West Okoboji Lake - Smiths Bay IA 06-LSR-2067

Assessment Comments

Assessment is based on results of (1) IDNR beach monitoring from 2002-04, (2) ISU lake survey from 2000-04, (3) surveys by IDNR Fisheries Bureau, (4) ISU reports on lake plankton communities from 2000-05, (5) results of U.S. EPA/IDNR fish tissue monitoring in 2004, and (6) atrazine monitoring in 1998.

Basis for Assessment

[Note:  For the 2006 Section 305(b)/303(d) cycle, West Okoboji Lake was subdivided into the following segments for purposes of developing water quality assessments:  main lake basin south, Emersons Bay, Smiths Bay, Millers Bay, and main lake basin north.   These segments were created to better use the water quality data collected as part of the IDNR beach monitoring program which includes monitoring at four of this lake’s beaches.   Developing separate assessments for the main lake basin as well as for the Lake’s bays represents a more accurate characterization of water quality conditions than did past assessments which developed only one assessment for the entire lake.]  

SUMMARY:  The Class A (primary contact recreation) uses of West Okoboji Lake's Smiths Bay are assessed (monitored) as "fully supporting" based on results of results of ambient lake monitoring from 2000-2004 that show exceptional water quality.   The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting.”  The Class C drinking water uses remain assessed (evaluated) as "fully supported."  Fish consumption uses are assessed (monitored) as "fully supported" 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 conducted from 2000 through 2004 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on plankton communities collected from 2000 through 2005 for the ISU lakes survey, (4) results of the 1998 "Iowa Voluntary Atrazine Monitoring Program", and (5) results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring in 2004.  

EXPLANATION:  Results from the ISU statewide survey of Iowa lakes suggest “full support” of the Class A uses designated for West Okoboji 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 48, 39, and 34, respectively.   According to Carlson (1977), the index value for total phosphorus places this lake in the upper range of mesotrophic lakes; the index value for chlorophyll-a is in the upper range of oligotrophic lakes, and the index value for secchi depth is in the middle range of oligotrophic lakes.   These index values suggest extremely low levels of phosphorus, extremely low (and less than expected) levels of chlorophyll-a, and exceptionally good water transparency.   Of the 131 lakes sampled from 2000 through 2004 as part of the ISU survey, West Lake Okoboji ranked third best for median total phosphorus (21 ug/l) and best for median chlorophyll-a (2 ug/l) and median Secchi depth (6.0 meters).   These results suggest (1) that this lake does not have threats or impairments to the Class A uses due to either high levels of chlorophyll or to inorganic turbidity and (2) that this lake has the best physical water quality (as measured by water transparency) of any Iowa lake.  

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 results of ISU monitoring from 2000-04 do not immediately suggest a non-phosphorus limitation that would account for the low chlorophyll-a concentration (TSI = 39) relative to that for total phosphorus (TSI = 48).   Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Lake West Okoboji is 33; this ratio suggests that nitrogen availability does not limit algal production at this lake.  

In terms of all Iowa lakes sampled, data from the ISU survey show very small populations of zooplankton species at this lake that graze on algae.   Although sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 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 (51 mg/l) was the 32nd lowest of the 131 lakes sampled.   These results suggest little if any non-phosphorus limitation due to zooplankton grazing at West Okoboji Lake.  

The levels of inorganic suspended solids at this lake are extremely low for Iowa lakes and do not suggest the potential for impairing designated uses.   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 West Lake Okoboji was 0.9 mg/l.   This was the lowest (best) median level of inorganic suspended solids of the 131 lakes sampled.  

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 West Okoboji Lake (6.7 mg/l) was the 34th lowest of the 131 lakes sampled.   This level ranks at the 25th percentile 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 ISU lake survey data also show good chemical water quality at this lake.   These data show no violations of the Class B(LW) criteria for dissolved oxygen in the 15 samples collected, or for pH in the 13 samples collected, at West Okoboji Lake during summers of 2000 through 2004.  

Drinking water uses are remain assessed (evaluated) as "fully supported" based on results of monitoring conducted in 1998 for the "Iowa Voluntary Atrazine Monitoring Program” (VMP) (this lake has not been part of the Iowa VMP since 1998).  However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively low confidence).  

Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at West Lake Okoboji in 2004.   The composite samples of fillets from common carp and yellow perch had low levels of contaminants.   Levels of primary contaminants in the composite sample of common carp fillets were as follows: mercury: 0.039 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm.   Levels of primary contaminants in the composite sample of yellow perch fillets were as follows: mercury: 0.068 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 West Lake Okoboji:  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.