Fogle Lake IA 05-GRA-1795

Assessment Comments

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

Basis for Assessment

SUMMARY:  The Class B(LW) aquatic life uses are assessed (monitored) as "fully supporting".   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 and (2) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey.  The 2004 assessment of this lake as “threatened” due to turbidity and to bluegreen algae were was in error.   The TSI values for chlorophyll-a and Secchi depth for that assessment (57 and 60, respectively) are well below the impairment trigger of 65 and do not suggest a threat or impairment of the Class B(LW) uses of this lake.

EXPLANATION:  Results from the ISU statewide survey of Iowa lakes suggest relatively good water quality at Fogle Lake and “full support” of its Class B(LW) uses.   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 62, 60, and 60, 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-and secchi depth are at the upper boundary of eutrophic lakes.   These index values suggest moderately high levels of phosphorus, relatively low levels of chlorophyll-a, and relatively good water transparency.  

Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Fogle Lake is 25.   This TN:TP ratio suggests that algal production at this lake is limited by availability of phosphorus.  

In terms of all Iowa lakes sampled, data from the ISU survey show large populations of zooplankton species at this lake that graze on algae.   Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 70% 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 (116 mg/l) was the 46th highest of the 131 lakes sampled.  

The levels of inorganic suspended solids at this lake are somewhat elevated and suggest the potential for attenuating light and contributing to 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 Fogle Lake was somewhat lower at 4.7 mg/l.   Despite the somewhat typical levels of inorganic suspended solids, the TSI values for chlorophyll-a and Secchi depth at Fogle Lake suggests relatively good water quality and full support of the Class B(LW) uses

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 (70%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Fogle Lake (13 mg/l) was the 71st lowest of the 131 lakes sampled.   This level is in the lowest two-thirds 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).  

The ISU lake survey data also show good chemical water quality at this lake.   These data show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected during summers of 2000 through 2004.   One of 15 samples exceeded the Class A,B(LW) criterion for pH (maximum = 9.01; minimum = 7.9 pH units).   Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A and Class B(LW) uses of this lake.   This violation likely reflects levels of primary productivity at Fogle Lake and does not reflect the input of pollutants into this lake.

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