Green Belt Lake IA 02-CED-548

Assessment Comments

Assessment is based on results of (1) ISU lake survey in 2000-01 and (2) ISU report on lake plankton communities.

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting."  The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting."  Fish consumption uses are "not assessed."  The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU) and (2) information on plankton communities at Iowa lakes in 2000 from Downing et al.  (2002).   EXPLANATION:  Results from the ISU statewide survey of Iowa lakes suggest neither impairments nor threats to full support of the Class A and Class B uses at this lake.   Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 59, 54, and 54, respectively.   According to Carlson (1977), the index values for total phosphorus places this lake at the upper range of eutrophic lakes; the index values for chlorophyll-a and secchi depth are in the middle range of eutrophic lakes.   These index values suggest relatively low levels of phosphorus, very low levels of chlorophyll-a, and very 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 or zooplankton grazing) other than phosphorus limits production of algae.   Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 18.   This TN:TP ratio suggests a slight possibility that nitrogen limitation suppresses the production of suspended algae at this lake.   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.   The levels of inorganic suspended solids at this lake are low and do not suggest the potential for impairing designated uses.   The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at Greenbelt Lake was 1.5 mg/l; this is the 6th lowest median level of the 130 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 less virtually 100% of the wet mass of the phytoplankton community in the three summer samples was bluegreen algae.   These values, however, given the relatively low levels of chlorophyll-a (i.e., suspended algae) at this lake, do not suggest significant problems with bluegreen algae.   Although results of ISU lake monitoring in 2000 and 2001 suggest full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses.   Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments.   Fish consumption uses are "not assessed" due to lack of fish contaminant monitoring at this lake.