Lake Sugema IA 04-LDM-1029

Assessment Comments

Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake phytoplankton communities in 2000, and (3) surveys by IDNR Fisheries Bureau.

Basis for Assessment

SUMMARY:  The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting" [this lake is not designated for Class A (primary contact recreation) uses in the Iowa Water Quality Standards].   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), (2) information from the IDNR Fisheries Bureau, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al.  (2002).  

EXPLANATION:  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 61, 59, and 55, 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 in the upper range of 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 relatively 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 or high levels of inorganic turbidity.   The nine-point increase in chlorophyll-a TSI values between the 2000-2001 period (TSI=50) and the 2000-2002 period (TSI=59) suggests some concern with an increase in algal production at this lake.

The general agreement between the TSIs for phosphorus, chlorophyll, and Secchi depth suggests that non-phosphorus limitations—which appear to occur at most Iowa lakes—do not occur at Lake Sugema.   Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Lake Sugema is 22; this ratio suggests that algal production is not limited by nitrogen availability.   The levels of inorganic suspended solids at this lake are somewhat elevated but do not suggest the potential for 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 Lake Sugema was 3.4 mg/l.   Data from Downing et al.  (2002) show relatively small 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.   The 2000 summer average mass of Cladocerans (2.8 mg/l) was the 23rd lowest of the 131 lakes, suggesting little impact of zooplankton grazing on algal populations.

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.   Despite the apparent dominance of bluegreens, the 2000 average summer mass of bluegreen algae (9.3 mg/l) was the 64th lowest of the 131 lakes sampled; this level does not suggest a strong possibility of a threat or impairment of designated uses.

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.