Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake plankton communities in 2000, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting." The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to siltation problems and due to a poor recreational fishery. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. 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 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 from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 67, 50, and 52, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is at the boundary between mesotrophic and eutrophic lakes, and the index value for Secchi depth is in the lower range of eutrophic lakes. These index values suggest that, despite the moderately 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 conditions due either to blooms of algae or high levels of inorganic turbidity. 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) other than phosphorus limits production of algae. The ISU lake data suggest that zooplankton grazing and possibly nitrogen limitation limits algal production at Lake Manteno. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Manteno Lake is 13. This ratio is low and suggests the possibility 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. The 2000 average summer mass of Cladoceran grazers (156.9 mg/l) was the fourth highest of the 131 lakes sampled. These results suggest the possibility that algal production at this lake is likely 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 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l; the median level at Manteno Lake was 2.0 mg/l; this is the 34th lowest median level of the 131 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. The 2000 average summer mass of bluegreen algae (0.7 mg/l) was the ninth lowest of the 131 lakes sampled.
Despite the indications of excellent 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 and due a poor recreational fishery. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
