Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau, and (3) ISU reports on lake plankton communities from 2000-05.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supporting" due to very poor water transparency that violates Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. This poor water transparency is caused by a combination of inorganic turbidity (inorganic suspended solids) and algal blooms. Also, the presence of large populations of bluegreen algae suggests an additional potential impairment of these uses. Frequent violations of the state criterion for pH represent an additional water quality impairment. The Class B(LW) aquatic life uses remain assessed (monitored) as "partially supporting" due to excessive nutrient loading to the water column, nuisance blooms of algae, siltation in the lake, and frequent violations of the state pH criterion. 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 of monitoring conducted by ISU from 2000 through 2004 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses are impaired due to poor water transparency at this lake. Using the median values from the Iowa State University statewide surveys of Iowa lakes from 2000 through 2004 (approximately 15 samples), Carlson’s trophic state indices for total phosphorus, chlorophyll-a, and secchi depth at White Oak Lake are 76, 64, and 70, respectively. According to Carlson (1977), the index values for total phosphorus and Secchi depth place this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in middle range between eutrophic and hyper-eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, moderately high (but less than expected) production of suspended algae, and very poor water transparency.
According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data suggest that non-algal particles do likely contribute to limiting algal production at White Oak Lake. 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. Of 131 lakes sampled, White Oak Lake had the 63rd highest median level of inorganic suspended solids (4.8 mg/l), thus suggesting that non-algal turbidity can, at times, limit the production of algae as well as contribute to reductions in water transparency.
These conditions suggest an suggest impairment to the Class A (primary contact) uses primarily due to presence very poor water transparency that violates Iowa’s narrative water quality standards protecting against aesthetically objectionable conditions. This poor water transparency is likely caused by a combination of inorganic and algal turbidity. The IDNR Fisheries Bureau concurs that turbidity-related impairments exist at this lake.
Apparently, other factors, possibly nitrogen limitation or zooplankton grazing, also tend to limit production of algae in this lake. Based on the 2000-2004 data, the TN/TP ratio for this lake is 10. This ratio is relatively low and does suggest that algal production at this lake is, at times, limited by nitrogen availability.
In terms of all Iowa lakes sampled, data from the ISU survey show only moderately large populations of zooplankton species at this lake that graze on algae. Although sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 65% 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 (83 mg/l) was only the 69th highest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at White Oak Lake.
The presence of nuisance (=noxious) algal species (i.e., bluegreen algae) may also present an impairment of the Class A uses at White Oak Lake. Data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion of this lake’s summertime phytoplankton community. Summer sampling during this period showed the percent wet mass of the total phytoplankton community in bluegreens (cyanobacteria) was approximately 80%. Also, the median per summer sample mass of bluegreen algae at this lake (28.8 mg/l) was the 33rd highest of the 131 lakes sampled. This median is in the worst 25% of the 131 Iowa lakes sampled. The presence of a relatively large population of bluegreen algae at this lake suggests the 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 greater than the 75th percentile of this distribution (~29 mg/l) were arbitrarily chosen by IDNR staff to represent the condition of “potential impairment: partially supported.” 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 hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses are "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and siltation in the lake. The ISU lake survey data for chemical water quality of White Oak Lake suggest impairment of the Class A and Class B(LW) uses. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 15 samples collected during summers of 2000 through 2004. Eight of 15 samples, however, exceeded the Class B(LW) criterion for pH (maximum = 9.8; minimum = 7.8 pH units). Based on IDNR’s assessment methodology, these results suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria. Thus, these results suggest an impairment (partial support/monitored) of the Class A and Class B(LW) uses of this lake. These violations likely reflect the high levels of primary productivity at White Oak Lake and do not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
