Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-02, (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 remain assessed (evaluated) as "fully supporting". Nutrient loading to the water column, high levels of non-algal turbidity, and siltation, however, remain concerns 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 that high levels of non-algal (inorganic) turbidity may adversely affect the Class A and Class B(LW) uses of Meyers 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 72, 60, and 63, 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 upper boundary of eutrophic lakes, and the index value for secchi depth is in the middle range between eutrophic and hyper-eutrophic lakes. These index values suggest (1) very high levels of phosphorus in the water column, (2) relatively low (and much less than expected) levels of chlorophyll-a (suspended algae), and (3) marginally good 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 a combination of nitrogen limitation and zooplankton grazing may limit algal production at Meyers Lake.
The levels of inorganic suspended solids at this lake are very low and suggest little potential for contributing to in-lake turbidity and limiting production of suspended algae. 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 Meyers Lake (2.6 mg/l) was the 21st lowest level of the 131 lakes sampled.
Nitrogen limitation may also limit algal production at this lake. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Meyers Lake is 12. This TN:TP ratio is low and suggests that nitrogen availability may, at times, limit algal production at this lake.
The presence of very large populations of zooplankton at Meyers Lake that graze on algae may explain the discrepancy between the TSI value for phosphorus (72) and that for chlorophyll-a (60). 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 about 60% 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 (151 mg/l) was the 36th highest of the 131 lakes sampled. This large population of zooplankton grazers suggests the potential for this type of non-phosphorus limitation on algal production at Meyers Lake.
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 (over 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 Meyers Lake (15.8 mg/l) was the 81st lowest of the 131 lakes sampled. This level is in the lower two-thirds 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 the Class B(LW) aquatic life uses should be assessed as "fully supported". Nutrient loading to the water column, high levels of non-algal turbidity, and siltation in the lake, however, remain water quality concerns at this lake. The ISU lake survey data show generally good water quality at this lake. These data show no violations of the Class B(LW) criteria for dissolved oxygen in the 13 samples collected at Meyers Lake during summers of 2000 through 2004. Three of 15 samples exceeded the Class B(LW) criterion for pH (maximum = 9.4; minimum = 8.2 pH units). Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A and Class B(LW) uses of this lake. These violations likely reflect the high levels of primary productivity at Meyers 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.
