Assessment Comments
Assessment is based on results of (1) ISU statewide survey of Iowa lakes from 2000-04, (2) information from the IDNR Fisheries Bureau, and (3) ISU studies on plankton communities at Iowa lakes from 2000-05.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supporting" due to poor water transparency that violates Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The poor water transparency at this lake is caused by a combination of moderately high levels of inorganic turbidity and suspended algae. Also, the presence of large populations of bluegreen algae suggests an additional impairment of these uses due to a violation of Iowa’s narrative water quality standard protecting against presence of nuisance aquatic life. The Class B(LW) aquatic life uses are assessed (evaluated) as "partially supporting" due to sediment-related turbidity. 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.
Note: A TMDL for turbidity and nutrients at Williamson Pond was prepared by IDNR and approved by EPA in 2006. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle (turbidity and noxious aquatic plants) are potentially addressed by the TMDL, this waterbody is moved from IR Category 5a from the 2004 assessment/listing cycle to IR Category 4a (impaired; TMDL approved) for the 2006 cycle.
EXPLANATION Results of monitoring conducted at this lake by ISU in 2000 and from 2002 through 2004 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses and Class B(LW) aquatic life uses are impaired.
Using the median values from the Iowa State University statewide surveys in 2000 and in 2002 through 2004 (12 samples), Carlson's trophic state indices for total phosphorus, chlorophyll-a, and secchi depth at Williamson Pond are 80, 61, and 68, respectively. According to Carlson (1977), the index value for total phosphorus is in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the lower range between eutrophic and hyper-eutrophic lakes, and the index value for Secchi depth is in the upper range between eutrophic and hyper-eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, moderately high (but much less than expected) production of suspended algae, and relatively poor water transparency.
According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively lower 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. The ISU lake data suggest that algal production at Williamson Pond is limited by a combination of nitrogen availability, zooplankton grazing, and by inorganic turbidity. Based on median values from ISU sampling from 2000-04, the ratio of total nitrogen to total phosphorus for this lake is 10; this ratio is low and suggests 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 moderately 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 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 (119 mg/l) was the 43rd highest of the 131 lakes sampled. These results suggest the potential for non-phosphorus limitation on algal production due to zooplankton grazing at Williamson Pond.
Data on inorganic suspended solids from the ISU survey suggest that this lake is also subject to moderately high levels of non-algal turbidity that may limit algal production. 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 of inorganic suspended solids at Williamson Pond (5.5 mg/l) was the 61st highest of the 131 lakes, thus suggesting that non-algal turbidity may contribute to limiting the production of algae.
These conditions indicate impairments to the Class A (primary contact) uses through presence of very poor water transparency caused by a combination of moderately high levels of inorganic suspended solids and algae that reduces water transparency. The IDNR Fisheries Bureau concurs that high levels of non-algal turbidity present an impairment of the designated uses at this lake.
Data from the ISU survey from 2002-2004 suggest that the presence of nuisance aquatic species (i.e., bluegreen algae) likely present an impairment of the Class A uses at this lake. Summer sampling during this period showed the percent wet mass of the total phytoplankton community in bluegreens was approximately 80%. In addition, Williamson Pond had the 10th highest median per summer sample mass of bluegreen algae of the 131 lakes sampled: 81 mg/l. This median is in the worst 10 % of the 131 Iowa lakes sampled. The presence of an extremely large population of bluegreen algae at this lake suggests 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 greater than the 90th percentile of this distribution (~59 mg/l) were arbitrarily chosen by IDNR staff to represent the condition of “potential impairment; not supported.” No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the , 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, of necessity, remains "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 sediment-related turbidity, probably related to the relatively large watershed:lake area ratio (common carp are not present in the lake). The ISU lake survey data show generally good chemical water quality at this lake. Results of this monitoring show that only one of the 12 samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that only two of the 12 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 9.3; minimum = 7.4 pH units). Based on IDNR’s assessment methodology, these results do not suggest violation frequencies are significantly greater than 10% for either parameter and thus do not suggest impairment of either the Class A and Class B(LW) uses of Williamson Pond.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
