Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau, and (3) ISU reports on lake plankton communities from 2000-05.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting". The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting ". Large populations of bluegreen algae (nuisance (=noxious) aquatic life) at this lake, however, are a concern for support of the Class B(LW) uses. 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 “full support” of the Class A and Class B uses of Lake Minnewashta. 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 67, 57, and 56, 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 values for chlorophyll-a and Secchi depth are in the middle range of eutrophic lakes. These index values suggest moderately high levels of phosphorus in the water column, relatively low (and less than expected) levels of chlorophyll-a, and relatively good (and better than expected) 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) other than phosphorus limits production of algae. The ISU lake data suggest that algal production at Lake Minnewashta may be limited by a combination of non-algal turbidity, nitrogen limitation, and inorganic turbidity. None of these factors alone, however, would appear to explain the apparent non-phosphorus limitation on algal production at this lake.
The levels of inorganic suspended solids at this lake are low relative to other Iowa lakes and, while not suggesting the potential for impairing designated uses, do suggest the potential for limiting algal production. 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 Lake Minnewashta was 3.9 mg/l.
Nitrogen limitation may also serve as a non-phosphorus limitation to algal production at this lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Lake Minnewashta is 16. This TN:TP ratio suggests the possibility that nitrogen limitation may, at times, suppress the production of suspended algae at this lake.
The presence of moderately large populations of zooplankton at Lake Minnewashta that graze on algae, however, may explain the discrepancy between the TSI value for phosphorus (67) and that for chlorophyll-a (57). 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 55% 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 (153 mg/l) was the 35th 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 Lake Minnewashta.
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 the majority (approximately 80%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Lake Minnewashta (6.7 mg/l) was the 35th lowest of the 131 lakes sampled. This levels is in the lowest 25% 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. 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 less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes. No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the ability to characterize the levels of bluegreen algae at this lake 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, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence. Information from the IDNR Fisheries Bureau suggests that Lake Minnewashta has high levels of bluegreen algae that impair the Class B(LW) aquatic life uses of this lake. Based on the relatively low levels of bluegreen algae reported as part of the ISU lake survey in 2000, however, the Class B(LW) uses will remain assessed as “fully supported”.
Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. Results of this monitoring show no violations of the Class A,B(LW) criteria for pH in the 14 samples collected during summers of 2000 through 2004 (maximum = 8.8; minimum = 8.2 pH units). One of 13 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 Lake Minnewashta.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
