Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities, 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 (evaluated) as "fully supporting / threatened." The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened." Fish consumption uses remain assessed as "fully supported." The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) results of U.S. EPA / IDNR fish tissue monitoring in 1997. Further monitoring and investigation are needed to better characterize water quality conditions at this lake.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of algal turbidity may adversely affect the Class A and Class B(LW) uses of Lake Cornelia. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 74, 58, and 59, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth place this lake in the upper range of eutrophic lakes. These index values suggest (1) very high levels of phosphorus in the water column, (2) relatively low (and much less than expected) levels of chlorophyll-a, (3) relatively 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 or zooplankton grazing) other than phosphorus limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Lake Cornelia is 12. This TN:TP ratio suggests the possibility that nitrogen limitation suppresses the production of suspended algae at this lake. In addition, data from Downing et al. (2002) show a 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 levels of inorganic suspended solids at this lake are moderately low and do not suggest the potential for impairing designated uses. 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 at Lake Cornelia was 5.3 mg/l. The very high TSI value for total phosphorus, and the moderately high TSI value for chlorophyll-a, suggest potential impairments to the Class A (primary contact) and Class B(LW) 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 summertime phytoplankton community of this lake, especially in mid to late summer when greater than 90% of wet mass is comprised of bluegreen algae. Although bluegreen algae appear to dominate the summertime phytoplankton community, the production of chlorophyll is low at this lake, and the presence of bluegreen algae likely does not present 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 above). Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing a more accurate assessment of support of these uses. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments.
