Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau, and (3) ISU reports on lake plankton communities from 2000-05.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses remain assessed (monitored) as "partially supporting" due to poor water transparency caused by algal blooms and high levels of inorganic turbidity. In addition, frequent violations of the Class B(LW) pH criterion also suggest impairment of the Class B(LW) uses; this is a new impairment for this lake. Nutrient loading to the water column and siltation are additional concerns. In addition, levels of bluegreen algae are sufficiently high to suggest a potential 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. The water quality conditions at this lake, however, would indicate impairment of Class A uses as well.] 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 sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, and (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey.
Note: A TMDL for algae and turbidity at Littlefield Lake was prepared by IDNR and approved by EPA in 2006. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle (algal growth, turbidity, and pH) are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).
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 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 75, 64, and 70, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the middle range between eutrophic and hyper-eutrophic lakes, and the index value for secchi depth is at the boundary 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 relatively 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 2004 was 5.2 mg/l. The median ISS value at Littlefield Lake was 9.3 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 13. This TN:TP ratio is low and suggests that algal production at this may, at times, be limited by nitrogen availability.
The presence of relatively large populations of zooplankton at Littlefield Lake that graze on algae, however, may explain the discrepancy between the TSI value for phosphorus (75) and that for chlorophyll-a (64). Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 55% of the dry mass of the zooplankton community of this lake. The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (139 mg/l) was the 39th highest of the 131 lakes sampled.
These conditions indicate impairments to the Class B(LW) uses through presence of poor water transparency that violates Iowa’s narrative water quality criterion protecting against aesthetically objectionable conditions. Based on the ISU monitoring data, the cause of this poor transparency appears to be a combination of high levels of inorganic suspended solids and high levels of suspended algae.
Data from the ISU survey from 2002-2004 suggest that the presence of nuisance aquatic species (i.e., bluegreen algae) may present an impairment of the Class A uses at this lake. Summer sampling during this period showed the percent wet mass of the total phytoplankton community in bluegreens was approximately 90%. In addition, Littlefield Lake had the third highest median per summer sample mass of bluegreen algae of the 131 lakes sampled: 144 mg/l. This median is in the worst 10 % of the 131 Iowa lakes sampled. The presence of a large population of bluegreen algae at this lake suggests a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels greater than the 90th percentile of this distribution (~59 mg/l) were arbitrarily chosen by IDNR staff to represent the condition of “potential impairment; not supported.” No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the , while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless, of necessity, remains "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
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.
The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected during summers of 2000 through 2004. Four of 14 samples, however, exceeded the Class B(LW) criterion for pH (maximum = 10.6; minimum = 7.7 pH units). Based on IDNR’s assessment methodology, these results suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria. Thus, these results suggest an impairment (partial support/monitored) of the Class B(LW) uses of this lake. These violations likely reflect the high levels of primary productivity at Littlefield Lake and do not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
