Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities. See attached document for details.
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 moderately high levels of phosphorus may threaten full support of the Class A uses of Central Park Lake. 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 69, 54, and 60, 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 middle and upper range, respectively, between eutrophic and hyper-eutrophic lakes. These index values suggest relatively high levels of phosphorus, low levels of chlorophyll-a, and relatively 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 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 Central Park Lake is 13.3. This ratio suggests that algal production at this lake is limited by the amount of nitrogen. The levels of inorganic suspended solids at this lake are relatively low and do not suggest a strong 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 Central Park Lake was 3.4 mg/l. The relatively high TSI value for total phosphorus suggests a threat to the full support of the Class A (primary contact) uses through potential presence of aesthetically objectionable blooms of algae and through presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of this lake. In 2000, greater than 80% of wet mass was bluegreen algae during samplings in early, mid, and late summer. Although bluegreen algae appear to dominate the summertime phytoplankton community, the production of chlorophyll is low at this lake, and the presence of bluegreen algae likely does not present a significant water quality impact at this time. 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. 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 / threatened." According to IDNR Fisheries, 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. The upper ends of the lake, however, were already shallow (1-2 foot) from the siltation before the marsh was established. 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 plantonic algae from time to time. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
