Dale Maffitt Reservoir IA 04-RAC-1119

Assessment Comments

The sources of data for this assessment include (1) results of the statewide surveys of Iowa lakes conducted from 2000 through 2004 by Iowa State University (ISU) and (2) information from ISU plankton surveys at Iowa lakes from 2000-05.

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 continues to demonstrate exceptional water quality.   The Class C (drinking water) uses are “not assessed” due to lack of sufficient parameter coverage to allow development of an assessment.   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:  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 2004 (approximately 15 samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 47, 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 2004, 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 the ISU survey show relatively small populations of zooplankton species at this that graze on algae.   Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 50% of the dry mass of the zooplankton community and that Dale Maffit Lake had the 12th lowest median summer sample level of zooplankton (Cladoceran) grazers (23.9 mg/l) of the 131 lakes sampled.  These results suggest little potential for zooplankton grazers to limit algal production at this lake.  

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 131 lakes sampled for the ISU lake survey from 2000 through 2004 was 5.2 mg/l; the median level at Dale Maffitt Lake was 3.8 mg/l.   This median value is the 50th lowest 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) dominate (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 Dale Maffit Lake  was the 25th lowest of the 131 lakes sampled:  4.9 mg/l.   This levels is in the lowest 25% of the 131 Iowa lakes sampled.   The presence of a relatively small population of bluegreen algae at this lake 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 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).   Results of chemical water quality monitoring conducted as part of the ISU lakes survey also suggest good water quality at this lake.   The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 13 samples collected, or for pH in the 14 samples collected, at Dale Maffit Reservoir during summers of 2000 through 2004.

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 2000-04 show that nitrate levels are extremely low at this lake (maximum value = 0.6 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.