Assessment Comments
Assessment is based on (1) surveys by DNR Fisheries Bureau, (2) results of Syngenta's "Iowa Voluntary Atrazine Monitoring Program" from 2000-02, (3) ISU lake survey from 2000-02, and (4) ISU report on lake phytoplankton communities in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting / threatened" due to slight reductions in water transparency. The moderately high levels of bluegreen algae (=noxious aquatic plants) also threaten full support of these uses. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to excessive nutrient loading to the water column. The Class C (drinking water) uses are assessed as “fully supported / threatened” due to a single sample value for atrazine that exceeded Iowa Water Quality Standards. Fish consumption uses are assessed (evaluated) as "fully supported" based on fish contaminant monitoring in 1994. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000-2002 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), (4) results of Syngenta’s voluntary atrazine monitoring program (VMP) from 2000-2002, and (5) results of U.S. EPA/Iowa DNR fish contaminant (RAFT) monitoring in 1994.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that the somewhat elevated levels of inorganic suspended solids contribute to non-algal turbidity that reduces water transparency and thus threatens full support of the Class A uses of Little River 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 64, 55, and 60, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the middle range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a is in the middle range of eutrophic lakes, and the index value for secchi depth is at the upper range of eutrophic lakes. These index values suggest somewhat elevated levels of phosphorus, low (and less than expected) production of suspended algae, and marginally good water clarity. According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data suggest that the somewhat elevated levels of inorganic suspended solids (non-algal turbidity) do likely limit algal production at Little River Lake as well as contribute to non-algal turbidity that reduces water transparency. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000-02 was 4.8 mg/l; the median level at Little River Lake was 3.9 mg/l. Neither nitrogen limitation nor zooplankton grazers appear likely to limit algal production at this lake. Based on median values from ISU sampling from 2000-02, the ratio of total nitrogen to total phosphorus for Little River Lake is 20. This ratio suggests that nitrogen limitation does not influence algal production at this lake. Data from Downing et al. (2002) showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 50% of the dry mass of the zooplankton community in the late July sample and 60% of the late August sample. The average summer 2000 mass of zooplankton that graze on algae (14.6 mg/l) is low relative to other Iowa lakes and does not suggest a strong potential to limit algal production.
The somewhat elevated populations of nuisance (=noxious) algal species (i.e., bluegreen algae) may present an additional threat to full support of designated uses at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of this lake, especially in mid to late summer. In 2000, greater than 80% of wet mass was bluegreen algae during samplings in late July and late August. The average summer mass of bluegreen algae at this lake (12.7 mg/l) was the 53rd highest of the 131 lakes sampled. This level is believed to potentially threaten full support of the Class A uses.
The hyper-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 excessive nutrient loading to the water column.
The assessment of support of the Class C (drinking water) uses (“fully supported / threatened”) was based primarily on the results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" (VMP) in 2000, 2001 and 2002. Data from the ISU statewide lake survey was also used for this assessment. Results of the Syngenta VMP showed that the time-weighted mean levels of atrazine in samples collected from the this lake in all three calendar years were below the MCL of 3.0 ug/l. The time-weighted mean of atrazine in 2000 (N=31) was 2.32 ug/l, in 2001 (N=31) was 2.05 ug/l, and in 2002 was 2.08 mg/l (N=15). 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. Thus, the annual time-weighted means of atrazine for all three years round to 2 ug/l. None of these time-weighted annual means exceed the MCL for atrazine. One of the 77 samples collected during this three-year period exceeded the MCL of 3.0 ug/l (3.6 ug/l on July 2, 2001). Based on DNR's Section 305(b) assessment methodology, if the average contaminant level in source water is less than the MCL, but a level in one or more samples is above the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported / threatened." None of the moving annual averages for atrazine for the years 2000 through 2002 at Little River Lake exceeded the MCL (maximum average = 2.3 ug/l). In addition, results of the ISU lake survey show that nitrate levels are extremely low at this lake (maximum value = 0.7 mg/l; median = 0.3 mg/l).
Fish consumption uses remain assessed (evaluated) as "fully supported" based on results of the U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1994. However, because these data are considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively low confidence).
