Assessment Comments
Assessment is based on results of (1) ISU statewide survey of Iowa lakes from 2000-02, (2) surveys by the IDNR Fisheries Bureau, (3) information on plankton communities at Iowa lakes in 2000, (4) results of EPA/DNR fish tissue (RAFT) monitoring in 1994, and (5) results of TMDL monitoring conducted by DNR in 2002.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting / threatened" due to somewhat elevated algal populations and due to presence of only marginally good water transparency. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to excessive nutrient loading to the water column, high levels of algal turbidity, and siltation. Fish consumption uses are assessed (evaluated) as “fully supported” based on results of fish contaminant monitoring in 1994. 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, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), (4) results of U.S. EPA / IDNR fish contaminant monitoring in 1994, and (5) results of TMDL monitoring conducted by Iowa DNR in 2002.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of algal turbidity may threaten full support of the Class A uses and may impair the Class B(LW) uses of Lake Meyer. 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 66, 59, and 63, respectively. According to Carlson (1977), the index values for total phosphorus and Secchi depth are in the range between eutrophic and hyper-eutrophic lakes. The level of chlorophyll-a is in the upper range of eutrophic lakes. These index values suggest moderately high levels of phosphorus, relatively low levels of chlorophyll-a, and 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 indicates 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 Meyer may be limited by zooplankton grazing; neither nitrogen limitation nor inorganic turbidity appear to limit algal production at this lake. Downing et al. (2002) show that Cladoceran taxa (e.g., Daphnia) comprised approximately 45% of the dry mass of the zooplankton community in the early August sample and nearly 50% of the mid-September sample. The 2000 average summer mass of Cladoceran grazers at Lake Meyer (91.1 mg/l) was the 19th highest of the 131 lakes sampled and thus suggests the potential for a non-phosphorus limitation to algal production due to Cladoceran grazers. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 20. This TN:TP ratio suggests that algal production at this lake is limited by phosphorus as opposed to nitrogen. The levels of inorganic suspended solids at this lake are low and do not suggest a strong 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 Lake Meyer was 2.3 mg/l; this was the 24th lowest level of the 131 lakes sampled. The somewhat elevated TSI values for Secchi depth suggest a threat to full support of the Class A (primary contact) uses through presence of aesthetically objectionable conditions (turbidity). The results of ISU monitoring, however, do not immediately suggest a cause of this turbidity. Based on information from the IDNR Fisheries Bureau, the slight reductions in water transparency at Lake Meyer will be attributed to algae and algal blooms.
The levels of nuisance algal species (i.e., bluegreen algae) at Lake Meyer are moderately high and are of concern, but they likely do not threaten or impair designated uses. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a relatively large portion of the summertime phytoplankton community, with greater than 60% wet mass in bluegreens in early summer increasing to greater than 80% bluegreens in late summer. The 2000 average summer mass of bluegreen algae (11 mg/l) was the 57th highest of the 131 lakes sampled.
The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should remain assessed as "partially supported" due to excessive nutrient loading to the water column, high levels of algal turbidity, and siltation in the lake. Despite these impairments of Class B(LW) uses, results of monitoring conducted by IDNR/UHL in support of TMDL development at this lake from March to October 2002 showed no violations of Class B(LW) water quality criteria for dissolved oxygen or ammonia-nitrogen in the 10 samples collected. One of the 10 samples exceeded the Iowa water quality criterion for pH; this violation does not suggest either a threat or impairment to the designated uses of Lake Meyer. Fish consumption uses are assessed (evaluated) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 1994 (see assessment for the 1996 report). However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively low confidence).
