Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau (3) ISU report on lake phytoplankton communities in 2000, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1998.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses remain assessed (monitored) as "partially supported" due to algal blooms and high levels of inorganic turbidity. In addition, the large populations of bluegreen algae at this lake likely impair the Class A uses as well. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supported" due to nutrient loading, algal blooms, and re-suspension of sediment. Fish consumption uses remain assessed (monitored) as "fully supported" based on results of fish contaminant monitoring in 1998. 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) and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1998. The dredging project at this lake was conducted after the 2000-2002 data collection period used for this assessment.
EXPLANATION: Results of monitoring conducted by ISU from 2000 through 2002 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses are only "partially supported" due to algal blooms and high levels of inorganic turbidity. 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 82, 71, and 79, respectively, for Little Wall Lake. According to Carlson (1977), these index values place this lake in the range of hyper-eutrophic lakes and suggest (1) extremely high levels of phosphorus in the water column, (2) very high, although somewhat less than expected, levels of suspended algae, and (3) extremely poor water transparency. 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 non-algal particles do likely limit algal production, at least to some degree, at Little Wall Lake, although other non-phosphorus limitations (e.g., nitrogen limitation and zooplankton grazers) may limit algal populations as well. Data on inorganic suspended solids from the ISU survey suggest that this lake is subject to occasional episodes of high levels of non-algal turbidity. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l. The median level of inorganic suspended solids at Little Wall Lake (21.0 mg/l) was the ninth highest of the 131 lakes, thus strongly suggesting that non-algal turbidity limits the production of algae as well as contributes to inorganic turbidity that impairs beneficial uses. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 11. This ratio is low and suggests the potential for nitrogen availability to also limit algal production. In addition, data from Downing et al. (2002) show relatively large populations of zooplankton species (Cladocerans) at this lake that graze on algae. The average summer mass of these zooplankton grazers (40.4 mg/l) in 2000 was the 33rd highest of the 131 lakes sampled. These conditions indicate impairments to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and high levels of inorganic turbidity related to re-suspension of sediment. These high levels of turbidity, related both to algal production and to re-suspension of sediment (inorganic material), violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The IDNR Fisheries Bureau agrees that high levels of suspended sediment increase turbidity and reduce water transparency but that hydraulic dredging scheduled for 2003 should improve this problem.
An additional impairment may exist due to presence of a large population 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 Little Wall Lake. Sampling in summer 2000 showed that from between 70 and 80% of the wet mass of the phytoplankton in the three summer samples from this lake was in bluegreen algae. The 2000 average summer mass of bluegreen algae at this lake (37.7 mg/l) was the 26th highest of the 131 lakes sampled. This level of bluegreen algae is moderately to very high and suggests a potential impairment to the designated uses of this lake due to a violation of Iowa’s narrative water quality standard protecting against presence of “nuisance aquatic life.” Although results of plankton sampling in 2000 suggest this impairment of designated uses, the amount of data available for characterizing algal populations at this lake (one season) 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.
The hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses are "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. Fish consumption remain "fully supported" based on results of U.S. EPA/IDNR fish tissue monitoring in 1998 (see assessment for the 2000 report).
