Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02 and (2) ISU report on lake plankton communities in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "partially supported" due to presence of large populations of nuisance aquatic (algal) life (bluegreen algae). Despite potential problems with bluegreen algae, the lake has relatively low levels of phosphorus and very good water transparency. 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 conducted from 2000 through 2002 by Iowa State University (ISU) and (2) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002).
EXPLANATION: Using the median values from the ISU survey from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 58, 55, and 57, respectively. According to Carlson (1977), all three index values are in the middle to upper range of eutrophic lakes. These index values suggest relatively low levels of phosphorus, very low levels of chlorophyll-a, and 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 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 availability of phosphorus versus nitrogen. Data from Downing et al. (2002) suggest very small populations of zooplankton species at Greenbelt Lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised less than 10% of the dry mass of the zooplankton community in the early August and late August samples. According to Downing et al. (2002), the average Cladoceran mass for summer 2000 was less than 1 mg/l (i.e., extremely low) and suggests little potential for non-phosphorus limitation due to zooplankton grazing at this lake. The levels of inorganic suspended solids at this lake are very low and do not suggest the potential for either threatening or 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 Greenbelt Lake was 1.3 mg/l; this is the 6th lowest median level of the 131 lakes sampled.
Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) completely dominate the summertime phytoplankton community of this lake. Sampling in 2000 showed that virtually 100% of the wet mass of the phytoplankton community in the three samples was bluegreen algae. The 2000 summer average mass of bluegreen algae at Green Belt Lake (33 mg/l) was the 29th highest of the 131 lakes sampled and suggests a potential impairment due to violation of Iowa’s narrative standard protecting against presence of nuisance aquatic life. The amount of data available for characterizing algal populations at this lake (one season), however, is not sufficient for developing a more accurate assessment of support of these uses. Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to improve the accuracy of future water quality assessments for this lake. Fish consumption uses remain "not assessed" due to lack of fish contaminant monitoring at this lake.
