Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) ISU report on lake phytoplankton communities, and (3) surveys by IDNR Fisheries Bureau. See attached document for details.
Basis for Assessment
For the 2002 report: SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting." The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting." Fish consumption uses remain "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), (2) information from the IDNR Fisheries Bureau, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002). EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that the Class A uses of Manteno Lake are "fully supported." 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, 50, and 52, 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 at the boundary between mesotrophic and eutrophic lakes, and the index value to secchi depth is in the lower range of eutrophic lakes. These index values suggest that, despite the very high levels of phosphorus, the production of suspended algae is extremely low and water transparency is very good and is much better than expected. Thus, this lake does not appear to have impairments due to aesthetically objectionable blooms of algae. 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 Manteno Lake is 11.7). This ratio suggest that algal production is limited by the availability of nitrogen. In addition, data from Downing et al. (2002) show relatively large populations of zooplankton species at Manteno Lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised over 90% of the dry mass of the zooplankton community in the mid-July sample and over 59% of the early September sample. These results suggest the possibility that algal production at this lake is also limited by zooplankton grazing. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for impairing designated uses. The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at Manteno Lake was 3.3 mg/l; this is the 34th lowest median level of the 130 lakes sampled. This lake does not appear to have problems due to presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a very small portion of the summertime phytoplankton community of this lake. Sampling in 2000 showed that bluegreen algae were not present in the mid-June and mid-July samples and comprised only 20% of the wet mass of phytoplankton in the early August sample. Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses. 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.
Despite the indications of good water quality at this lake, information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses at Manteno Lake should be assessed as "partially supporting" due to siltation problems in the lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
