Lake Sugema IA 04-LDM-1029

Assessment Comments

Assessment is based on results of (1) ISU lake survey from 2000-04, (2) ISU reports on lake plankton communities from 2000-05, 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 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.  

EXPLANATION:  Using the median values from this survey from 2000 through 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 61, 58, and 58, 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 values for chlorophyll-a and Secchi depth are in the upper 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 relatively 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 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 2004, the ratio of total nitrogen to total phosphorus for Lake Sugema is 21; 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 2004 was 5.2 mg/l; the median level at Lake Sugema was 3.1 mg/l.  

In terms of all Iowa lakes sampled, data from the ISU survey show very extremely small populations of zooplankton species at this lake that graze on algae.   Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 25% of the dry mass of the zooplankton community of this lake  The average per summer sample mass of Cladoceran taxa (9 mg/l) was the 3rd lowest level of the 131 lakes sampled.   These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Lake Sugema.  

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 (approximately 75%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Lake Sugema (15 mg/l) was only the 52nd highest of the 131 lakes sampled.   This levels is in the lowest two-thirds of the 131 Iowa lakes sampled.   The presence of this 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).  

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."  The ISU lake survey data also suggest relatively good water quality at Lake Sugema.   Results of monitoring during summers of 2000 through 2004 show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected, or for pH in the 15 samples collected.  

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