Assessment Comments
Assessment based on (1) beach monitoring by Nebraska DEQ, (2) surveys by IDNR Fisheries Bureau, (3) fish tissue monitoring conducted by the Nebraska DEQ, (4) results of ISU lake survey in 2000-01, and (5) ISU study of phytoplankton communities.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported." The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting." Fish consumption uses remain assessed as "not supported." The sources of data for this assessment include (1) results of monitoring for indicator bacteria conducted by the Nebraska Department of Environment Quality in summer of 2000, (2) the listing of fish consumption advisories for the state of Nebraska, (3) results of Iowa State University (ISU) lake surveys in 2000-01, (4) surveys by IDNR Fisheries Bureau, and (4) ISU report on lake phytoplankton communities. EXPLANATION: Levels of indicator bacteria at Carter Lake were monitored once per week during the primary contact recreation season (May through September) of 2000 as part of ambient monitoring by the Nebraska DEQ. According to U.S. EPA guidelines for determining support of primary contact recreation uses (U.S. EPA 1997b, page 3-35), the geometric mean of fecal coliform bacteria levels from at least five samples collected over a 30-day period is compared to the water quality standard of 200 fecal organisms/100ml. If a 30-day geometric mean exceeds 200 orgs/100 ml, the primary contact recreation uses are assessed as "not supported." In addition, the U.S. EPA guidelines state that if more than 10% of the total samples taken during any 30-day period has a bacterial density that exceeds 400 fecal coliform organsims/100 ml, the primary contact recreation uses are assessed as "partially supported." Due to the relatively low numbers of samples collected during any 30-day period (N=5), the use of single-sample maximum values to assess beaches is problematic. With less than 10 samples collected during any 30-day period, the occurrence of a single level of bacteria above the single-sample maximum value will result in more than 10% violation of the single-sample maximum value and thus suggest impairment of the primary contact recreation uses. The use of less than 10 samples in an assessment based on a critical value of 10% results in large probabilities (approximately 60%) of incorrectly concluding that an impairment exists. For this reason, the single-sample maximum value is not used to assess support of primary contact recreation uses with data from the IDNR beach monitoring program. At Carter Lake, six of the 19 thirty-day geometric means for summer 2000 exceeded the Iowa Water Quality Standard of 200 orgs/100 ml (maximum geometric mean = 2,823 orgs/100 ml; maximum sample value = 12,500 orgs/100 ml). According to U.S. EPA guidelines for Section 305(b) reporting, if a 30-day geometric mean based on at least five samples is greater than 200 orgs/100ml, the primary contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, based on multiple geometric mean values greater than 200 orgs/100 ml in summer 2001, the Class A uses of this lake are assessed as "not supported." Results of monitoring conducted by ISU in 2000 and 2001 as part of the statewide survey of Iowa lakes also suggest impairment of the Class A (primary contact recreation) uses at Carter Lake. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlsons's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 79, 76, and 74 respectively. According to Carlson (1977), these index values place this lake in the range of hyper-eutrophic lakes and suggest excessive levels of phosphorus in the water column, excessive production of suspended algae, and poor water transparency. These conditions indicate impairments to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the phytoplankton community of Carter Lake. Sampling in summer 2000 showed that greater than 95% of the wet mass of phytoplankton in the three summer samples from this lake was in bluegreen algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 12. Data from the ISU survey suggest that this lake has moderately high levels of inorganic suspended solids and thus has potential problems with high levels of non-algal turbidity. The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l. The median level of inorganic suspended solids at Carter Lake was 6.5 mg/l, thus suggesting that non-algal turbidity may present some limitation to the production of suspended algae. The hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses are "not supported" due to excessive nutrient loading to the water column and nuisance blooms of algae. Fish consumption uses remained assessed (monitored) as "not 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://www.ngpc.state.ne.us/fish/fishguide/FGadvisories.html).
