Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) ISU report on lake phytoplankton communities, and (3) surveys by IDNR Fisheries Bureau.
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 "not assessed." 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, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002). EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that the Class A uses of Rodgers Park Lake are "fully supported / threatened." 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 71, 48, and 60, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the lower range of hyper-eutrophic lakes; the index values for chlorophyll-a is in the upper range of mesotrophic lakes, and the index value for secchi depth is in the upper range of eutrophic lakes. These index values suggest that, despite the very high levels of phosphorus, the production of suspended algae is extremely low, and water transparency is relatively good (and better than expected). Thus, this lake does not appear to have impairments due to aesthetically objectionable blooms of algae. The very high levels of phosphorus, however, remain a concern and suggest the potential for aesthetically objectionable blooms of algae to occur. 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) limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Rodgers Park Lake is 70.3; this ratio does not suggest that production of algae at this lake is limited by the availability of nitrogen. The levels of inorganic suspended solids at this lake are relatively 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 Rodgers Park Lake was 3.8 mg/l; this median level is the 42nd lowest of the 130 lakes sampled. Data from Downing et al. (2002) show moderately large populations of zooplankton species at Rodgers Park Lake that graze on algae; these presence of these populations may help explain the low levels of algal production seen at this lake. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 40% of the dry mass of the zooplankton community in the early August sample but only 10% of the early September sample. Although monitoring data suggest that levels of suspended algae are extremely low, the Class A uses of this lake are threatened by the 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. Sampling in 2000 showed that bluegreen algae comprised from 70 to nearly 100% of the wet mass of phytoplankton in the three summer samples taken.
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." This assessment represents an improvement over previous assessments (see assessments for previous reports above). According to the IDNR Fisheries Bureau, the upper end of Rodgers Lake was dredged out several years ago and has greatly improved the water quality. A marsh has been established on the main tributary to the lake. The primary problem is that the lake has an unfavorably large watershed to surface area ratio if 85:1. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
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 an 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.
