Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02 and (2) ISU report on lake plankton communities in 2000.
Basis for Assessment
SUMMARY: Despite the indications of very low nutrient levels and excellent water transparency at this lake, the Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened" due to moderately large populations of bluegreen algae (=noxious aquatic plants) at this lake. [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 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 B(LW) uses of South Prairie Lake are "fully supported / threatened." 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 56, 51, and 53, 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. 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, zooplankton grazing, or some other factor) limits production of algae. 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 inorganic turbidity may limit algal production at this lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 18, 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 lowest median total nitrogen level (0.6 mg/l) of the 131 lakes sampled from 2000-02 as part of the ISU survey, thus again suggesting a potential for nitrogen limitation. Data from Downing et al. (2002) show relatively small populations of zooplankton species at this 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 early July samples. The average 2000 summer mass of Cladoceran grazers (4.8 mg/l) was the 33rd lowest of the 131 lakes sampled. At these population levels, grazing of algae by zooplankters likely does not affect water transparency. The levels of inorganic suspended solids at this lake are somewhat elevated but 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 2002 was 4.8 mg/l; the median level at South Prairie Lake was 3.3 mg/l. The somewhat elevated levels of inorganic suspended solids suggest at least the potential for limiting algal production through light attenuation.
The levels of bluegreen algae appear to be moderately high at South Prairie Lake and present a threat full support of the Class B(LW) aquatic life uses. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of this lake, especially in mid to late summer. Sampling in 2000 showed that bluegreen algae comprised approximately 10% of the wet mass of the phytoplankton community in the early June sample but increased to 70% and 90% in the early August and early September samples, respectively. The average 2000 summer mass of bluegreen algae at this lake (20.9 mg/l) was the 43rd highest of the 131 lakes sampled. This level suggests a threat to full support of Class B(LW) uses. The occurrence of these moderately large populations of bluegreen algae in a lake with otherwise excellent water quality is puzzling, although the low TN:TP ratio may contribute to the success of bluegreen algae at this lake. Fish consumption uses remain"not assessed" due to the lack of fish contaminant monitoring at this lake.
