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 supporting" due to very poor water transparency caused primarily by high levels of inorganic suspended solids. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting". The Class C (drinking water) uses are “not assessed” due to the lack of recent information upon which to base an assessment. Fish consumption uses remain assessed (evaluated) as "fully supported" based on results of fish contaminant monitoring in 1999. 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, (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.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of primarily non-algal turbidity adversely affect the Class A and Class B(LW) uses of Silver 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 76, 54, and 70, respectively. According to Carlson (1977), the index values total phosphorus and secchi depth place this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the middle range of eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, very low (and much less than expected) levels of chlorophyll-a, and very 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 suggest that non-algal particles do likely limit algal production at Silver 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, Silver Lake had the 28th highest median level of inorganic suspended solids (10.5 mg/l), thus suggesting that non-algal turbidity is the primary factor that limits the production of algae and that contributes to the turbidity-related impairments of both the primary contact recreation and aquatic life uses.
Nitrogen limitation does not appear to limit algal production at this lake. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for this lake is 20. This ratio suggest that algal production at Silver Lake is limited by the availability of phosphorus.
The presence of very large populations of zooplankton at Silver Lake that graze on algae, however, may explain the majority of the discrepancy between the TSI value for total phosphorus (76) and that for chlorophyll-a (54). In terms of all Iowa lakes sampled, data from the ISU survey show 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 approximately 60% 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 (169 mg/l) was the 25th highest of the 131 lakes sampled. This population of zooplankton grazers suggests the potential for this type of non-phosphorus limitation on algal production at Silver Lake.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (almost 80%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Silver Lake (4.7 mg/l) was the 23rd lowest of the 131 lakes sampled. This level is in the lowest 25% of the 131 Iowa lakes sampled. The presence of a relatively 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. 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 less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes. No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, 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 remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
Based on information from the IDNR Fisheries Bureau, the Class B(LW) aquatic life uses are assessed as "fully supported". Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 13 samples collected during summers of 2000 through 2004. Two of 13 samples, however, (13%) exceeded the Class B(LW) criterion for pH (maximum = 9.3; minimum = 8.2 pH units). Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A and Class B(LW) uses of this lake. These violations likely reflect primary productivity at Silver Lake and do not reflect the input of pollutants into this lake.
The Class C (drinking water) uses are “not assessed” due to the lack of recent information upon which to base an assessment. In previous 305(b) assessment cycles, the Class C uses were assessed as "fully supported" based on results of sampling conducted by UHL in January 1995 (see assessment for the 1996 and 1998 reports). However, these data are now considered too old (nearly greater than ten years) to characterize current water quality conditions. 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 low at this lake (maximum value = 5.8 mg/l; median = 1.5 mg/l), these data are not sufficient for developing a more recent assessment of support of the Class C uses.
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.
