Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau, (3) ISU reports on lake plankton communities from 2000-05, 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 an adverse trend in levels of chlorophyll-a (suspended algae) and to only marginally good water transparency. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting". Elevated levels of nutrients and algal turbidity, however, remain concerns for support of the Class B(LW) uses at this lake. Fish consumption uses remain assessed (evaluated) 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 sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (4) results of U.S. EPA / IDNR fish contaminant monitoring in 1997. Although not originally 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 suggested adverse trends in water quality.
Note: A TMDL for algae and turbidity at Lake Cornelia was prepared by IDNR and approved by EPA in 2006. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle (algal growth and turbidity) are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that moderately 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 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 68, 62, and 64, 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) relatively 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 2004, the ratio of total nitrogen to total phosphorus for Lake Cornelia is 18. This TN:TP ratio suggests that algal production at this lake is most often limited by the availability of phosphorus.
The presence of extremely large populations of zooplankton at Lake Cornelia that graze on algae, however, may explain the discrepancy between the TSI value for phosphorus (68) and that for chlorophyll-a (62). Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised nearly 70% of the dry mass of the zooplankton community of this lake. The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (246 mg/l) was the 12th highest of the 131 lakes sampled.
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 2004 was 5.2 mg/l; the median level at Lake Cornelia was 6.9 mg/l. Relative to other Iowa lakes, this median level of inorganic suspended solids is relatively high, thus suggesting the potential to contribute to in-lake turbidity.
The somewhat elevated TSI values for chlorophyll-a and Secchi depth suggest potential impairments of the Class A (primary contact) uses, and suggest concerns for full support of the Class B(LW) uses, through presence of turbidity and blooms of algae that could potentially lead to aesthetically objectionable conditions. A comparison of annual TSI values for the 2000-2004 period at Lake Cornelia suggests a potential adverse trend in water quality. Annual summer TSI values for Secchi depth have generally been stable over the five-year period: 2000: 67; 2001: 54; 2002 = 67; 2003 = 66; and 2004 = 64. Annual summer TSI values for chlorophyll-a, however, have generally worsened over the five-year period: 2000: 53 (approximately 10 ug/l); 2001: 59; 2002 = 62; 2003 = 63; and 2004 = 64 (approximately 30 ug/l). The trend in the annual TSI values for chlorophyll-a at Lake Cornelia is approaching the impairment trigger of 65, thus suggesting the potential for impairment conditions in the near future. These conditions are in contract to previous characterizations of water quality conditions at Lake Cornelia. In 2002, the IDNR Fisheries Bureau noted that Lake Cornelia had shown improvements in water quality over the past 15 years. Secchi disc readings during early summer over the past 5 years had varied from 1 ½ feet to 3 feet. Despite these improvements, the lake continued to have excessive nutrients which stimulated summer algae blooms which were of a large enough magnitude that desirable growth of submergent vegetation did not increase substantially in the lake. IDNR Fisheries attributed much of the improvement in water quality at Lake Cornelia to completion of a sanitary sewer system around this lake in the mid-1980's.
The presence of nuisance (=noxious) algal species (i.e., bluegreen algae) may also present an impairment of the Class A uses at Lake Cornelia. Data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion of this lake’s summertime phytoplankton community. Sampling from 2000 through 2004 showed that approximately 90 percent of the wet mass of the phytoplankton community was bluegreen algae. Also, the median per summer sample mass of bluegreen algae at this lake (44.9 mg/l) was the 18th highest of the 131 lakes sampled. This median is in the worst 25% of the 131 Iowa lakes sampled. The presence of a large population of bluegreen algae at this lake suggests the potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels greater than the 75th percentile of this distribution (~29 mg/l) were arbitrarily chosen by IDNR staff to represent the condition of “potential impairment: partially supported.” No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "fully supported". Nutrient loading to the water column and high levels of algal turbidity in the lake, however, remain concerns for support of the Class B(LW) uses. The ISU lake survey data suggest good chemical water quality at Lake Cornelia: no violations of the Class B(LW) criteria for dissolved oxygen occurred in the 14 samples collected, or for pH in the 15 samples collected, at this lake during summers of 2000 through 2004.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Lake Cornelia in 1997. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes. Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa. In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health, in cooperation with Iowa DNR, developed a risk-based advisory protocol. This protocol went into effect in January 2006 (see http://www.iowadnr.gov/fish/news/consump.html for more information on Iowa’s revised fish consumption advisory protocol). Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest either a threat to, or impairment of, fish consumption uses. This scenario, however, does not apply to the fish contaminant data generated from the 1997 RAFT sampling conducted Lake Cornelia: levels of all contaminants from this monitoring were below advisory trigger levels, thus suggesting the continued “full support” of fish consumption uses.
