Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake plankton communities in 2000, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting / threatened" due to slight reductions in water transparency related inorganic turbidity caused by slightly elevated levels of inorganic suspended solids. The Class B(LW) aquatic life uses are also assessed (evaluated) as "fully supporting / threatened" due to siltation impacts. Fish consumption uses remain "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), (2) information from the IDNR Fisheries Bureau, and (3) 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 very good water quality at this lake. The slightly elevated levels of non-algal turbidity, however, may threaten full support of the Class A uses of Slip Bluff Lake. Using the median values from the Iowa State University statewide surveys of Iowa lakes from 2000 through 2002 (approximately nine samples), Carlson's trophic state indices for total phosphorus, chlorophyll-a, and Secchi depth at Slip Bluff Lake are 65, 40, and 55 , respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the middle range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is at the boundary between mesotrophic and oligotrophic lakes, and the index value for secchi depth is in the middle range of eutrophic lakes. These index values suggest moderately high levels of phosphorus, extremely low (and much less than expected) production of suspended algae, and very good water transparency. According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data suggest that non-algal particles do likely limit algal production at Slip Bluff Lake. Based on median values from the ISU survey from 2000-02, the levels of inorganic suspended solids at this lake are somewhat elevated and suggest the potential for limiting algal production and contributing to the slightly elevated levels of in-lake turbidity. 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 Slip Bluff Lake was 3.2 mg/l; this is the 39th lowest median value of the 131 lakes sampled. Nitrogen limitation may also limit algal production at Slip Bluff lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Slip Bluff Lake is 9. This ratio is low and suggests that the availability of nitrogen limits production of algae (this lake had the second lowest median total nitrogen (0.6 mg/l) of the 131 lakes sampled for the ISU survey from 2000-02). Data from Downing et al. (2002) show relatively small populations of zooplankton species at Slip Bluff Lake that graze on algae, thus suggesting a limited ability of zooplankton to reduce algal production through grazing. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised less than 5% of the dry mass of the zooplankton community in the late July sample but increased to approximately 30% in the late August sample. The summer 2000 average mass of Cladoceran grazers (8.2 mg/l) was the 44th lowest of the 131 lakes sampled. Despite the indications of good water quality in terms of low chlorophyll-a, the slightly elevated TSI value for Secchi depth suggests a threat to the full support of the Class A (primary contact) uses through presence of non-algal turbidity.
The presence of nuisance algal species (i.e., bluegreen algae) does not appear to be a significant problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta)in general, comprise a relatively small portion of the summertime phytoplankton community of this lake. Sampling in 2000 showed no bluegreens present in the late June, an increase to over 70% of wet mass in bluegreens in late July, and a decrease to approximately 40% of wet mass in late August. The 2000 average summer mass of bluegreens at this lake (0.76 mg/l) was the 10th lowest of the 131 lakes sampled, thus suggesting the lack of any significant problems with bluegreen algae at this lake.
The slightly eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported" threatened" due to siltation impacts at the lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
