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 2002 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 remain assessed (monitored) as "fully supporting."  The Class B(LW) aquatic life uses also remain assessed (evaluated) as "fully supporting."  In terms of nutrient condition and water transparency, this lake has exceptional water quality.   Fish consumption uses are "not assessed" due to the lack of fish contaminant monitoring at this lake.   The Class C (drinking water) uses are “not assessed” due to lack of sufficient parameter coverage to allow development of an assessment.   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:  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 from 2000 through 2002 (approximately nine samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 49, 40, and 43, respectively.   According to Carlson (1977), the index values for all three parameters place this lake in the range of mesotrophic lakes.  These index values suggest extremely low levels of phosphorus and chlorophyll-a and suggest exceptional water transparency for an Iowa lake.   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 lower 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, however, do not immediately suggest a non-phosphorus limitation to algal production.   Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Dale Maffitt Lake is 30.   This ratio suggests that algal production is not limited 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 2000 average summer mass of zooplankton grazers (1.5 mg/l) was the 15th lowest of the 131 lakes sampled, thus suggesting little potential for zooplankton grazers to limit algal grazers.   The levels of inorganic suspended solids at this lake are very low and do not suggest the potential for 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 Dale Maffitt Lake was 2.1 mg/l.   This median value is the 15th lowest of the 131 lakes sampled.  

This lake does not appear to have significant problems due to presence of nuisance aquatic 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).   The 2000 average summer mass of bluegreen algae at Dale Maffit Lake (9.4 mg/l), while not excessive, ranked as the 65th highest of the 131 lakes sampled.  

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 (e.g., Bachmann et al.  1980, Bachmann et al.  1994)  The Class C (drinking water) uses remain "not assessed" due to lack of 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 2000-02 show that nitrate levels are extremely low at this lake (maximum value = 0.4 mg/l; median = 0.2 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.