Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) ISU reports on lake plankton communities from 2000-5, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting". The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting". Siltation impacts, however, remain a concern at this lake. 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 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.
Note: A TMDL for siltation at Slip Bluff Lake was prepared by IDNR and approved by EPA in 2001. Through implementation of this TMDL, the siltation problem at this lake was addressed. The Class A,B(LW) uses of this lake were assessed as "fully supported" for the 2004 and the current (2006) assessment/listing cycle. Thus, this lake was placed into IR Category 2a (fully supported) for the 2004 assessment/listing cycle and remains in IR Category 2a for the 2006 cycle.
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 2004 (approximately 15 samples), Carlson's trophic state indices for total phosphorus, chlorophyll-a, and Secchi depth at Slip Bluff Lake are 53, 43, and 55 , respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the lower range of eutrophic lakes; the index value for chlorophyll-a is in the lower range of mesotrophic lakes, and the index value for secchi depth is in the middle range of eutrophic lakes. These index values suggest very low 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 indicates that 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 2000-04, 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 2004 was 5.2 mg/l; the median level at Slip Bluff Lake was 3.7 mg/l; this is the 46th 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 2004, the ratio of total nitrogen to total phosphorus for Slip Bluff Lake is 22. This ratio suggests that the algal production at this lake is likely limited by the availability of phosphorus. Despite the moderately high TN:TP ration, 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.
In terms of all Iowa lakes sampled, data from the ISU survey show very small populations of zooplankton species at this lake that graze on algae. Although sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 60% 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 (39 mg/l) was the 19th lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Slip Bluff Lake.
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) comprise a significant portion (almost 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 Slip Bluff Lake (6 mg/l) was the 31st lowest of the 131 lakes sampled. This level is in the lowest 25 percent 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).
A comparison of annual TSI values for the 2000-2004 period at Slip Bluff Lake suggests improving trends in levels of phosphorus and Secchi depth. Based on ISU lake survey monitoring data, annual summer TSI values for total phosphorus have generally improved over the five-year period: 2000: 69; 2001: 68; 2002 = 47; 2003 = 51; and 2004 = 50. The biggest improvement occurred between summers of 2001 and 2002 when TSI values dropped from the high 60s to the upper 40s and low 50s. A similar improvement can be seen for Secchi depth where TSIs were in the low 60s in 2000 and 2001 and have been in the mid-50s since 2002. As levels of phosphorus have declined and as water transparency has improved, levels of chlorophyll have increased somewhat. From 2000 to 2002, the chlorophyll TSI levels were extremely low and were in the upper range of oligotrophic lakes (a truly atypical situation for an Iowa lake). Since 2002, the chlorophyll TSI values have been in the high 40s and low 50s; these values, however, continue to suggest very low levels of chlorophyll and “full support” of the Class A uses at Slip Bluff Lake .
Information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed (evaluated) as "fully supported". Siltation impacts at the lake, however, remain a concern. The ISU lake survey data show generally good chemical water quality at Slip Bluff Lake. No violations of the Class B(LW) criteria for dissolved oxygen or pH occurred in the approximately 14 samples collected during summers of 2000 through 2004.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
