Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-02, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities.
Basis for Assessment
For the 2004 report: SUMMARY: The Class B(LW) aquatic life uses are assessed (monitored) 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 from 2000 through 2002 by Iowa State University (ISU), (2) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (3) information from the IDNR Fisheries Bureau. This lake is not yet designated for beneficial uses but has beenn proposed for addition by IDNR as a Class B(LW) waterbody.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that moderately 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 from 2000 through 2002 (approximately six samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 73, 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 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, zooplankton grazing, or some other factor) limits production of algae. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 9. This TN:TP ratio strongly suggests that algal production at this lake is limited by nitrogen. 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; this lake had the 25th highest mass of Cladoceran grazers of any of the 131 lakes sampled. 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 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l. The median ISS value at Avenue of the Saints Lake was 9.1 mg/l; this was the 31st highest value of the 130 lakes sampled. This relatively high level of inorganic turbidity may be related to resuspension of sediment by common carp which entered the lake in 1999 during a high-flow event. 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 IDNR Fisheries Bureau feels that this lake has relatively good water quality and was surprised that the ISU lake survey data suggest impairments. IDNR Fisheries reports recent Secchi depths of from 16" to 26" in recent years; these Secchi depths, however, are consistent with TSI values greater than 65 that suggest an impairment due to aesthetically objectionable conditions.
The presence of nuisance (=noxious) algal species (i.e., bluegreen algae) may also contribute to impairment at 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 average mass of bluegreen algae at this lake in 2000 (93.8 mg/l) ranked 10th highest of the 131 lakes sampled.
The hyper-eutrophic conditions at this lake suggest that the Class B(LW) aquatic life uses should be assessed as "partially supported" due to the moderately high levels of algal and non-algal turbidity which violate Iowa’s narrative water quality standard protecting against aethetically objectionable conditions. Fish consumption uses are "not assessed" due to the lack of fish contaminant monitoring at this lake.
