Assessment Comments
Assessment is based on results of (1) ISU survey of Iowa lakes from 2000-04, (2) information from the IDNR Fisheries Bureau, (3) ISU reports on plankton communities at Iowa lakes, 2000-05, (4) results of U.S. EPA/IDNR fish contaminant monitoring in 1996, and (5) results of TMDL monitoring conducted by Iowa DNR in 2002.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting". Minor reductions in water transparency due to turbidity from algae and/or inorganic sources, however, remain a concern at this lake. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to nutrients in the water column and siltation impacts, especially in the upper portions of the lake. Fish consumption uses remain assessed (evaluated) as “fully supported” based on fish contaminant monitoring in 1996. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000 through 2004 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on plankton communities at Iowa lakes in 2000 through 2005 from the ISU lake survey, (4) results of U.S. EPA/IDNR fish contaminant monitoring in 1996, and (5) results of TMDL monitoring conducted by Iowa DNR in 2002.
Note: A TMDL for organic enrichment and siltation at Don Williams Lake was prepared by IDNR and approved by EPA in 2005. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).
EXPLANATION: Using the median values from the ISU lake survey from 2000 through 2004 (approximately 15 samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 67, 59, and 59, respectively. According to Carlson (1977), the index value for total phosphorus is in the middle range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and Secchi depth are in the upper range of eutrophic lakes. Although these index values suggest high levels of phosphorus in the water column, the relatively low index levels for chlorophyll-a and Secchi depth suggest relatively low levels of suspended algae and relatively good water transparency.
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) limits production of algae. The ISU lake data suggest that algal production at Don Williams Lake is limited by zooplankton grazing and possibly by inorganic turbidity. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for this lake is 156, thus suggesting an overabundance of nitrogen in this lake. This ratio suggests that production of algae at this lake is limited by phosphorus. This very high TN:TP ratio reflects the often exceptionally high levels of total nitrogen in impoundments of the Des Moines Lobe region of north-central Iowa. For example, Don Williams Lake had the fourth highest median total nitrogen concentration of the 131 lakes sampled.
The levels of inorganic suspended solids at this lake during the 2000-04 period were somewhat elevated and suggest at least some potential for increasing in-lake turbidity and limiting production of suspended algae. 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, Don Williams Lake had the 58th highest median level of inorganic suspended solids (5.8 mg/l), thus suggesting that non-algal turbidity may limit algal production as well as cause slight reductions in water transparency.
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 45% of the dry mass of the zooplankton community and that Don Williams Lake had the 48th lowest median per summer sample level of zooplankton (Cladoceran) grazers (65.4 mg/l) of the 131 lakes sampled. This small population of zooplankton grazers suggests little potential for non-phosphorus limitation on algal production at this lake.
The overall (2000-2004) TSI values for chlorophyll-a and Secchi depth (both at 59) suggests relatively low levels of suspended algae and relatively good water transparency, thus indicating “full support” of the Class A (primary contact) uses.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. Data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a relatively small portion (about 20%) of this lake’s summertime phytoplankton community. Also, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Don Williams Lake (0.6 mg/l) was the third lowest of the 131 lakes sampled. This levels is in the lowest 10% of the 131 Iowa lakes sampled. The presence of this small 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.
The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed (evaluated) as "partially supported" due to excessive nutrient loading to the water column, high levels of non-algal turbidity, and siltation in the lake. Despite these impairments of Class B(LW) uses, results of monitoring conducted by IDNR/UHL in support of TMDL development at this lake from March to October 2002 showed no violations of Class B(LW) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the 10 samples collected. In addition, the ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 11 samples collected during summers of 2000 through 2004. One of 14 samples violated the Class A,B(LW) criterion for pH (maximum = 9.1; minimum = 8.1 pH units). Based on IDNR’s assessment methodology, this violation does not suggest impairment of the Class A,B(LW) uses. This violation likely reflects the primary productivity at Don Williams Lake and does not reflect the input of pollutants into this lake.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Don Williams Lake in 1996. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes. Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa. In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health, in cooperation with Iowa DNR, developed a risk-based advisory protocol. This protocol went into effect in January 2006 (see http://www.iowadnr.gov/fish/news/consump.html for more information on Iowa’s revised fish consumption advisory protocol). Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest either a threat to, or impairment of, fish consumption uses. This scenario, however, does not apply to the fish contaminant data generated from the 1996 RAFT sampling conducted at this lake: levels of all contaminants from this monitoring were below advisory trigger levels, thus suggesting the continued “full support” of fish consumption uses.
