Dale Maffitt Reservoir IA 04-RAC-1119

Assessment Comments

The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000 through 2001 by Iowa State University (ISU) and (2) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002).

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 Class C (drinking water) 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 that the Class A uses of Dale Maffitt Lake are “fully supported.”  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 67, 34, and 43, respectively.   According to Carlson (1977), the index value for total phosphorus places this lake in the range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is in the middle range of oligotrophic lakes, and the index value for secchi depth is in the lower range of mesotrophic lakes.   These index values suggest that, despite the moderately high levels of phosphorus, the production of suspended algae is extremely low (the lowest two-year median chlorophyll-a of the 130 lakes sampled), and water transparency is exceptional for Iowa lakes.   These results suggest that this lake does not have impairments due to aesthetically objectionable blooms of algae.   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) limits production of algae.   Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Dale Maffitt Lake is 11.8.   This ratio suggests that algal production is limited, at least in part, by nitrogen availability.   Data from Downing et al.  (2002) show relatively small populations of zooplankton species at Dale Maffitt Lake that graze on algae.   Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised only about 10% of the dry mass of the zooplankton community in the late July sample and about 30% of the late August sample.   The levels of inorganic suspended solids at this lake are relatively 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 Dale Maffitt Lake was 2.1 mg/l.   This median value is the 13th lowest of the 130 lakes sampled.  

This lake does not appear to have problems due to presence of nuisance algal species (i.e., bluegreen algae).   Data from Downing et al.  (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of this lake.   Sampling in 2000 showed that bluegreen algae comprised over 95% of the wet mass of the phytoplankton community all three samplings (late June, late July, and late August).   Although bluegreen algae appear to dominate the summertime phytoplankton community, the production of chlorophyll is extremely low at this lake, and the presence of bluegreen algae likely does not present a water quality impact.  

The water quality conditions at this lake suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported." This lake, however, is not identified as one of Iowa’s “significant publicly-owned lakes” (SPOLs) and thus has not been assessed as part of previous lake surveys conducted either by Iowa DNR or Iowa State University.   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 remain "not assessed" due to the lack of fish contaminant monitoring at this lake.