Roberts Creek Lake IA 04-LDM-1019
Marion County S4T76NR19W 6 mi NE of Knoxville.
- Cycle
- 2016
- Release Status
- Final
- Overall IR
- 5 - Water is impaired or threatened and a TMDL is needed.
- Trophic
- Eutrophic
- Trend
- Stable
- Created
- 6/10/2016 9:07:02 AM
- Updated
- 12/19/2016 11:06:14 AM
| Cycle Added | Class | Cause | Data Source | Rationale |
|---|---|---|---|---|
| 2008 | Class A1 | Algal Growth: Chlorophyll a | Ambient monitoring: Iowa DNR-lakes | New data: WQ improvement (chemical / physical / bacterial) |
The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to poor water transparency caused by high levels of inorganic turbidity and algae. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” (IR 3b) due to high levels of inorganic turbidity. Fish consumption uses remain “not assessed.” Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2010 through 2014 by Iowa State University (ISU), and (2) information from the IDNR Fisheries Bureau.
For the 2016 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Roberts Creek Lake are assessed (monitored) as "not supported" due to poor water transparency 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 70, 62, and 70 respectively for Roberts Creek Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Roberts Creek Lake in between the Eutrophic and the Hypereutrophic categories. These values suggest moderately 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. IDNR's assessment methodology indicates that at least two assessment/listing cycles with a TSI value less than or equal to 63 are necessary to suggest de-listing of an impairment. The chlorophyll a TSI value for the 2008 assessment listing cycle was 66, which based on IDNR's assessment/listing methodology indicated an impairment based on the narrative criteria protecting against aesthetically objectionable conditions. Because the TSI value for the 2014 assessment/listing cycle was less than the trigger of 65 (2014 listing for chlorophyll a was 61), and because the TSI value for the current listing cycle is also 62, based on IDNR's methodology, the impairment for chlorophyll a is suggested for delisting for the 2016 assessment/listing cycle. The level of inorganic suspended solids was high at Roberts Creek Lake, and does suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Roberts Creek Lake (11.4 mg/L) was ranked 124th among the 138 lakes by the ISU lake survey. Data from the 2010-2014 ISU lake survey suggest a small population of cyanobacteria exists at Roberts Creek Lake. These data show that cyanobacteria comprised 73% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (11.2 mg/L) was ranked 32nd of the 138 lakes sampled. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” based on information from the IDNR Fisheries Bureau and results from the ISU lake surveys. Based on information from the IDNR Fisheries Bureau, inorganic turbidity contributes to impairment of the Class B(LW) uses of Roberts Creek Lake due to common carp. However, the results of the ISU lake survey from 2010-2014 show there were no violations of the criterion for ammonia in 15 samples(0%), 3 violations of the criterion for dissolved oxygen in 15 samples(20%), 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 Roberts Creek Lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.