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 phytoplankton communities in 2000.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses remain assessed (monitored) as "partially supporting" due to nutrient loading to the water column, siltation, algal blooms, and sediment re-suspension (inorganic turbidity). In addition, levels of bluegreen algae are sufficiently high to suggest an additional impact to the designated uses at this lake. [Note: As of July 2003, this lake was not designated for Class A uses in the Iowa Water Quality Standards.] 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 that high levels of algal and non-algal turbidity may adversely affect the Class B(LW) uses of Littlefield 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 77, 69, and 69, 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 upper range between eutrophic and hyper-eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, moderately high 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, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. The ISU lake data suggest that non-algal particles (inorganic suspended solids) do likely limit algal production, at least to some degree, at Littlefield Lake, although other non-phosphorus limitations (e.g., nitrogen limitation and zooplankton grazers) may limit algal populations as well. The median level of inorganic suspended solids (ISS) in the 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l. The median ISS value at Littlefield Lake was 7.6 mg/l thus suggesting the potential that non-algal turbidity may limit the production of algae as well as contribute to turbidity-related impairments of designated uses for primary contact recreation or aquatic life. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Littlefield Lake is 10. This TN:TP ratio is low and suggests that algal production at this may be limited by nitrogen availability. In addition, data from Downing et al. (2002) show a relatively large populations of zooplankton species (Cladocerans) 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. The 2000 average summer mass of zooplankton grazers (46.8 mg/l) was the 28th highest of the 131 lakes sampled, thus suggesting the potential to limit algal production. These water quality conditions suggest impairment to the Class B(LW) uses of this lake due to high levels of algal and inorganic turbidity that violate Iowa’s narrative water quality standards protecting against aesthetically objectionable conditions.
The presence of nuisance (=noxious) algal species (i.e., bluegreen algae) may also contribute to the impairment of the designated uses of this lake. 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 2000 average summer mass of bluegreen algae (85.8 mg/l) was the 11th highest of the 131 lakes sampled; this mass suggests an impairment due to violation of Iowa’s narrative water quality standard protecting against nuisance aquatic life. The amount of data available for characterizing algal populations at this lake (one season), however, is not sufficient for developing a more accurate assessment of support of these uses. Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to improve the accuracy of future water quality assessments for this lake.
Information from the IDNR Fisheries Bureau, suggest that the Class B(LW) is consistent with results of ISU monitoring and suggests that the aquatic life uses should remain assessed (monitored) as "partially supported" due to siltation, excessive nutrient loading to the water column, algal blooms, and moderately high levels of turbidity. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
