Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04 and (2) 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 supported". The lake has relatively low levels of phosphorus and relatively good water transparency. Levels of bluegreen algae are somewhat elevated but do not suggest an impairment of designated uses at this lake. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting." Fish consumption uses are "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 the ISU survey from 2000 through 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 57, 57, and 59, respectively. According to Carlson (1977), all three index values are in the upper range of eutrophic lakes. These index values suggest relatively low levels of phosphorus and chlorophyll-a and suggest relatively good water transparency.
According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The respective TSI values for Green Belt Lake, however, are similar and suggest only a minor non-phosphorus limitation. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for this lake is 21. This TN:TP ratio suggests that algal production at this lake is limited by availability of phosphorus versus nitrogen.
In terms of all Iowa lakes sampled, data from the ISU survey show very small populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised just under 20% 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 (2 mg/l) was the lowest level of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Green Belt Lake.
The levels of inorganic suspended solids at this lake are extremely low and do not suggest the potential for either threatening or impairing designated uses through contribution to turbidity. 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 Green Belt Lake was 1.6 mg/l; this is the 4th lowest median level of the 131 lakes sampled.
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 most (~90%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Green Belt Lake (19 mg/l) was the 46th highest of the 131 lakes sampled. This level is in the lowest two-thirds of the 131 Iowa lakes sampled. The presence of a moderately large population of bluegreen algae at this lake likely 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 show good chemical water quality at this lake. These data show no violations of the Class B(LW) criteria for dissolved oxygen in the 13 samples collected, or for pH in the 15 samples collected, at Green Belt Lake during summers of 2000 through 2004.
Fish consumption uses remain "not assessed" due to lack of fish contaminant monitoring at this lake.
