Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake plankton communities in 2000, and (3) the 1995 UHL survey of Iowa water supply reservoirs (Miller and Kennedy 1995).
Basis for Assessment
[NOTE: This lake was added to the surface water classifications of the Iowa Water Quality Standards in 2004 as a Class B(LW) (aquatic life uses) and Class C uses (drinking water uses) waterbody.]
SUMMARY: The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened" due to slightly elevated levels of turbidity related to algal and non-algal sources. The Class C (drinking water) uses remain assessed (evaluated) as "fully supporting / threatened" based on results of sampling conducted by UHL in January 1995. 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 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), and (4) results of a drinking water study conducted by UHL in January 1995.
EXPLANATION: 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 68, 57, and 57, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth are in the upper 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 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 results of ISU monitoring suggest that inorganic turbidity and/or zooplankton grazers may limit algal production at this lake; nitrogen limitation is likely minor at this lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Greenfield Lake is 14. This TN:TP ratio suggests some potential for nitrogen limitation at this lake. In addition, data from Downing et al. (2002) show relatively large populations of zooplankton species at this lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 50% of the dry mass of the zooplankton community in the mid-July and early August samples. The summer 2000 average mass of zooplankton grazers at this lake (45.8 mg/l) was the 29th highest of the 131 lakes sampled and suggests the potential for limiting algal production. The levels of inorganic suspended solids at this lake are moderately high and also suggest the potential for contributing to in-lake turbidity and limiting algal production. 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 Greenfield Lake (6.9 mg/l) was the 42nd highest of the 131 lakes sampled. Thus, the TSI values for the 2000-02 period for Greenfield Lake suggest relatively good water quality; the levels of chlorophyll-a and Secchi depth suggest only minor threats to full support of the designated uses at this lake.
The presence of nuisance aquatic (algal) species (i.e., bluegreen algae) does not appear to be a significant problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a very low portion of the summertime phytoplankton community of this lake. In 2000, less than 5% of the wet mass of the phytoplankton community was bluegreen algae in samples collected in mid-June, mid-July, and early August. The summer 2000 average mass of bluegreen algae at this lake (0.9 mg/l) was the 11th lowest of the 131 lakes sampled.
The Class C (drinking water) uses remain assessed (evaluated) as "fully supported / threatened" based on results of sampling conducted by UHL in January 1995 (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). The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU survey from 2000-02 show that nitrate levels are very low at this lake (maximum value = 1.8 mg/l; median = 1.2 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
