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.
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 phosphorus may adversely affect the Class A and Class B(LW) uses of Oldham 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 65, 57, and 51, respectively. According to Carlson (1977), the index value for total phosphorus is in the middle range between eutrophic and hyper-eutrophic lakes; the value for chlorophyll-a is in the upper range of eutrophic lakes, and the value for secchi depth is in the lower range of eutrophic lakes. These index values suggest relatively high levels of phosphorus, relatively low (and less than expected) levels of chlorophyll-a, and much better than expected 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) other than phosphorus limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Oldham Lake is 16. This TN:TP ratio suggests a possibility that nitrogen limitation suppresses the production of suspended algae at this lake. In addition, data from Downing et al. (2002) show relatively large populations of zooplankton species at this lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 75% of the dry mass of the zooplankton community in the mid-July sample and 70% of the early August sample. 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 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at Oldham Lake was 2.2 mg/l. The relatively high TSI values for total phosphorus suggests a potential threat to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae. Presence of nuisance algal species (i.e., bluegreen algae), however, does not appear to be a significant problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise relatively small portions of the phytoplankton community. Sampling in 2000 showed the percent wet mass of bluegreens ranged from less than 5% in the late June sampling, to approximately 35% in the mid-July sampling, and down to approximately 20% in the early August sampling. The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should reamin assessed as "fully supported / threatened" due to excessive nutrient loading to the water column and siltation. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this 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.
