Lake Sugema IA 04-LDM-1029

Assessment Comments

Assessment is based on results of (1) ISU lake survey in 2000-01, (2) ISU report on lake phytoplankton communities, and (3) surveys by IDNR Fisheries Bureau.

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting."  The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting."  Fish consumption uses are "not assessed."  The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 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 that the Class A uses of Lake Sugema are "fully supported."  Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 61, 50, and 54, respectively.   According to Carlson (1977), the index value for total phosphorus places this lake in the lower range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is at the boundary between mesotrophic and eutrophic lakes, and the index value for secchi depth is in the middle range of eutrophic lakes.   These index values suggest that, despite the somewhat elevated levels of phosphorus, the production of suspended algae is extremely low and that water transparency is very good for Iowa lakes.   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 low values for chlorophyll-a and secchi depth indicate that some factor (e.g., nitrogen limitation or zooplankton grazing) limits production of algae.   Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Lake Sugema is 21.5).   This ratio suggest the possibility that algal production at this lake is, in part, limited by the availability of nitrogen.   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 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at Lake Sugema was 3.7 mg/l.   This lake does not appear to have serious problems due to presence of nuisance algal species (i.e., bluegreen algae).   Data from Downing et al.  (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of this lake, especially in mid and late summer.   Sampling in 2000 showed that bluegreen algae comprised approximately 30% of the wet mass of the phytoplankton community in the late June sample but increased to approximately 80% in the late July and mid- August samples.   Although bluegreen algae comprise a significant portion of the summertime phytoplankton community, the production of chlorophyll is very low at this lake, and the presence of bluegreen algae likely does not present a water quality impact.   In addition, data from Downing et al.  (2002) show moderately large populations of zooplankton species at Lake Sugema that graze on algae.   Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia), while not present in the late July sample, comprised approximately 45% of the dry mass of the zooplankton community in the mid-August.   Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses.   Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments.  

The water quality 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."  Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.