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 remain 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). Further monitoring and investigation are needed to better characterize water quality conditions at this lake. EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of nutrients in the water column may adversely affect the Class A and Class B(LW) uses of Nelson 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 61, 57, and 55, respectively. According to Carlson (1977), the index value for total phosphorus is in the lower range between eutrophic and hyper-eutrophic lakes. The index values for chlorophyll-a and secchi depth are in the middle of eutrophic lakes. These index values suggest moderately high levels of phosphorus, relatively low levels of chlorphyll-a, and relatively good (and somewhat better than expected) water transparency. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 22, thus suggesting the phosphorus is the limiting nutrient for algal production at this lake. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for increasing lake turbidity. 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 Nelson Park Lake was 4.2 mg/l. The relatively high TSI values for total phosphorus and chlorophyll-a suggest potential threats to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and through presence of nuisance algal species. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of Nelson Park Lake, with approximately 90% of the wet mass in bluegreens in early and late-summer. Sampling in 2000 showed the percent wet mass of bluegreens ranged from approximately 90% in the late June sampling, to approximately 40% in the mid-July sampling, and up to approximately 95% in the late August sampling. These values, however, given the relatively low levels of chlorophyll-a (i.e., suspended algae) at this lake, do not suggest significant problems with bluegreen algae. 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" due to relatively high nutrient loading to the water column and siltation in the lake. 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 a more 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. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
