Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities in 2000, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1997.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supported" due to slightly elevated turbidity related to levels of algae and inorganic suspended solids at this lake. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened" due to nutrients and algal turbidity. Fish consumption uses remain assessed (monitored) as "fully supported" based on results of fish contaminant monitoring in 1997. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) results of U.S. EPA / IDNR fish tissue monitoring in 1997. Although not orginally assessed as "impaired" by IDNR for the 2004 Section 305(b) cycle, this lake was added to Iowa's 2004 Section 303(d) list at the recommendation of U.S. EPA Region 7 based on ISU lake monitoring data from 2003 and 2004 that suggest adverse trends in water quality.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of turbidity, related to both algal and non-algal sources, may adversely affect the Class A and Class B(LW) uses of Lake Cornelia. 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 69, 60, and 63, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth are in the lower range between eutrophic and hyper-eutrophic lakes. These index values suggest (1) moderately high levels of phosphorus in the water column, (2) relatively low (and less than expected) levels of chlorophyll-a, and (3) marginally good water transparency. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low values for chlorophyll-a and secchi depth indicate that some factor (e.g., nitrogen limitation, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Lake Cornelia is 16. This TN:TP ratio suggests the possibility that nitrogen limitation, at times, may suppress the production of suspended algae at this lake. In addition, data from Downing et al. (2002) show relatively large populations of zooplankton species at Lake Cornelia that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 30% of the dry mass of the zooplankton community in the late July sample and approximately 45% of the late August sample. The 2000 average summer mass of Cladocerans at Lake Cornelia (23.5 mg/l) was 54th highest of the 131 lakes sampled and suggests at least the potential for zooplankton grazing to limit algal populations. The levels of inorganic suspended solids at this lake are somewhat elevated and suggest the potential for contributing to in-lake 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 at Lake Cornelia was 5.7 mg/l. Thus, the somewhat elevated TSI values for chlorophyll-a and Secchi depth suggest potential impairments of the Class A (primary contact) uses, and suggest potential threats to full support of the Class B(LW) uses, through presence of turbidity and blooms of algae that could potentially lead to aesthetically objectionable conditions.
The levels of nuisance algal species (i.e., bluegreen algae) at Lake Cornelia are relatively low and do not appear to either threaten or impair the designated uses of this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of this lake, especially in mid to late summer when greater than 90% of wet mass is comprised of bluegreen algae. The 2000 average summer mass of bluegreen algae at this lake (6.1 mg/l), however, is low relative to most other Iowa lakes (47th lowest of the 131 lakes sampled) and does not suggest a significant water quality impact. These conclusions are supported by the following information from the IDNR Fisheries Bureau in 2002.
"Lake Cornelia has definitely shown improvement over the past 15 years. Secchi disc readings during early summer over the past 5 years have varied from 1 ½ feet to 3 feet. Although suspended solids do not appear to impair water quality, the lake still has excessive nutrients which stimulate summer algae blooms. Nuisance blue-green algae blooms have not been a problem in recent years. Algae blooms have been on a large enough magnitude, however that desirable growth of submergent vegetation has not increased substantially. Much of the improvement in water quality can be attributed to completion of a sanitary sewer system around this lake in the mid-1980's."
The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported / threatened," with primary threats including excessive nutrient loading to the water column and high levels of algal turbidity in the lake. Fish consumption uses remain assessed as "fully supported" based on results of U.S. EPA /IDNR fish tissue (RAFT) monitoring in 1997 (see assessment for the 2000 report).
