Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau (3) ISU reports on lake plankton communities from 2000-05, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1999.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported" due to (1) extremely poor water transparency that violates Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions and (2) frequent violations of Iowa's water quality criterion for pH. In addition, the presence of very large populations of nuisance aquatic life (bluegreen algae) likely represents an additional impairment to the Class A uses. The Class B(LW) aquatic life uses are assessed (monitored) as "not supported" due to violations of state water quality criteria for dissolved oxygen and for pH. The Class B(LW) uses are also assessed (evaluated) as “not supported” due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. The impairments due to violations of Iowa's Class A and Class B(LW) pH criterion are new for this lake. Fish consumption remain assessed (evaluated) as "fully supported" based on results of fish contaminant monitoring in 1999. These assessments are consistent with those developed for the 2000, 2002, and 2004 reports. 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, (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (4) results of U.S. EPA/IDNR fish contaminant monitoring in 1999.
Note: A TMDL for siltation and organic enrichment at Silver Lake was prepared by IDNR and approved by EPA in 2002; thus, this waterbody was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Because not all of the Section 303(d) impairments identified for the 2006 assessment/listing cycle (algal growth, pH, organic enrichment, and turbidity) are addressed by the TMDL, this waterbody is moved from IR Category 4a to IR Category 5a (impaired; TMDL required) for the 2006 assessment/listing cycle.
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 "not supported" due to the extremely poor water transparency caused by algal blooms and by high levels of non-algal turbidity. 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 88, 79, and 82, respectively, for Silver Lake. According to Carlson (1977), these index values place this lake in the range of hyper-eutrophic lakes and suggest (1) extremely high levels of phosphorus in the water column (third highest of the 131 lakes sampled), (2) extremely high levels of chlorophyll-a (second highest of the 131 lakes), and (3) extremely poor water transparency (third poorest of the 131 lakes sampled). These TSI values suggest that Silver Lake continues to have some of the poorest water quality of any lake in the state.
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 Silver Lake is limited by nitrogen availability. Nonetheless, both the levels of inorganic suspended solids and the relatively low ratio of total nitrogen to total phosphorus may, at times, limit algal production at this lake. The median level of inorganic suspended solids at Silver Lake is typical for Iowa lakes sampled as part of the ISU lakes survey. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 5.2 mg/l. The median level of inorganic suspended solids at Silver Lake (5.3 mg/l) was the 63rd highest of the 131 lakes, thus suggesting that non-algal turbidity may limit the production of algae.
Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for this lake is 8; this ratio is relatively low and does suggest the potential for nitrogen availability to limit algal production at this lake.
In terms of all Iowa lakes sampled, 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 35% 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 (67 mg/l) was the 50th lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Silver Lake.
Thus, the water quality conditions at Silver Lake indicate impairments to the Class A (primary contact) uses through presence of extremely poor water transparency that violates Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The causes of the extremely poor water transparency at this lake is primarily blooms of suspended algae.
Data from the ISU survey from 2002-2004 suggest that the presence of nuisance aquatic species (i.e., bluegreen algae) may 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 90%. In addition, Silver Lake had the 7th highest median per summer sample mass of bluegreen algae of the 131 lakes sampled: 100 mg/l. This median is in the worst 10 % of the 131 Iowa lakes sampled. The presence of a 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 "not supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. The IDNR Fisheries Bureau states that, although the lake was renovated in fall 2003 (fish populations were removed), water quality conditions have not noticeably improved.
The ISU lake survey data for chemical water quality of Silver Lake also suggest “nonsupport” of the Class B(LW) uses. Four violations of the Class B(LW) criteria for dissolved oxygen occurred in the 12 samples (33%) collected during summers of 2000 through 2004 (violations were 2.8 mg/l on August 13, 2003, 2.0 mg/l on June 9, 2004, 4.0 mg/l on July 14, 2004, and 4.7 mg/l on August 11, 2004). Interestingly, these four violation occurred in the last four samples collected during the 2000-2004 period. Based on IDNR’s assessment methodology, these results suggest that significantly more than 10% of the samples exceed Iowa’s criterion for dissolved oxygen and thus suggest an impairment of the Class B(LW) uses of this lake. In addition, nine of 13 samples (69%) exceeded the Class B(LW) criterion for pH (maximum = 10.3; minimum = 8.3 pH units). Based on IDNR’s assessment methodology, these results suggest that significantly more than 10% of the samples exceed Iowa’s pH criteria and thus suggest an impairment of the Class A and Class B(LW) uses of this lake. These violations, however, likely reflect the extremely high levels of primary productivity at Silver Lake and do 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 Silver Lake in 1999. 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 1999 RAFT sampling conducted at Silver Lake: levels of all contaminants from this monitoring were below advisory trigger levels, thus suggesting the continued “full support” of fish consumption uses.
