Assessment Comments
Assessment is based on (1) surveys by DNR Fisheries Bureau, (2) results of the "Iowa Voluntary Atrazine Monitoring Program" in 99, 00 & 01, (3) ISU lake survey in 2000-01, and (4) ISU report on lake phytoplankton communities.
Basis for Assessment
[Note: The original 2002 assessment for Little River Watershed Lake was incomplete. This assessment was updated in August 2004 and the corrected/updated information has been entered into ADB+.]
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened." The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting." The Class C (drinking water) uses are assessed as “fully supported / threatened.” Fish consumption uses are assessed (evaluated) as "fully supported." 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, (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 conducted in 1999, 2000, and 2001 by Syngenta Crop Protection, Inc. (formerly Novartis Crop Protection, Inc.)
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of algal turbidity may threaten full support of the Class A uses of Little River 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 70, 49, and 60, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the lower range of hyper-eutrophic lakes; the index values for chlorophyll-a is in the upper range of mesotrophic lakes, and the index value for secchi depth is at the upper range of eutrophic lakes. These index values suggest very high levels of phosphorus, extremely low (and much less than expected) production of suspended algae, and relatively good (and somewhat better than expected) water clarity. 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) limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Little River Lake is 14.6. The relatively high proportion of Daphnia in the zooplankton population of this lake suggest the potential for grazing to reduce populations of suspended algae and thus levels of chlorophyll-a. Data from Downing et al. (2002) show relatively large populations of zooplankton species at Little River Lake that graze on algae. Sampling in 2000 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 levels of inorganic suspended solids at this lake are relatively low and do not suggest a strong potential for impairing designated uses due to high turbidities. 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 Little River Lake was 5.1 mg/l. The high TSI value for total phosphorus suggest threats to the full support of the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and due to presence of nuisance algal species (i.e., bluegreen algae). 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 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. Fish consumption uses remain assessed as "fully supported" based on results of the U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1994. However, because these data are now 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).
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 an 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.
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 1999, 2000, and 2001. This monitoring 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 1999 (N=31) was 1.7 ug/l, in 2000 (N=31) was 2.32 ug/l and in 2001 (N=31) was 2.05 ug/l. 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 mean of atrazine for 1999 (0.0017 mg/l) rounds up to 0.002 mg/l (= 2 ug/l); for 2000 (0.00232 mg/l) rounds down to 0.002 mg/l (= 2 ug/l), and for 2001 (0.00205 mg/l) rounds down to 0.002 mg/l (= 2 ug/l). None of these time-weighted annual means exceed the MCL for atrazine. One of the 93 samples collected during this three-year period exceeded the MCL of 3.0 ug/l (3.6 ug/l in 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."
Fish consumption uses are considered "not assessed" due to the lack of recent fish contaminant monitoring at this lake. The most recent fish tissue monitoring was conducted in 1994 as part of the U.S. EPA / IDNR fish tissue (RAFT) monitoring program. These data are now more than five years old and are thus considered too old for characterizing current water quality conditions.
