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 2000, 2002, and 2004.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported" due to extremely poor water transparency that violates Iowa’s narrative water quality criterion protecting against “aesthetically objectionable conditions”. This poor water transparency is caused by a combination of algal blooms and 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 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 2000, 2002, and 2004.
Note: A TMDL for algae, turbidity, and chlordane at Ottumwa Lagoon was prepared by IDNR and approved by EPA in 2005. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle (algal growth, turbidity, chlordane) are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).
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 2004 as part of the statewide survey of Iowa lakes. These data, however, continue to suggest severe impairment of the Class A (primary contact) uses due to aesthetically objectionable conditions (extremely poor water transparency) caused by algal blooms and high levels of inorganic turbidity.
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 86, 73, and 80, 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 2004 was 5.2 mg/l. Of 131 lakes sampled, Ottumwa Lagoon had the 4th 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 2004, 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.
In terms of all Iowa lakes sampled, data from the ISU survey show extremely small populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised only about 20% of the dry mass of the zooplankton community of this lake; the average per summer sample mass of Cladoceran taxa (9.4 mg/l) was the 4th lowest level of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Ottumwa Lagoon.
These conditions indicate impairments (“non-support”) to the Class A (primary contact) uses due to poor water transparency that violates Iowa’s narrative water quality criterion protecting against aesthetically objectionable conditions. The poor water transparency is caused by a combination of very high levels (blooms) of suspended algae and extremely high levels of inorganic suspended solids.
Despite the generally poor water quality at this lake, the levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at Ottumwa Lagoon do not suggest an impairment of Class A uses. This lake, however, had the highest measured biovolume of bluegreen algae of any of the 131 lakes monitored over the 2000-04 period: 31,674 mg/l on June 30, 2004. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (approximately 60%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Ottumwa Lagoon (7.3 mg/l) was the 38th lowest of the 131 lakes sampled. This level is in the lowest third 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 "not supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and organic enrichment. Despite extremely high levels of nutrients, suspended algae, and inorganic suspended solids at this lake, results of ISU monitoring do not suggest significant chemical water quality problems at this lake. Only one minor violation (4.9 mg/l) of the Class B(LW) criteria for dissolved oxygen (5.0 mg/l) occurred in the 13 samples collected (7%) during summers of 2000 through 2004. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), however, a violation frequency of less than 10 % for conventional parameters such as dissolved oxygen nonetheless suggests "full support" of aquatic life uses. Thus, the percentage of violations of the dissolved oxygen criterion at this station does not suggest an impairment of aquatic life uses at this lake. One of 14 samples (7%) violated the Class A,B(LW) criterion for pH (maximum = 9.1; minimum = 7.8 pH units).
Fish consumption uses were assessed (monitored) as “not supported” based the existence of a fish consumption advisory at this lake and on results of ongoing U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Ottumwa Lagoon in 2000, 2002 and in 2004. 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 (IDPH), 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 applies to the fish contaminant data generated from the RAFT sampling conducted in Ottumwa Lagoon: levels of chlordane in samples of channel catfish from consecutive samplings in 2000 and 2002 exceeded an advisory trigger level, thus suggesting the need for a consumption advisory. According to IDNR’s assessment methodology, the existence of a consumption advisory indicates that fish consumption uses should be assessed as “not supported”.
A fish consumption advisory has existed at Ottumwa Lagoon since 2001. Levels of chlordane in channel catfish fillets had exceeded the previous trigger level (FDA action level) of 0.3 ppm used by the Iowa DNR in two consecutive samplings. In January 2006, Iowa adopted a revised protocol, as developed jointly by the Iowa Department of Public Health and the Iowa DNR, for issuing consumption advisories. The existing data for contaminants in fish from all Iowa lakes and rivers were reviewed to determine whether the new protocol would justify the continuance of existing advisories or justify the issuance of new advisories. The new trigger level for technical chlordane is 0.6 ppm (for a one meal per week advisory). A review of the existing data for Ottumwa Lagoon justified continuance of the consumption advisory for channel catfish issued in 2001. Because of this advisory, Ottumwa Lagoon has been monitored as part of the RAFT program on an every-other-year basis.
The level of technical chlordane in the composite sample of fillets from channel catfish collected for the 2000 RAFT program was 0.87 ppm; this level is greater than the IDPH/INDR trigger level of 0.6 ppm for a one meal per week consumption advisory. For the 2002 RAFT sampling, the composite samples of fillets from common carp and channel catfish had generally low levels of contaminants. 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 chlordane in the sample of channel catfish fillets (0.78 ppm), however, exceeded the IDNR/IDPH trigger level of 0.60 ppm for a one meal per week consumption advisory (this levels is well below the “do not eat” trigger level for chlordane of 5.0 ppm). Because the level of chlordane in the 2000 sample of channel catfish fillets (0.87 ppm) had also exceeded this trigger level, continuance of the existing advisory was justified. As per Iowa’s revised protocol, the type of advisory, however, was changed from a “do not eat” to a “one meal per week” advisory.
The 2004 RAFT follow-up sampling at Ottumwa Lagoon showed that levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: <0.0181 ppm; total PCBs: 0.12 ppm; and technical chlordane: 0.45 ppm. The level of chlordane in this sample of (0.45 ppm), however, was below the IDNR/IDPH trigger level of 0.60 ppm for a one meal per week consumption advisory. According to the IDNR/IDPH advisory protocol, if two consecutive samplings show that contaminant levels are below the trigger level in fillet samples, an existing consumption advisory can be removed. Although the “one meal per week” advisory for channel catfish continues at Ottumwa Lagoon, RAFT monitoring will again be conducted in 2006 to determine whether this consumption advisory should be rescinded.
