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 A (primary contact recreation) uses are assessed (monitored) as "fully supporting". The Class B(LW) aquatic life uses remain 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.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that the Class A uses of Willow Lake are "fully supported"; this lake has some of the best trophic state index numbers of any lake in the state. 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 54, 42, and 46, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the middle range of eutrophic lakes; the index values for chlorophyll-a and Secchi depth are in the range of mesotrophic lakes. These index values suggest very low levels of phosphorus, extremely low levels of chlorophyll-a (the fourth lowest of the 131 lakes sampled), and water transparency that is exceptional for Iowa lakes. These results suggest that this lake does not have impairments due to aesthetically objectionable conditions related either to blooms of algae or to high levels of inorganic turbidity.
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, zooplankton grazing, or some other factor ) other than phosphorus limits production of algae. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Willow Lake is 21. This ratio suggests little potential that algal production at this lake is limited by nitrogen availability.
In terms of all Iowa lakes sampled, data from the ISU survey show relatively 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 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 (59 mg/l) was the 42nd lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Willow Lake.
The levels of inorganic suspended solids at this lake are extremely low relative to other Iowa lakes and thus do not suggest the potential for either limiting algal production or 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 Willow Lake was 2.2 mg/l. This median value was the 13th lowest of the 131 lakes sampled.
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 (61%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Willow Lake (13.2 mg/l) was only the 60th highest of the 131 lakes sampled. This level is approximately equal to the median summer mass for all of 131 Iowa lakes sampled (12.2 mg/l). The presence of a moderately large 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". Siltation impacts, however, remain a concern at this lake. The ISU lake survey data show good chemical water quality at Willow Lake. Results of this monitoring show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected during summers of 2000 through 2004 and show that only one of 15 samples (7%) exceeded the Class B(LW) criterion for pH (maximum = 9.0; minimum = 8.3 pH units).
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
