Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) ISU reports on lake plankton communities from 2000-05, and (3) surveys by the IDNR Fisheries Bureau.
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 (monitored) as "fully supporting ". The Class C (drinking water) uses are “not assessed” due to the lack of information upon which to base an assessment. 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: 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 68, 58, and 56, 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 mid to upper range of eutrophic lakes. These index values suggest relatively 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 2004, 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. The presence of relatively large populations of zooplankton at Greenfield Lake that graze on algae may also explain the discrepancy between the TSI value for phosphorus (68) and that for chlorophyll-a (58). 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 (169 mg/l) was the 26th highest of the 131 lakes sampled.
Relative to other Iowa lakes, the levels of inorganic suspended solids at this lake are in the range from “typical” to “moderately high” and thus 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 2004 was 5.2 mg/l; the median level at Greenfield Lake (6.4 mg/l) was the 52nd highest of the 131 lakes sampled. Thus, levels of ISS may contribute to the non-phosphorus limitation on algal production at this lake. Given the relatively good water transparency at this lake (overall TSI for Secchi depth = 56), however, the levels of ISS do not appear to adversely affect this lake’s water transparency.
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 (approximately 50%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Greenfield Lake (12.7 mg/l) was the 70th lowest of the 131 lakes sampled. This level is just slightly greater than the overall median of 12.2 mg/l for 131 Iowa lakes sampled. The presence of a moderately 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).
The ISU lake survey data also show good chemical water quality at this lake. These data show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected, or for pH in the 14 samples collected, at Greenfield Lake during summers of 2000 through 2004.
The Class C (drinking water) uses are not assessed due to the lack of recent information upon which to base an assessment. 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 2000-04 show that nitrate levels are very low at this lake (maximum value = 0.7 mg/l; median = 0.1 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.
