Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting / threatened" due to occasional blooms of algae and levels of inorganic suspended solids (turbidity) that reduce water transparency. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened" due to continuing, but improving, problems with delivery of silt and nutrient to the lake; occasional blooms of algae are also of concern for support of the Class B(LW) uses. 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: 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, 58, and 65, 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 to moderately-poor 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 that algal production at Central Park Lake is potentially limited by nitrogen availability, by zooplankton grazing, and by inorganic turbidity. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Central Park Lake is 13. This ratio suggests the potential that algal production at this lake is limited by nitrogen availability. Also, data from Downing et al. (2002) show relatively large populations of zooplankton species at Central 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 both summer samples. The 2000 average summer mass of Cladocerans at this lake (32 mg/l) was the 40th highest of the 131 lakes sampled, thus suggesting that zooplankton grazing may limit algal production. The levels of inorganic suspended solids at this lake are relatively low but are sufficiently high to potentially limit algal production and to contribute to increased lake turbidity. 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 Central Park Lake was 3.8 mg/l. Thus, 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 high levels of inorganic turbidity. These conditions suggest a threat to full support of the Class A uses due to moderately poor water transparency related primarily to high levels of inorganic suspended solids and secondarily to algal turbidity.
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. The 2000 average summer mass of bluegreen algae (11 mg/l) was 58th highest of the 131 lakes sampled. The amount of data available for characterizing algal populations at this lake (one season), however, is not sufficient for developing a more accurate assessment of support of these uses. Thus, the assessment category is considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). 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 for this lake.
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 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. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
