Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake plankton communities in 2000, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "partially supported” due to large populations of nuisance aquatic life (bluegreen algae). The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened" due to nutrient loading to the water column. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. 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, 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" due to slight reductions in water transparency. 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 71, 50, and 58, 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 at the boundary between mesotrophic and eutrophic 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 very low, and water transparency is relatively good (and better than expected). Thus, this lake does not appear to have impairments due to aesthetically objectionable conditions due either to blooms of algae or to high levels of inorganic turbidity. 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. None of these potential non-phosphorus limitations, however, appear to explain the lack of correspondence between the TSI value for total phosphorus and those for chlorophyll-a and Secchi depth. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Rodgers Park Lake is 61; this ratio is high and 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 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l; the median level at Rodgers Park Lake was 2.8 mg/l; this median level is the 45th lowest of the 131 lakes sampled. Data from Downing et al. (2002) show very low populations of zooplankton species at Rodgers Park Lake that graze on algae. 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. The summer 2000 average mass of Cladoceran grazers at this lake (1 mg/l) was the 13th lowest of the 131 lakes sampled; this level does not suggest the potential for zooplankton grazers to limit algal production. Thus, the water quality conditions at this lake suggest a threat to full support of designated uses due to slight reductions in water transparency.
Although monitoring data suggest that levels of suspended algae are extremely low, the Class A uses of this lake are impaired by the presence of nuisance algal species (i.e., bluegreen algae) at levels that may violate Iowa’s narrative water quality standard protecting against presence of nuisance aquatic life. 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 average summer mass of bluegreen algae at this lake (37 mg/l) was the 28th highest of the 131 lakes sampled, suggesting a potential impairment to the designated uses at this lake. Although results of plankton sampling in 2000 suggest that designated uses are potentially impaired by nuisance aquatic (e.g., algal) life, the amount of data available for characterizing algal populations at this lake (one season) is not sufficient for developing a more accurate assessment of support of these uses. Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to improve the accuracy of future water quality assessments.
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). 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.
