Assessment Comments
Assessment is based on (1) results of the Iowa Voluntary Atrazine Monitoring Program from 2000-02, (2) ISU lake survey from 2000-02, (3) ISU report on lake plankton communities in 2000, & (4) surveys of the DNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses remain assessed (monitored) as "fully supporting / threatened" (minor impacts) due to slight reductions in water transparency primarily caused by inorganic suspended solids. Additional threats to full support of the Class A uses include excessive nutrient loading to the water column and siltation in the lake. The Class C (drinking water) uses are assessed (monitored) as "not supported" due to violations of the Iowa water quality standard for atrazine. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. Note: this lake is not designated for Class A (primary contact recreation) uses. 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, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) the results of the Iowa Voluntary Atrazine Monitoring Program (VMP) conducted in 2000, 2001, and 2002 by Syngenta Crop Protection, Inc. (formerly Novartis Crop Protection, Inc.).
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that the Class B(LW) uses at this lake are threatened by slight reductions in water transparency. 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, 55, and 59, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth, however, place this lake in the mid to upper range, respectively, of eutrophic lakes. These index values suggest (1) extremely high levels of phosphorus, (2) relatively low (and much less than expected) levels of chlorophyll-a, and (3) 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, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. The results of the ISU lake survey suggest that algal production at this lake may be limited by nitrogen availability; however, levels of inorganic suspended solids and zooplankton grazers are low at this lake and likely do not influence algal populations. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for West Lake Osceola is 13. This ratio is low and suggests the possibility that algal production at this lake is nitrogen-limited. The levels of inorganic suspended solids at this lake are somewhat elevated and thus suggest a slight potential for both limiting algal production and contribution to reductions in water transparency that may threaten full support of 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 West Lake Osceola was 3.2 mg/l. Data from Downing et al. (2002) show that the summer 2000 mass of zooplankton species that graze on algae (Cladocerans) was 7.9 mg/l; this was the 42nd lowest of the 131 lakes sampled an thus does not suggest the potential for zooplankton grazers to limit algal production at West Lake Osceola. Thus, results of ISU monitoring from 2000-02 suggest a threat to the full support of the Class B(LW) uses due to slight reduction in water transparency caused by the somewhat elevated levels of inorganic suspended solids.
The presence of nuisance algal species (i.e., bluegreen algae) does not appear to be a significant problem at West Lake Osceola. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise approximately half of the summertime phytoplankton community of this lake but do not dominate to the degree seen in many other Iowa lakes. The summer 2000 average mass of bluegreen algae (7.7 mg/l) was the 54th lowest of the 131 lakes sampled; this level is not believed to present a significant water quality impact at this lake.
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 in the lake. The assessment of support of the Class C (drinking water) uses was based on the results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" in 2000, 2001, and 2002. This monitoring showed that the time-weighted mean level of atrazine in samples collected from West Lake Osceola in calendar year 2000 was below the MCL of 3 ug/l. The time-weighted mean of atrazine in 2000 was 3.34 ug/l (N=29), in 2001 was 3.33 ug/l (N=30), and in 2002 was 2.09 mg/l. [Note: Rounding is required when comparing levels of atrazine to the MCL. Because the MCL, as published by U.S. EPA, is 0.003 mg/l, any values compared to the MCL must be in the proper MCL units and compared at the same number of significant figures. For example, the annual time-weighted means of atrazine for 2001 (0.00333 mg/l) rounds down to 0.003 mg/l (= 3 ug/l); this value does not exceed the MCL for atrazine. The time-weighted annual mean for 2000 (3.34 ug/l) similarly rounds down to 0.003 mg/l. Yearly maximum values at West Lake Osceola were 9.8 ug/l in 1999, 4.5 ug/l in 2000, and 9.0 ug/l in 2001. Samples collected during the period June through August tended to exceed the MCL.
Tweny-one of the 46 moving annual averages for the 2000-2002 period exceed the state water quality criterion and the U.S. EPA MCL of 3 ug/l (range: 2.8 to 4.1 ug/l). Based on DNR's Section 305(b) assessment methodology, if the average contaminant level in source water is greater than the MCL, the Class C (drinking water) uses of the source water should be assessed as "not supported." Thus, based on the moving annual average values that exceed the atrazine MCL, the Class C uses of West Lake Osceola remain assessed as "not supported." Fish consumption uses remain "not assessed due to the lack of fish contaminant monitoring at this lake.
