Assessment Comments
Assessment based on (1) results of the IDNR beach monitoring program from 2002-04, (2) surveys by IDNR Fisheries Bureau, (3) results from the ISU lake survey from 2000-04, & (4) ISU reports on lake plankton communities, 2000-05.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (monitored) as "not supported" due to (1) aesthetically objectionable conditions (poor water clarity) caused by high levels inorganic turbidity and (2) results of IDNR beach monitoring that show high levels of indicator bacteria at this lake’s beach area. The Class B(LW) aquatic life uses remain assessed (evaluated) as "not supporting" primarily due to non-algal turbidity and siltation. The Class C (drinking water) uses and fish consumption uses remain "not assessed" due to the lack of water quality information upon which to base an assessment. Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake. The sources of data for this assessment include (1) results of IDNR/UHL beach monitoring from 2000-2002, (2) results of the statewide survey of Iowa lakes sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (3) surveys by IDNR Fisheries Bureau, and (4) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey.
Note: A TMDL for siltation at Bob White Lake was prepared by IDNR and approved by EPA in 2001. Because the Section 303(d) impairment for indicator bacteria was not addressed in the TMDL, this waterbody was placed into IR Category 5a (impaired; TMDL required) for the 2004 and 2006 assessment/listing cycles.
EXPLANATION: Results of IDNR beach monitoring from 2000 through 2002 suggest that the Class A uses are "not supported" (this beach was not monitored for indicator bacteria during recreational seasons of 2003 and 2004) Levels of indicator bacteria at Bob White Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2000 (18 samples), 2001 (16 samples), and 2002 (29 samples) as part of the IDNR beach monitoring program. According to IDNR’s assessment methodology, two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A (primary contact recreation) uses: (1) all thirty-day geometric means for the three-year assessment period are less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml and (2) not more than 10 % of the samples during any one recreation season exceeds the state’s single-sample maximum value of 235 E. coli orgs/100 ml. If a 5-sample, 30-day geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period, the Class A uses should be assessed as “not supported”. Also, if more than 10% of the samples in any one of the three recreation seasons exceed Iowa’s single-sample maximum value of 235 E. coli orgs/100 ml, the Class A uses should be assessed as “partially supported”. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35of U.S. EPA 1997b).
At Bob White Lake beach, the geometric means for 8 of the 12 thirty-day periods during the summer recreation seasons of 2001exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml; none of the geometric means exceeded this standard during the recreational seasons of 2000 (14 geometric means) or 2002 (25 geometric means). Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was significantly greater than 10% in the following recreation seasons: 2002 (4 of 18 samples or 22%) and 2003 (7 of 16 samples or 44%). Only 1 of the 29 samples (3%) collected during the 2004 recreational season exceeded this standard. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest impairment (“nonsupport”) of the primary contact recreation uses due to geometric mean values that exceed Iowa’s Class A criterion of 126 orgs/100 ml.
Results from the ISU statewide survey of Iowa lakes also suggest impairments to Class A uses. Data from this study suggest that high levels of non-algal turbidity may affect the Class A uses of Bob White Lake. 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 81, 58, and 80, respectively. According to Carlson (1977), the index values for total phosphorus and secchi depth place this lake in the upper range of hyper-eutrophic lakes; the index value for chlorophyll-a is low (and much less than expected) for Iowa lakes, and is in the upper range of eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, much less than expected production of suspended algae, and extremely 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 strongly suggest that non-algal particles limit algal production at Bob White 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, Bob White Lake had the sixth highest median level of inorganic suspended solids (25.7 mg/l), thus suggesting that non-algal turbidity limits the production of algae and is the primary cause of the extremely poor water transparency. These conditions suggest an suggest impairments to the Class A (primary contact) uses primarily due to presence high levels of inorganic turbidity that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The level of chlorophyll-a at Bob White Lake is relatively low (49th lowest of the 131 lakes sampled) and does not appear to contribute to impairments at this lakes. The IDNR Fisheries Bureau concurs that turbidity-related impairments exist at this lake. Other factors may limit algal production at Bob White Lake. Based on median values from ISU sampling from 2000-04, the ratio of total nitrogen to total phosphorus for this lake is 8; this ratio suggests the possibility that algal production at this lake is limited by nitrogen availability.
Levels of zooplankton grazers at Bob White Lake to not likely suppress algal production. Data from zooplankton monitoring conducted from 2000-2005 as part of the ISU lakes survey show only moderately large zooplankton populations at this lake with the population of species known as algal grazers comprising 26 percent of the total zooplankton biomass. The per summer sample mass of zooplankton grazers at this lake during the 2000-2004 period (30.3 mg/l) was the 14th lowest of the 131 lakes sampled. The extremely high level of inorganic suspended solids at this lake, however, remains the primary non-phosphorus limitation to algal production at this lake.
Nuisance aquatic (algal) species (i.e., bluegreen algae) appear to be a potential problem at Bob White Lake. Data from the ISU survey from 2000-04 suggest that bluegreen algae (Cyanophyta) comprise approximately 70% of the summertime phytoplankton community of this lake. The average per sample mass (biovolume) of bluegreen algae in summers of 2000 through 2004 at this lake (28.7 mg/l) was the 34th highest of the 131 lakes sampled. These conditions suggest the potential for impairments due to presence of nuisance aquatic (e.g., algal) species. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median (2000-2004) levels greater than the 75th percentile of this distribution (28.8 mg/l) were arbitrarily considered to represent “potential impairment.” No criteria exist, however, upon which to base an accurate identification of impairments due to bluegreen algae. Thus, while the collection of additional data has improved our ability to accurately estimate the lake-specific levels of bluegreen algae, the exact levels at which these organisms begin to violate Iowa’s narrative criteria protecting against “nuisance aquatic life” or “aesthetically objectionable conditions” remains unknown. Thus, the assessment category for assessments based on level of bluegreen algae of necessity remains "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
These results showing large populations of bluegreen algae stand in contrast to the assessment of bluegreen algae at Bob White Lake developed for the previous (2004) Section 305(b) reporting cycle. The 2004 assessments stated that levels of bluegreen algae at Bob White Lake in 2000 (Downing et al. 2002) suggested that bluegreen algae (Cyanophyta) comprised a relatively small portion (approximately 4%) of the summertime phytoplankton community of this lake: the average mass of bluegreen algae in summer 2000 at this lake (1.7 mg/l) was the 17th lowest of the 131 lakes sampled. The increase in the levels of bluegreen algae, however, is demonstrated in the sample results of zooplankton monitoring for the years 2002 through 2004.
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 "not supported" due to high levels of non-algal turbidity and siltation in the lake. Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. Results of this monitoring show that only 1 of the 15 samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that only 1 of the 14 samples collected exceeded Iowa’s Class A,B(LW) criterioa for pH. Based on IDNR’s assessment methodology, these results do not suggest violation frequencies of significantly greater than 10% for either parameters and thus do not suggest impairment of either the Class A or Class B(LW) uses of Bob White Lake.
The Class C (drinking water) uses and fish consumption uses remain "not assessed" due to lack of information upon which to base an assessment of these uses. The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU survey from 2000-04 show that nitrate levels are very low at this lake (N = 15; maximum value = 4.9 mg/l; median = 1.2 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.
