Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-04, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities, 2000-05.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses are assessed (monitored) as "partially supporting" due to poor water transparency and due to high levels of pH that exceed state criteria. 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 2004 sponsored by IDNR and conducted by Iowa State University (ISU), (2) information on plankton communities at Iowa lakes collected from 2000 through 2005 as part of the IDNR/ISU statewide lake survey, and (3) information from the IDNR Fisheries Bureau. This lake is not yet designated for beneficial uses but has been 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 2004 (approximately 15 samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 74, 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 2004, the ratio of total nitrogen to total phosphorus for this lake is 11. This TN:TP ratio suggests the potential that algal production at this lake is, at times, limited by nitrogen.
Data from the ISU survey also show relatively large populations of zooplankton species at Avenue of the Saints Lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 55% of the dry mass of the zooplankton; this lake had the 21st highest per summer sample mass of Cladoceran grazers (202.1 mg/l) 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 2004 was 5.2 mg/l. The median ISS value at Avenue of the Saints Lake was 9.0 mg/l; this was the 34th highest median ISS value of the 130 lakes sampled. This relatively high level of inorganic turbidity may be related to re-suspension 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 the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion of this lake’s summertime phytoplankton community. Sampling from 2000 through 2004 showed that the percent wet mass of bluegreen algae in the phytoplankton community of this lake in summers of the 2000-2004 period was approximately 84%. Also, the median summer mass of bluegreen algae at this lake was the 17th highest of the 131 lakes sampled: 45.5 mg/l. This median is in the worst 25% 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 75th percentile of this distribution were arbitrarily considered to represent “potential impairment: partially supported.” No other criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the accuracy of this (2006) assessment 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 remains "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
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 aesthetically objectionable conditions. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 12 samples collected during summers of 2000 through 2004. Four of 15 samples, however, exceeded the Class B(LW) criterion for pH (maximum = 9.6; 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 A and Class B(LW) uses of this lake. These violations likely reflect the high levels of primary productivity at Avenue of the Saints 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.
