Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) ISU reports on lake plankton communities from 2000-05, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supported”. The Class B(LW) aquatic life uses are assessed (monitored) as "fully supported". 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 sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, and (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that the Class A uses of Rodgers Park Lake are "fully supported ". 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 70, 56, 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 in the middle range of 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 relatively 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 64; 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 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 2004 was 5.2 mg/l; the median level at Rodgers Park Lake was 3.5 mg/l; this median level is the 38th lowest of the 131 lakes sampled.
In terms of all Iowa lakes sampled, data from the ISU survey show relatively small populations of zooplankton species at this lake that graze on algae. Although sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised almost 60% 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 (75 mg/l) was the 56th lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Rodgers Park Lake.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (approximately 60%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Rodgers Park Lake (8.2 mg/l) was the 44th lowest of the 131 lakes sampled. This level is in the lowest third of the 131 Iowa lakes sampled. The presence of a relatively small population of bluegreen algae at this lake does not suggest a 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 less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes. 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". 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.
Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. Results of this monitoring show that two of the 15 samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that only one of the 15 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 9.1; minimum = 8.2 pH units). Based on IDNR’s assessment methodology, these results do not suggest violation frequencies are significantly greater than 10% for either parameter and thus do not suggest impairment of either the Class A and Class B(LW) uses of Rodgers Park Lake. The violation of the pH criterion likely reflects the influence of primary productivity and does not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
