Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake phytoplankton communities, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supported." The Class B(LW) aquatic life uses are assessed (evaluated) as "partially supported." Fish consumption are assessed (monitored) as "not supported." Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) results of U.S. EPA/IDNR fish tissue monitoring in 2000. EXPLANATION: The information used previously to assess support of the Class A uses as "partially supported" is the observations of combined sewer overflows discharging to the Ottumwa Lagoon by DNR staff in June 1995 (see above assessment for the 1996 report). Although these conditions likely continue, this information is now well over five years old and is no longer appropriate for characterizing current water quality conditions in the context of a "monitored" assessment. Results of monitoring conducted by ISU in 2000 and 2001 as part of the statewide survey of Iowa lakes, however, suggest that the Class A (primary contact) uses are only "partially supported." Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlsons's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 84, 71, and 77, respectively, for Ottumwa Lagoon. According to Carlson (1977), these index values place this lake in the range of hyper-eutrophic lakes and suggest excessive phosphorus loading to the water column, high levels of chlorophyll-a and production of suspended algae, and very poor water transparency. These conditions indicate impairments to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that Ottumwa Lagoon has a relatively diverse summertime phytoplankton community but that bluegreen algae (Cyanophyta) tend to dominate in late summer. The TSI value for chlorophyll-a (71) is low relative to the TSI's for both total phosphorus (84) and secchi depth (77), suggesting that fators other than phosphorus limitation, including nitrogen limitation and/or non-algal turbidity, limit production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 8, thus suggesting the potential for nitrogen limitation at this lake. Levels of inorganic suspended solids in this lake are relatively high. Data on inorganic suspended solids from the ISU survey suggest that this lake is subject to occasional episodes of high levels of non-algal turbidity. The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l. The median level of inorganic suspended solids at Ottumwa Lagoon (17.4 mg/l) was the nineteenth highest of the 130 lakes. The six summertime values for inorganic suspended solids collected during the ISU survey in 2000 were 7.3, 29.6, 7.7 mg/l and in 2001 were 27.1, 37.4, and 2.8 mg/l, thus suggesting that non-algal turbidity may, at times, limit the production of algae as well as impair beneficial uses. 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 "not supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and organic enrichment. Fish consumption uses remained assessed as "not supporting based on results of recent EPA/DNR fish tissue (RAFT) monitoring." RAFT monitoring in 2000 showed that levels of technical chlordane in bottom feeding fish from this lake remain a problem (see assessments for the 1998 and 2000 reports above). The level of technical chlordane in the composite sample of fillets of common carp contained 0.24 ppm of technical chlordane. Although below the FDA action level of 0.300 ppm, this level is above one-half the FDA action level, thus suggesting a threat to full support of fish consumption uses and the need to continue to conduct follow-up monitoring. The level of technical chlordane in the composite sample of fillets from channel catfish, however, was 0.87 ppm; this level is nearly three time the FDA action level of 0.300 ppm. These results were used to justify issuance of a fish consumption advisory in July 2001 for channel catfish taken from Ottumwa Lagoon. Levels of other contaminants in the samples of common carp and channel catfish from RAFT monitoring in 2000 were well below one-half of the respective FDA action levels and IDNR levels of concern. Additional fish contaminant monitoring will be conducted as part of the U.S. EPA/IDNR "RAFT" program on an every-other-year basis to determine any changes in contaminant levels that would require adjustment to the existing consumption advisory.
