Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities in 2000, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 2000 and 2002.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported" due to aesthetically objectionable conditions caused by algal blooms and by high levels of inorganic turbidity. The Class B(LW) aquatic life uses are assessed (evaluated) as "not supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and organic enrichment. Fish consumption are assessed (monitored) as "not supported" based on results of fish contaminant monitoring in 2000 and 2002 and on the existence of a fish consumption advisory at this lake. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) results of U.S. EPA/IDNR fish tissue monitoring in 2000 and 2002.
EXPLANATION: The information used previously to assess support of the Class A uses as "partially supported" was the observation 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 approaching ten years old and is no longer appropriate for characterizing current water quality conditions in the context of a "monitored" assessment. The current assessments is based on results of monitoring conducted by ISU from 2000 through 2002 as part of the statewide survey of Iowa lakes. These data, however, continue to suggest impairment of the Class A (primary contact) uses due to aesthetically objectionable conditions caused by algal blooms and high levels of inorganic turbidity. Using the median values from this survey from 2000 through 2002 (approximately nine samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 84, 74, and 79, respectively, for Ottumwa Lagoon. According to Carlson (1977), these index values place this lake in the range of hyper-eutrophic lakes and suggest extremely high levels of phosphorus in the water column, very high levels of chlorophyll-a and production of suspended algae, and extremely poor water transparency. 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 particles do likely limit algal production at Ottumwa Lagoon. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l. Of 131 lakes sampled, Ottumwa Lagoon had the 19th highest median level of inorganic suspended solids (27.1 mg/l), thus suggesting that non-algal turbidity limits the production of algae through light attenuation as well as contributes to the extremely poor water transparency at this lake. Algal production may also be limited by nitrogen availability. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Ottumwa Lagoon is 7. This ratio is very low and suggests a strong possibility that algal production at this lake is limited by nitrogen availability. Data from Downing et al. (2002) show relatively small populations of zooplankton species at this lake that graze on algae (Cladocerans). The 2000 average summer mass of Cladocerans at Ottumwa Lagoon (5.0 mg/l) was the 35th lowest of the 131 lakes sampled, thus suggesting that zooplankton grazing does not limit algal production at this lake. These conditions indicate impairments (“non-support”) to the Class A (primary contact) uses due to blooms of algae and very high levels of inorganic suspended solids that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions.
Data from Downing et al. (2002) suggest that Ottumwa Lagoon has a relatively diverse summertime phytoplankton community and that bluegreen algae (Cyanophyta) tend to dominate this community in late summer. The summer 2000 average mass of bluegreen algae (11.7 mg/l), however, is not particularly high relative to levels in other Iowa lakes and thus is not believed to either threaten or impair the Class A uses designated for this lake. The amount of data available for characterizing algal populations at this lake (one season), however, is not sufficient for developing a more accurate assessment of support of these uses. Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to update this assessment and improve the accuracy of future water quality assessments for this lake.
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 (monitored) as "not supporting” based on results of recent U.S. 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). 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. As part of RAFT follow-up monitoring, a composite sample of common carp and channel catfish fillets were collected in August 2002. Results show that levels of technical chlordane in this sample are high relative to other RAFT sample sites in Iowa. Levels of primary contaminants in the composite common carp fillets were low and do not suggest a fish contaminant problems (mercury: 0.095 ppm; total PCBs: 0.106 ppm; and technical chlordane: 0.240 ppm). With the exception of chlordane, levels of primary contaminants in the composite sample of channel catfish fillets were also relatively low (mercury: 0.021 ppm; total PCBs: 0.143 ppm) The level of technical chlordane (0.780 ppm) in this sample, however, was more than twice the FDA action level; thus, the fish consumption advisory issued in 2001 for channel catfish from this lake will be continued.
