
- Trend
- Unknown
- Created
- 5/7/2019 3:17:01 PM
- Updated
- 6/11/2019 1:48:18 PM
The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to poor water transparency and nuisance algae blooms that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions as well as violations of the state’s pH criterion. The Class B(LW) (aquatic life) uses are assessed (monitored) as “not supported” due to violations of the state’s criterion for DO, pH and excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. Fish consumption uses are considered “not assessed” due to the age of the data upon which the previous assessment (“fully supported”) was based. Sources of data for this assessment include (1) results of IDNR/UHL beach monitoring from 2014 through 2016, (2) results of the statewide survey of Iowa lakes conducted from 2012 through 2016 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, (4) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2003, and (5) results of a fish kill investigation in July 2015.
For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Swan Lake are assessed (monitored) as "not supported" due to poor water transparency, aesthetically objectionable conditions caused by algae blooms and due to frequent violations of the state criterion for pH based on information from the ISU lake survey. Using the median values from these surveys from 2012-2016 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 65, 67, and 86 respectively for Swan Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Swan Lake in between the Eutrophic and the Hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, poor water transparency, and extremely high levels of phosphorus in the water column. The data show 4 violations of the Class A1 criterion for pH in 15 samples (27%). The level of inorganic suspended solids was moderately high at Swan Lake, and does suggest that non-algal turbidity may contribute to the impairment at this lake. The median level of inorganic suspended solids in Swan Lake (2.9 mg/L) was ranked 65th among the 138 lakes by the ISU lake survey. Data from the 2012-2016 ISU lake survey suggest a moderately large population of cyanobacteria exists at Swan Lake. These data show that cyanobacteria comprised 37% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (18.8 mg/L) was ranked 80th of the 138 lakes sampled. The Class B(LW) (aquatic life) uses are assessed (monitored) as "not supported" due to violations of the Class B(LW) criterion for DO, pH,and results of a fish kill investigation in 2015. Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 15 samples(0%), 4 violations of the criterion for dissolved oxygen in 15 samples(27%), and 4 violations of the criterion for pH in 15 samples(27%). Based on DNR's assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment (not supported/monitored) of the Class B(LW) uses of Swan Lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
Although attributed to "natural causes", the occurrence of fish kill in this lake in October 2015is consistent with the assessment of aquatic life uses as “not supported.”The kill occurred on or before October 23, 2015. The cause of the kill was identified as dissolved oxygen that dropped to lethal levels. An estimated 1,000 fish were killed. No estimate of the value of the fish killed was provided. According to DNR’s assessment/listing methodology, 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 (2012-2014) 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 DNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated.”Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5). Waterbodies affected by such fish kills will be placed in IR subcategories 3b and will be added to the state list of waters in need of further investigation. Thus, this assessment segment will be placed in Category 3b of Iowa’s 2018 Integrated Report.