Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04 and (2) ISU reports on lake plankton communities from 2000-5.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses are assessed (monitored) as "fully supporting". [Note: This lake is not designated for Class A (primary contact recreation) uses.] 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 and (2) 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 indicate good water quality at this lake and suggest that the Class B(LW) uses of South Prairie 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 55, 51, and 54, respectively. According to Carlson (1977), the index values for all three parameters are in the middle to lower range of eutrophic lakes. These index values suggest relatively low levels of phosphorus in the water column, very low levels of chlorophyll-a, and very good water transparency. These results suggest that this lake has above average water quality and does not have threats or impairments due to aesthetically objectionable conditions related either to blooms of algae or to high levels of inorganic turbidity.
The generally good agreement between the TSI values for phosphorus, chlorophyll, and Secchi depth suggests that significant non-phosphorus limitations on algal production (e.g., nitrogen limitation, inorganic turbidity, or zooplankton grazing) do not exist at this lake. However, both nitrogen-limitation and zooplankton grazing may limit algal production at this lake. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for this lake is 20, thus suggesting that phosphorus (versus nitrogen) is the nutrient that most often limits production of suspended algae at this lake. In the assessment developed for the 2002 report, however, a TN:TP ratio of 9 was calculated from median values from ISU lake surveys in 2000 and 2001. This tendency for a low TN:TP ratio does suggest, however, a potential for nitrogen limitation to occur at this lake. In addition, this lake had the sixth lowest median total nitrogen level (0.7 mg/l) of the 131 lakes sampled from 2000-04 as part of the ISU survey, thus again suggesting a potential for nitrogen limitation.
In addition, the presence of moderately large populations of zooplankton at South Prairie Lake that graze on algae may explain the slight discrepancy between the TSI value for phosphorus (55) and that for chlorophyll-a (51). Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 50% of the dry mass of the zooplankton community of this lake. The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (84 mg/l) was the 68th highest of the 131 lakes sampled.
The levels of inorganic suspended solids at this lake are relatively 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 2004 was 5.2 mg/l; the median level at South Prairie Lake was 3.6 mg/l.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of designated uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (73%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at South Prairie Lake (8 mg/l) was the 42nd lowest of the 131 lakes sampled. This level is in the lowest third of the 131 Iowa lakes sampled. Thus, 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.
The ISU lake survey data also show generally good chemical water quality at this lake. Only one violation of the Class B(LW) criteria for dissolved oxygen occurred in the 14 samples collected (7%) during summers of 2000 through 2004. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), however, a violation frequency of less than 10% for conventional parameters such as dissolved oxygen suggests "full support" of aquatic life uses. Thus, the percentage of violations of the dissolved oxygen criterion at this station does not suggest an impairment of aquatic life uses at this lake. Two of 15 samples exceeded the Class A,B(LW) criterion for pH (maximum = 9.4; minimum = 7.7 pH units). Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A and Class B(LW) uses of this lake. These violations likely reflect the high levels of primary productivity at South Prairie Lake and do not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
