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 surveys from 2000-04, (5) ISU reports of plankton communities from 2000-05 and (6) an IDNR investigation of a fish kill in April 2004.
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 by the state of Nebraska 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 may 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, nutrient loading to the water column, and occurrence of a fish kill in April 2004. 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 2000-2004, (4) surveys by IDNR Fisheries Bureau, (5) ISU reports on lake plankton communities in summers of 2000 through 2005, and (6) results of an IDNR investigation of a fish kill in April 2004.
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 2004 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 2004 (approximately 15 samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 79, 74, and 76 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, very 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 through 2004, 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.
The results of ISU plankton monitoring from 2000 through 2005 show a relatively small population of zooplankton grazers at Carter Lake. Carter Lake had the seventh lowest per summer sample mass of zooplankton (Cladoceran) grazers of the 131 lakes sampled. This low level of zooplankton grazers would likely be unable to suppress algal production.
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 2004 was 5.2 mg/l. The median level of inorganic suspended solids at Carter Lake during this period was 7.5 mg/l. This moderately high level suggests that non-algal turbidity presents 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.
Nuisance aquatic (algal) species (i.e., bluegreen algae) appear to be a potential problem at Carter Lake. Data from the ISU survey from 2000-04 suggest that bluegreen algae (Cyanophyta) comprise approximately nearly the entire (98% )of the summertime phytoplankton community of this lake. The average per summer sample mass (biovolume) of bluegreen algae in summers of 2000 through 2004 at this lake (59.4 mg/l) was the 13th highest of the 131 lakes sampled. These conditions suggest the potential for impairments due to presence of nuisance aquatic (e.g., algal) species. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median (2000-2004) levels greater than the 75th percentile of this distribution (28.8 mg/l) were arbitrarily considered to represent “potential impairment: partially supported.” Median levels greater than the 90th percentile of this distribution (59.4 mg/l) were arbitrarily considered to represent “potential impairment; not supported.” No criteria exist, however, upon which to base an accurate identification of impairments due to bluegreen algae. Thus, while the collection of additional data has improved our ability to accurately estimate the lake-specific levels of bluegreen algae, the exact levels at which these organisms begin to violate Iowa’s narrative criteria protecting against “nuisance aquatic life” or “aesthetically objectionable conditions” remains unknown. The assessment category for assessments based on level of bluegreen algae, of necessity, remains "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
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. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected during summers of 2000 through 2004. One of 15 samples (7%), however, violated the Class A,B(LW) criterion for pH (maximum = 9.1; minimum = 7.9 pH units). This violation likely reflects the high levels of primary productivity at Carter Lake and does not reflect the input of pollutants into this lake.
Although attributed to "natural causes", the occurrence of a fish kill in this lake in April 2004 is consistent with the assessment of aquatic life uses as only "partially supported". This kill 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. According to IDNR’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 (2002-2005) 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 IDNR 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.
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). 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 IDNR’s assessment methodology, the existence of a consumption advisory indicates that fish consumption uses should be assessed as “not supported”.
