Assessment Comments
Assessment is based on results of (1) ISU lake surveys from 2000-04, (2) surveys by IDNR Fisheries Bureau, and (3) ISU reports on lake plankton communities from 2000-05.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting”. The Class B(LW) aquatic life uses are assessed (monitored) as "fully supporting" based on information from the IDNR Fisheries Bureau and based on results of chemical/physical monitoring conducted as part of the ISU statewide lakes survey. 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: 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, 60, and 63, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the lower range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the upper range of eutrophic lakes, and the value for secchi depth is in the middle range between eutrophic and hyper-eutrophic lakes. These index values suggest very high levels of phosphorus, relatively low levels of chlorophyll-a, and 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) limits production of algae. The ISU lake data suggest do not suggest an obvious factor that limits that algal production at Central Park Lake. Nitrogen availability, zooplankton grazing, and inorganic turbidity, however, are all potential factors contributing to the discrepancy in the TSI values between total phosphorus and chlorophyll-a.
Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Central Park Lake is 14. This ratio suggests a limited potential that algal production at this lake is limited by nitrogen availability.
Data from the ISU survey show moderately large populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 50% of the dry mass of the zooplankton community. Central Park Lake had the 65th highest per summer sample mass of zooplankton (Cladoceran) grazers of the 131 lakes sampled (i.e., the near the median mass of the 131 lakes). This level of zooplankton grazers may be able to suppress algal production and account for some of the discrepancy between the TSI values for total phosphorus (70) and chlorophyll-a (60).
The levels of inorganic suspended solids at this lake are relatively low but may be sufficiently high to potentially limit algal production and to contribute to 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 Central Park Lake was 3.4 mg/l.
Thus, the 2000-2004 TSI values for this lake are below impairment triggers and do not suggest impairments of the Class A uses. Very little change in these TSI values was apparent between the previous (2004) and current (2006) assessment cycles.
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 (80%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Central Park Lake (12.0 mg/l) was the 65th lowest of the 131 lakes sampled (i.e., the near the median average per sample mass of the 131 lakes). This levels is in the lower two-thirds 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 (~20 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 assessment of Class B(LW) aquatic life uses should be upgraded from "partially supported" (see previous assessments above) to "fully supported". According to the IDNR Fisheries Bureau, Jones County has constructed a marsh on the main tributary to the lake that now controls most of the silt and nutrients on that arm; water quality has improved as a result. Also, submergent vegetation has been controlled by grass carp for the last 10-15 years. Before that time, the lake had problems with dense growth of submergent vegetation. The lake does have nuisance blooms of planktonic algae from time to time, but improvements in water quality have reduced their severity. Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. Results of this monitoring show that 2 of the 14samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that 2 of the 14 samples collected exceeded Iowa’s Class A,B(LW) criterioa for pH (maximum = 9.1; minimum = 8.1 pH units). Based on IDNR’s assessment methodology, these results do not suggest violation frequencies of significantly greater than 10% for either parameter and thus do not suggest impairment of either the Class A and Class B(LW) uses of Central Park Lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
