Roberts Creek Lake IA 04-LDM-1019
Marion County S4T76NR19W 6 mi NE of Knoxville.
- Cycle
- 2018
- Release Status
- Final
- Overall IR
- 5 - Water is impaired or threatened and a TMDL is needed.
- Trophic
- Eutrophic
- Trend
- Stable
- Created
- 4/23/2019 8:03:22 AM
- Updated
- 4/23/2019 8:14:03 AM
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 2012 through 2016 by Iowa State University (ISU), (2) results of the TMDL monitoring conducted in 2015 by Iowa Department of Natural resources (IDNR), and (3) information from the IDNR Fisheries Bureau.
For the 2018 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 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, Carlson 's (1977) trophic state indices for Secchi depth (15 samples), chlorophyll a (27 samples), and total phosphorus (28 samples) were 70, 67, and 72 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 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 very 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 125th among the 138 lakes by the ISU lake survey. Data from the 2012-2016 ISU lake survey suggest a moderate population of cyanobacteria exists at Roberts Creek Lake. These data show that cyanobacteria comprised 44% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (7.1 mg/L) was ranked 41st of the 138 lakes sampled. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” based on information from the DNR Fisheries Bureau and results from the ISU lake surveys. Based on information from the DNR 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 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 no violations of the criterion for pH in 15 samples(0%). 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 Roberts Creek Lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.