Assessment Comments
Assessment based on (1) beach monitoring by Nebraska DEQ in summer 2000, (2) surveys by IDNR Fisheries Bureau, (3) fish tissue monitoring conducted by the Nebraska DEQ, (4) results of ISU lake survey from 2000-02, and (5) ISU study of plankton communities in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses remain assessed (monitored) as "not supported" due primarily to high levels of algal turbidity that reduce water transparency. Results of monitoring for indicator bacteria (fecal coliforms) in summer 2000 also suggests impairment of the Class A uses. Data from the ISU lake survey suggests that very large populations of bluegreen algae (noxious aquatic plant) at this lake also impair the Class A uses. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to nuisance blooms of algae and nutrient loading to the water column. Fish consumption uses remain assessed (monitored) as "not supported" due to the existence of a fish consumption advisory issued by the state of Nebraska. 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 from 2000-2002, (4) surveys by IDNR Fisheries Bureau, and (5) ISU report on lake plankton communities in summer 2000 (Downing et al. 2002).
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 thirty-day period is compared to the water quality standard of 200 fecal organisms/100ml. If a thirty-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 thirty-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 thirty-day period (N=5), the use of single-sample maximum values to assess beaches is problematic. With less than 10 samples collected during any thirty-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 having a collection frequency of less than 10 samples over a thirty-day period.
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 thirty-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 from 2000 through 2002 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 from 2000 through 2002 (approximately nine samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 79, 78, and 77 respectively. According to Carlson (1977), these index values place this lake in the range of hyper-eutrophic lakes and suggest extremely high levels of phosphorus in the water column, extremely high levels of chlorophyll-a, and extremely poor water transparency. The good agreement between the TSI values does not suggest the likelihood of a strong non-phosphorus limitation at this lake. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 11. This ratio suggests the possibility that algal production at this lake is limited by nitrogen availability. Although results of ISU plankton monitoring in 2000 show a moderately large zooplankton population at Carter Lake, a relatively small percentage of the zooplankton are grazers on algae. The 2000 average summer mass of Cladocerans (6.9 mg/l) was the 41st lowest of the 131 lakes sampled. 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 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l. The median level of inorganic suspended solids at Carter Lake was 6.4 mg/l, thus suggesting that non-algal turbidity may present some light-limitation to the production of suspended algae and may contribute to the poor water transparency at this lake. These conditions indicate impairments to the Class A (primary contact) uses through presence of the aesthetically objectionable conditions due to (1) blooms of algae and (2) very poor water transparency.
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. The 2000 average summer mass of bluegreen algae at this lake (51.3 mg/l) was the 19th highest of the 131 lakes sampled. The presence of this very large population of bluegreen algae suggests an impairment of support of the designated uses of this lake due violation of Iowa’s narrative water quality standard protecting against presence of nuisance aquatic life. Although results of plankton sampling in 2000 suggest that this narrative standard is violated such that designated uses are impaired, the amount of data available for characterizing algal populations at this lake (one season) is not sufficient for developing a more accurate assessment of support of these uses. Thus, the assessment category is considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to improve the accuracy of future water quality assessments.
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/fishing/guides/fishguide/fgadvisories.asp).
