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 "partially supported" due poor water transparency caused primarily by algal blooms. Large populations of bluegreen algae present an additional concern and potential impairment of the Class A uses. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supported". Nutrient loading to the water column, moderately high levels of algal turbidity, and siltation impacts, 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 and information from the DNR Fisheries Bureau suggest that high levels of algal turbidity impair the Class A and Class B(LW) uses of Meadow Lake. Using the median values from the Iowa State University statewide surveys of Iowa lakes from 2000 through 2004 (approximately 15 samples), Carlson's trophic state indices for total phosphorus and chlorophyll-a at Meadow Lake are 75, 64 and 65 , respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index values for chlorophyll-a and Secchi depth are in the middle range between eutrophic and hyper-eutrophic lakes. These index values suggest very high levels of phosphorus in the water column, somewhat elevated (but less than expected) levels of chlorophyll-a (suspended algae), and moderately poor 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 three factors (non-algal (inorganic) particles, nitrogen limitation, and zooplankton grazing) limit algal production at Meadow Lake.
The levels of inorganic suspended solids at this lake are moderately high and suggest at least the 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 Meadow Lake was slightly greater than this median (5.2 mg/l); this was the 59th highest level of the 131 lakes sampled.
Nitrogen availability 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 Meadow Lake is 12. This TN:TP ratio is low and suggests that nitrogen availability may, at time, limit algal production at this lake.
In addition, the presence of moderately large populations of zooplankton at Meadow Lake that graze on algae may explain the discrepancy between the TSI value for phosphorus (75) and that for chlorophyll-a (64). 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 (112 mg/l) was the 23rd highest of the 131 lakes sampled.
The water quality conditions at Meadow Lake suggest that the moderately high TSI values for chlorophyll-a and Secchi depth indicates the potential for impairing the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and poor water transparency. The IDNR Fisheries Bureau also notes that Meadow Lake is impaired by algae and that anglers complain about algae at this lake.
The presence of nuisance (=noxious) algal species (i.e., bluegreen algae) may also present an impairment of the Class A uses at Meadow Lake. Data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion of this lake’s summertime phytoplankton community. Summer sampling during this period showed the percent wet mass of the total phytoplankton community in bluegreens (cyanobacteria) was approximately 70%. Also, the median per summer sample mass of bluegreen algae at this lake (39 mg/l) was the 24th highest of the 131 lakes sampled. This median is in the worst 25% of the 131 Iowa lakes sampled. The presence of a large population of bluegreen algae at this lake suggests the 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 greater than the 75th percentile of this distribution (~29 mg/l) were arbitrarily chosen by IDNR staff to represent the condition of “potential impairment: partially supported.” 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, moderately high levels of algal turbidity, and siltation in the lake remain water quality concerns at this lake that may affect support of the Class B(LW) uses. Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. Results of this monitoring show that two of the 15 samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that 3 of the 15 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 9.6; minimum = 7.1 pH units). Based on IDNR’s assessment methodology, these results do not suggest violation frequencies are significantly greater than 10% for either parameter and thus do not suggest impairment of either the Class A and Class B(LW) uses of Meadow Lake. The violations of the pH criterion likely reflect the high levels of primary productivity at this lake and do not reflect the input of pollutants.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
