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 2004.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting ". The Class B(LW) aquatic life uses are also assessed (evaluated) as "fully supporting ". Fish consumption uses are assessed (monitored) as "fully supporting" based on results of fish contaminant monitoring in 2004. 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 2004.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that the designated Class A and Class B(LW) uses of East Okoboji Lake are “fully supported”. 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 67, 57, and 53, respectively. According to Carlson (1977), the index value for total phosphorus is in the middle range between eutrophic and hyper-eutrophic lakes; the values for chlorophyll-a and Secchi depth are in the middle to lower range of eutrophic lakes. These index values suggest moderately high levels of phosphorus in the water column, but relatively low (and somewhat less than expected) levels of chlorophyll-a, and suggest very good (and somewhat better than expected) water transparency. The TSI index values for chlorophyll-a and Secchi depth are well below the IDNR impairment threshold of 65.
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 or zooplankton grazing) other than phosphorus limits production of algae. Results of ISU monitoring suggest that the somewhat elevated levels of inorganic suspended solids and the large population of zooplankton grazers may serve 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 East Lake Okoboji is 14). This TN:TP ratio suggests a slight possibility that nitrogen limitation, at times, suppresses the production of suspended algae at this lake.
The presence of moderately large populations of zooplankton at East Lake Okoboji that graze on algae, however, may explain the large discrepancy between the TSI value for phosphorus (67) and that for chlorophyll-a (57). 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 50% 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 (100 mg/l) was the 58th 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 East Lake Okoboji.
The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2004 through 2002 was 5.2 mg/l; the median level at East Okoboji Lake was 3.0 mg/l, thus suggesting very low levels of ISS and little potential for limiting algal production 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 (approximately 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 East Lake Okoboji (11.7 mg/l) was the 62nd lowest of the 131 lakes sampled. This level is in the lower half of the 131 Iowa lakes sampled. The presence of this relatively small population of bluegreen algae at this lake does not suggest a 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).
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed (evaluated) as "fully supported". Nutrient loading to the water column and the potential for nuisance blooms of algae, however, remain water quality concerns at this lake. Water quality monitoring conducted as part of the ISU lakes survey from 2000-2004 suggests good chemical quality at East Okoboji Lake: no violations of the Class B(LW) criteria for dissolved oxygen occurred in the 14 samples collected, and no violations of criteria for pH occurred in the 15 samples collected.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at East Okoboji Lake in 2004. The composite samples of fillets from channel catfish and yellow perch had low levels of contaminants. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.032 ppm; total PCBs: 0.092 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of yellow perch fillets were as follows: mercury: 0.0.037 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. 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 2004 RAFT sampling conducted at this lake: the levels of contaminants do not exceed any of the new (2006) advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
