Carter Lake IA 06-WEM-1714

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Carter Lake are assessed (monitored) as "not supported" 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 49, 58, and 60 respectively for Carter Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Carter Lake in between the Mesotrophic and the Eutrophic categories. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, exceptional water transparency, and moderately high levels of phosphorus in the water column. The data show 5 violations of the Class A1 criterion for pH in 15 samples (33%).

DNR'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 Secchi TSI value for the 2012 assessment listing cycle was 76 and the Chlorophyll a TSI values for the2002 and 2004 assessment listing cycle were 76 and 78, which based on DNR's assessment/listing methodology indicated an impairment based on the narrative criteria protecting against aesthetically objectionable conditions. Because the Secchi TSI value for the 2016 and 2018 assessment/listing cycles were both less than the trigger of 65 (2016 listing was 53 and 2018 listing is 49) and becausethe Chlorophyll a TSI value for the 2016 and 2018 assessment/listing cycles were both less than the trigger of 65 (2016 listing was 60 and 2018 listing is 58), based on DNR's methodology, these values suggest "full support" of the Class A1 uses protecting against aesthetically objectionable conditions.

Note: A TMDL for algae/algal toxins, chlorophyll a, total phosphorus, total nitrogen, and pH at Carter Lake was prepared by DNR and the Nebraska Department of Environmental Management and approved by EPA in 2007. Because all Section 303(d) impairments identified for the 2016 assessment/listing cycle (pH) are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).

The level of inorganic suspended solids was low at Carter Lake, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Carter Lake (1.6 mg/L) was ranked 9th 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 Carter Lake. These data show that cyanobacteria comprised 90% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (12.7 mg/L) was ranked 56th of the 138 lakes sampled.

According to Iowa DNR fisheries biologists, Carter Lake continues to see drastically improved water clarity as a result of the 2010 fishery renovation, alum treatment, dredging, wetland constructionand100acreno wake zone. Toxic algae blooms have declined, phosphorous and nitrogen concentrations are lower, and water clarity has increased. Aquatic vegetation growth has been extreme and difficult to control particularly in the no-wake boating zone. The City of Carter Lake and Omaha have two mechanical weed harvesters. Eurasian Water Milfoil (EWM) was discovered in the lake in 2015 and a whole lake treatment with sonar a.s. was done in the spring of 2016 in an attempt to eradicate EWM. While common carp are back in the system and eventually will have an impact on aquatic plants and water clarity.

Although attributed to "natural causes", the occurrence of fish kills in this lake in April 2004and March 2010 is consistent with the assessment of aquatic life uses as “not supported.”The first of the kills occurred on April 22, 2004. The cause of the kill was identified as disease (virus). An estimated 5,000 fish were killed; the kill affected only yellow bass. No estimate of the value of the fish killed was provided. The second of the kills occurred on March 5, 2010 and was attributed to natural causes (winterkill). Approximately 100 fish were killed from what was suspected to be low dissolved oxygen due to thick ice and snow cover over the winter. No estimate of the value of the fish was made. 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 (2010-2012) 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 2b or 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 2b of Iowa’s 2012 Integrated Report.

Fish consumption uses remained assessed (monitored) as "partially supported" due to the continuation of a fish consumption advisory for this lake issued by the state of Nebraska due to high levels of PCBs (for more information, see the following web site:http://deq.ne.gov/publica.nsf/pages/WAT257. 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. According to DNR’s assessment methodology, the existence of a restricted consumption advisory indicates that fish consumption uses should be assessed as “partially supported.”