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 plankton communities.
Basis for Assessment
Lake not assessed as part of Section 305(b) reporting prior to the 2002 cycle.
SUMMARY: The Class B(LW) aquatic life uses are 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) and (2) information on phytoplankton 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 and non-algal turbidity may adversely affect the Class B(LW) uses of Avenue of the Saints 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 73, 64, and 66, 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 high levels of chlorophyll-a, 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 this lake is 13. 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 Avenue of the Saints Lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 40% of the dry mass of the zooplankton community in the late July sample and approximately 80% in 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 in 2001 was 5.27 mg/l. The median ISS value at Avenue of the Saints Lake was 9.1 mg/l; this was the 40th highest value of the 130 lakes sampled. This level of inorganic suspended solids suggests the potential that non-algal turbidity may limit either the production of algae or contribute to turbidity-related impairments of designated uses for aquatic life uses. 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. Sampling in 2000 showed the percent wet mass of bluegreens ranged from approximately 85% in the early July sampling, to approximately 65% in the late July sampling, and approximately 75% in the late August sampling. The hyper-eutrophic conditions at this lake suggest that the Class B(LW) aquatic life uses should be assessed as "partially supported" due excessive nutrient loading to the water column and moderately high levels of algal and non-algal 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. 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 are "not assessed" due to the lack of fish contaminant monitoring at this lake.
