Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) ISU reports on lake plankton communities from 2000-05, 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 sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, and (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey.
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 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 69, 58, and 56, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range between eutrophic and hyper-eutrophic lakes (borderline hyper-eutrophic); the index value for chlorophyll-a is in the upper range of eutrophic lakes, and the index value for Secchi depth is in the middle range of eutrophic lakes. These index values suggest that, despite the moderately high levels of phosphorus, the production of suspended algae is relatively low and water transparency is relatively good and is 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 2004, the ratio of total nitrogen to total phosphorus for Manteno Lake is 11. This ratio is low and suggests the possibility that algal production is limited by the availability of nitrogen.
In addition, the presence of very large populations of zooplankton at Manteno Lake that graze on algae may explain the discrepancy between the TSI value for phosphorus (69) and that for chlorophyll-a (58). In terms of all Iowa lakes sampled, data from the ISU survey show large populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately just about one-third of the dry mass of the zooplankton community of this lake. The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (154 mg/l) was the 34th highest of the 131 lakes sampled. This population of zooplankton grazers suggests the potential for this type of non-phosphorus limitation on algal production at Manteno Lake.
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 2004 was 5.2 mg/l; the median level at Manteno Lake was 3.5 mg/l; this is the 39th lowest median level of the 131 lakes sampled.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (almost 60%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Manteno Lake (1.4 mg/l) was the 9th lowest of the 131 lakes sampled. This level is in the lowest 10% of the 131 Iowa lakes sampled. The presence of a relatively small population of bluegreen algae at this lake does not suggest a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life.
Despite the indications of very 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 and due a poor recreational fishery. Regardless, data from the ISU lake survey suggests that this lake has generally good chemical water quality. Results of this monitoring show no violations of the Class A,B(LW) criteria for pH in the 15 samples collected during summers of 2000 through 2004 (maximum = 9.0; minimum = 7.8 pH units). One of 14 samples exceeded the Class B(LW) criterion for dissolved oxygen. Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s dissolved oxygen criteria and thus do not suggest an impairment of the Class B(LW) uses of Manteno Lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
