Assessment Comments
Assessment is based on results of (1) ISU lake surveys 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 1998.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supporting" due to moderately high levels of inorganic turbidity and poor water transparency that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting". Excessive nutrient loading to the water column and elevated levels of non-algal turbidity, however, remain of concern at this lake. Fish consumption uses remain assessed (evaluated) as "fully supported" based on fish contaminant monitoring in 1998. 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 and (4) results of U.S. EPA / IDNR fish tissue monitoring in 1998.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of non-algal turbidity may adversely affect the Class A and Class B(LW) uses of DeSoto Bend Lake. Using the median values from this survey from 2000 through 2004 (approximately 15samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 72, 61, and 67, respectively. According to Carlson (1977), the index value for total phosphorus places this lake 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 middle to upper range between eutrophic and hyper-eutrophic lakes. These index values suggest (1) very high levels of phosphorus in the water column, (2) relatively low (and less than expected) levels of chlorophyll-a (suspended algae), and (3) poor 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) other than phosphorus limits production of algae. The ISU lake data suggest that algal production at DeSoto Bend Lake is potentially limited by nitrogen availability, by zooplankton grazing, and by inorganic turbidity. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for DeSoto Bend Lake is 11. This TN:TP ratio suggest the possibility that algal production at this lake may at times be limited by nitrogen availability.
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 50% of the dry mass of the zooplankton community. DeSoto Bend Lake had the 62nd highest per summer sample mass of zooplankton (Cladoceran) grazers of the 131 lakes sampled (96.0 mg/l). This level of zooplankton grazers may be able to suppress algal production and account for some of the discrepancy between the TSI values for total phosphorus (72) and chlorophyll-a (61).
The levels of inorganic suspended solids at this lake are very high for Iowa lakes and thus suggest the potential for limiting production of suspended algae through light attenuation. 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 at DeSoto Bend Lake was 10.2 mg/l; this was the 30th highest level of ISS of the 131 lakes sampled.
Thus, the moderately high TSI value for secchi depth (67) suggests impairment of the Class A (primary contact) uses 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 primarily due to high levels of inorganic suspended solids. The relatively low TSI value for chlorophyll-a (61) suggests that aesthetically objectionable blooms of algae are not a serious problem at this lake. The IDNR Fisheries Bureau concurs that high levels of inorganic turbidity present an impairment of designated uses at this 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 (nearly 50%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at DeSoto Bend Lake was the 60th lowest of the 131 lakes sampled: 12.0 mg/l. This level is in the lower half 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).
The eutrophic to hyper-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 as "fully supported". Excessive nutrient loading to the water column and high levels of non-algal turbidity, however, remain of concern at this lake. In addition, results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake although periods of low dissolved oxygen do occur. Results of this monitoring show that 3 of the 14 samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that none of the 15 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 8.8; minimum = 8.2 pH units). Based on IDNR’s assessment methodology, the results for dissolved oxygen do not suggest that significantly more than 10 percent of the samples exceed Iowa’s dissolved oxygen criteria and thus do not suggest an impairment of the Class B(LW) uses of this lake.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at DeSoto Bend Lake in 1998. 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 1998 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.
