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 phytoplankton communities.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "partially supporting." The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting." Fish consumption uses are "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 algal turbidity may adversely affect the Class A and Class B(LW) uses of Littlefield 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 76, 64, and 67, 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 place this lake in the range between eutrophic and hyper-eutrophic lakes. These index values suggest excessive levels of phosphorus in the water column, moderately low levels of chlorophyll-a (suspended algae), and moderately poor 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) limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Littlefield Lake is 11). This TN:TP ratio suggest that algal production at this may be limited by nitrogen. In addition, data from Downing et al. (2002) show a relatively large populations of zooplankton species at Littlefield Lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 60% of the dry mass of the zooplankton community in the early August sample and 70% of the early September sample. Thus, both nitrogen limitation and zooplankton grazing may contribute to the lower than expected production of suspended algae at this lake. The moderately high level of inorganic suspended solids at this lake also suggests the potential for (1) limitation algal production and (2) contributions to in-lake turbidity. The median level of inorganic suspended solids (ISS) in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l. The median ISS value at Littlefield Lake was 8.1 mg/l thus suggesting the potential that non-algal turbidity may limit either the production of algae and contribute to turbidity-related impairments of designated uses for primary contact recreation or aquatic life. The presence of nuisance (=noxious) algal species (i.e., bluegreen algae) may also contribute to the impairment of the Class A uses of this lakes. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a significant portion of this lake's summertime phytoplankton community but dominate this community in mid and late summer. Sampling in 2000 showed the percent wet mass of bluegreens ranged from approximately 40% in the late June sampling, to 100% in the early August sampling, and greater than 95% in the early September sampling. 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 remain assessed as "partially supported" due to siltation, excessive nutrient loading to the water column, and moderately high levels of turbidity. Although results of ISU lake monitoring in 2000 and 2001 suggest that nutrient-related impairments may exist, neither the amount of data available for characterizing water quality nor the magnitude of the TSI values are sufficient for identifying impairments with the level of confidence necessary to justify adding this lake to Iowa's Section 303(d) list. Thus, further investigation and monitoring are needed to (1) better characterize current water quality conditions and (2) verify that such impairments exist. 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. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
