Assessment Comments
Assessment is based on results of (1) ISU statewide survey of Iowa lakes from 2000-02, (2) information from the IDNR Fisheries Bureau, and (3) ISU study on plankton communities at Iowa lakes in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "partially supporting" due to reductions in water transparency caused primarily by moderately high levels of inorganic turbidity caused by suspended solids. Also, the presence of large populations of bluegreen algae suggests an additional impairment of these uses. The assessment type is considered “evaluated” due to lack of data from summer 2001 when the lake was reported as “dry”. 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 conducted from 2000 through 2002 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002).
EXPLANATION Results of monitoring conducted at this lake by ISU in 2000 and 2002 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 2002 (approximately six samples), Carlson's trophic state indices for total phosphorus, chlorophyll-a, and secchi depth at Williamson Pond are 85, 57, and 68, respectively. According to Carlson (1977), the index value for total phosphorus is in the upper range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the upper range of 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, relatively low (and much less than expected) production of suspended algae, and moderately 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 2000-02, the ratio of total nitrogen to total phosphorus for this lake is 8; this ratio is low and strongly suggests that algal production at this lake is limited by nitrogen availability. In addition, data from Downing et al. (2002) show that the zooplankton community of this lake has a large population of species known as algal grazers; the median summer mass of zooplankton grazers (Cladocerans) at this lake in 2000 (99.2 mg/l) was the 13th highest of the 131 lakes sampled. 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 2002 was 4.8 mg/l. The median level of inorganic suspended solids at Williamson Pond (4.6 mg/l) was the 67th highest of the 131 lakes, thus suggesting that non-algal turbidity contributes to limiting the production of algae. These conditions indicate impairments to the Class A (primary contact) uses through presence of moderately high levels of inorganic suspended solids 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.
The presence of nuisance algal species (i.e., bluegreen algae) suggests an additional impairment to the designated uses of Williamson Pond. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta), dominate the summertime phytoplankton community of this lake. The summer 2000 average mass of bluegreen algae at this lake (60.5 mg/l) was the 16th highest of the 131 lakes sampled and thus suggests the potential for violating Iowa’s narrative water quality standard protecting against nuisance aquatic life.
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). Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
