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" based on results of lake monitoring from 2000 through 2002. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to 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. 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: Results from the ISU statewide survey of Iowa lakes suggest “full support” of the Class A uses at Oldham Lake. 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 62, 56, and 54, respectively. According to Carlson (1977), the index value for total phosphorus is in the lower range between eutrophic and hyper-eutrophic lakes; the value for chlorophyll-a is in the middle range of eutrophic lakes, and the value for Secchi depth is in the middle range of eutrophic lakes. These index values suggest somewhat elevated levels of phosphorus, relatively low (and less than expected) levels of chlorophyll-a, and very good (and 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, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. Results of ISU monitoring suggest that zooplankton grazing is the most significant non-phosphorus limitation on algal production at this lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Oldham Lake is 16. This TN:TP ratio does not suggest significant nitrogen limitation 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 summer 2000 average mass of Cladocerans at this lake (92.3 mg/l) was the 18th highest of the 131 lakes sampled and thus suggest the potential for zooplankton grazing to limit algal production. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential either limiting algal production or for impairing designated uses. 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 Oldham Lake was 2.2 mg/l. Thus, these results suggest above average water quality for an Iowa lake and “full support” of the lake’s Class A uses.
Presence of nuisance algal species (i.e., bluegreen algae) 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 2000 average summer mass of bluegreen algae (2.1 mg/l) was the 20th lowest of the 131 lakes sampled.
The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should remain 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.
