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 2001.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (monitored) as "not supported" due to extremely poor water transparency caused by high levels of inorganic turbidity and sediment re-suspension. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. This assessment is consistent with the assessments of support of aquatic life uses developed for previous Section 305(b) reports. Fish consumption uses are assessed (monitored) as "fully supported" based on results of fish contaminant monitoring in 2001. 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 2001.
Note: A TMDL for turbidity at Storm Lake was prepared by IDNR and approved by EPA in 2005. Because the primary Section 303(d) impairment identified for the 2006 assessment/listing cycle (turbidity) is addressed by the TMDL, this waterbody is moved from IR Category 5a from the 2004 assessment/listing cycle to IR Category 4a (impaired; TMDL approved) for the 2006 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 extremely poor water transparency that violates Iowa’s narrative criterion protecting against aesthetically objectionable conditions. This poor water transparency is related primarily to high levels of inorganic material in the water column 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 70, 60, and 79, respectively, for Storm Lake. According to Carlson (1977), the index values for total phosphorus and secchi depth places this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the upper range of eutrophic lakes. These index values suggest very high levels of phosphorus in the water column, relatively low (and much less than expected) levels of chlorophyll-a, and extremely poor water transparency (the three-year median Secchi depth was 0.27 m).
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 (inorganic) particles do likely serve as the primary non-phosphorus factor limiting algal production at Storm 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. The median level of inorganic suspended solids at Storm Lake (37.6 mg/l) was the second highest of the 131 lakes, thus suggesting that non-algal turbidity limits the production of algae as well as contributes to the extremely poor water transparency that impairs the Class A uses.
The ratio of total nitrogen to total phosphorus (12) is moderately low for Iowa lakes and suggests the potential for nitrogen availability to limit the production of chlorophyll at this lake. Zooplankton grazing, however, does not appear to limit algal production at Storm 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 one-third 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 (53 mg/l) was the 33rd lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Storm Lake.
These conditions suggest impairments to the Class A (primary contact) uses primarily due to the extremely poor water transparency that violates Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. According to the IDNR Fisheries Bureau, the ongoing dredging project at Storm Lake may reduce the levels of suspended sediment and improve water transparency. A comparison of annual TSI values for the 2000-2004 period at Storm Lake does not suggest any trends in water transparency. Based on ISU lake survey monitoring data, annual summer TSI values for Secchi depth have remained in the 70s (indicating impairment) over the five-year period: 2000: 79; 2001: 79; 2002 = 72; 2003 = 77; and 2004 = 76.
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 75%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Storm Lake (9.7 mg/l) was the 52nd lowest of the 131 lakes sampled. 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 hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses are only "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. Despite the problems with water transparency, the ISU lake survey data suggest relatively good chemical water quality at Storm Lake. Results of monitoring during summers of 2000 through 2004 show no violations of the Class B(LW) criteria for dissolved oxygen or pH 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 Storm Lake in 2001. The composite samples of fillets from channel catfish and walleye had low levels of contaminants. 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 2001 RAFT sampling conducted at Storm Lake: the levels of contaminants do not exceed any of the new (2006) advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory for this waterbody.
