An ecological risk assessment of the exposure and effects of 2,4-D acid to rainbow trout (Onchorhyncus mykiss).

Numerous state and federal agencies are increasingly concerned with the rapid expansion of invasive, noxious weeds across the United States. Herbicides are frequently applied as weed control measures in forest and rangeland ecosystems that frequently overlap with critical habitats of threatened and endangered fish species. However, there is little published chronic toxicity data for herbicides and fish that can be used to assess ecological risk of herbicides in aquatic environments. We conducted 96-h flowthrough acute and 30-day chronic toxicity studies with swim-up larvae and juvenile rainbow trout (Onchorhyncus mykiss) exposed to the free acid form of 2,4-D. Juvenile rainbow trout were acutely sensitive to 2,4-D acid equivalent at 494 mg/L (95% confidence interval [CI] 334-668 mg/L; 96-h ALC(50)). Accelerated life-testing procedures, used to estimate chronic mortality from acute data, predicted that a 30-day exposure of juvenile rainbow trout to 2,4-D would result in 1% and 10% mortality at 260 and 343 mg/L, respectively. Swim-up larvae were chronically more sensitive than juveniles using growth as the measurement end point. The 30-day lowest observable effect concentration (LOEC) of 2,4-D on growth of swim-up larvae was 108 mg/L, whereas the 30-day no observable effect concentration (NOEC) was 54 mg/L. The 30-day maximum acceptable toxicant concentration (MATC) of 2,4-D for rainbow trout, determined as the geometric mean of the NOEC and the LOEC, was 76 mg/L. The acute:chronic ratio was 6.5 (i.e., 494/76). We observed no chronic effects on growth of juvenile rainbow trout at the highest concentration tested (108 mg/L). Worst-case aquatic exposures to 2,4-D (4 mg/L) occur when the herbicide is directly applied to aquatic ecosystems for aquatic weed control and resulted in a 30-day safety factor of 19 based on the MATC for growth (i.e., 76/4). Highest nontarget aquatic exposures to 2,4-D applied following terrestrial use is calculated at 0.136 mg/L and resulted in a 30-day safety factor of 559 (e.g., 76/0.163). Assessment of the exposure and response data presented herein indicates that use of 2,4-D acid for invasive weed control in aquatic and terrestrial habitats poses no substantial risk to growth or survival of rainbow trout or other salmonids, including the threatened bull trout (Salvelinus confluentus).

An ecological risk assessment of the acute and chronic toxicity of the herbicide picloram to the threatened bull trout (Salvelinus confluentus) and the rainbow trout (Onchorhyncus mykiss).

We conducted acute and chronic toxicity studies of the effects of picloram acid on the threatened bull trout (Salvelinus confluentus) and the standard coldwater surrogate rainbow trout (Oncorhynchus mykiss). Juvenile fish were chronically exposed for 30 days in a proportional flow-through diluter to measured concentrations of 0, 0.30, 0.60, 1.18, 2.37, and 4.75 mg/L picloram. No mortality of either species was observed at the highest concentration. Bull trout were twofold more sensitive to picloram (30-day maximum acceptable toxic concentration of 0.80 mg/L) compared to rainbow trout (30-day maximum acceptable toxic concentration of 1.67 mg/L) based on the endpoint of growth. Picloram was acutely toxic to rainbow trout at 36 mg/L (96-h ALC50). The acute:chronic ratio for rainbow trout exposed to picloram was 22. The chronic toxicity of picloram was compared to modeled and measured environmental exposure concentrations (EECs) using a four-tiered system. The Tier 1, worst-case exposure estimate, based on a direct application of the current maximum use rate (1.1 kg/ha picloram) to a standardized aquatic ecosystem (water body of 1-ha area and 1-m depth), resulted in an EEC of 0.73 mg/L picloram and chronic risk quotients of 0.91 and 0.44 for bull trout and rainbow trout, respectively. Higher-tiered exposure estimates reduced chronic risk quotients 10-fold. Results of this study indicate that picloram, if properly applied according to the manufacturer's label, poses little risk to the threatened bull trout or rainbow trout in northwestern rangeland environments on either an acute or a chronic basis.

Chronic toxicity of un-ionized ammonia to early life-stages of endangered Colorado pikeminnow (Ptychocheilus lucius) and razorback sucker (Xyrauchen texanus) compared to the surrogate fathead minnow (Pimephales promelas).

Ammonia-contaminated groundwater enters the Upper Colorado River from beneath the abandoned Moab Uranium Mill Tailings Pile near Moab, Utah. This reach of the Upper Colorado River was designated as critical habitat for four endangered fish species because it is one of the few existing areas with known spawning and rearing habitats. Un-ionized ammonia (NH3) concentrations frequently exceed 1.00 mg/L in backwaters adjacent to the tailings pile, which exceeds the Utah 30-d average chronic water quality criterion for un-ionized ammonia (0.07 mg/L NH3; temperature 20 degrees C; pH 8.2) by a factor of more than 10. However, there is little published information regarding the sensitivity of endangered fishes to ammonia. We conducted 28-d static renewal studies with post-swim-up larvae to determine the relative sensitivity of Colorado pikeminnow (Ptychocheilus lucius), razorback sucker (Xyrauchen texanus), and the standard surrogate fathead minnow (Pimephales promelas) to NH3. Chronic values (ChVs) for mortality and growth were determined as the geometric mean of the no observed effect concentration and the lowest observed effect concentration based on analysis of variance. The ChVs for growth of fathead minnow, Colorado pikeminnow, and razorback sucker were 0.43, 0.40, and 0.67 mg/L NH3, respectively. The ChVs for mortality of fathead minnow, Colorado pikeminnow, and razorback sucker were 0.43, 0.70, and 0.67 mg/L NH3, respectively. Therefore, the ChVs for mortality and growth were similar for fathead minnow and razorback sucker; however, the ChV for growth was lower than the ChV for mortality for Colorado pikeminnow. Maximum likelihood regression was used to calculate 28-d lethal concentrations (LCx) for each species. The 28-d LC50, LC20, and LC1 values for fathead minnow were 0.69, 0.42, and 0.13 mg/L NH3, respectively. The 28-d LC50, LC20, and LC1 values for Colorado pikeminnow were 0.76, 0.61, and 0.38 mg/L NH3, respectively. The 28-d LC50, LC20, and LC1 values for razorback sucker were 0.54, 0.38, and 0.25 mg/L NH3, respectively. The fathead minnow, Colorado pikeminnow, and razorback sucker are relatively similar in sensitivity and rank at the 35th, 49th, and 31st percentiles, respectively, of the theoretical chronic fish sensitivity distributions for NH3. The existing water quality criteria for NH3, if met by remediation activities at the Moab site, would be protective of these endangered fishes even if fish sensitivity is based on the conservative LC1 value.

Assessing contaminant sensitivity of endangered and threatened aquatic species: part I. Acute toxicity of five chemicals.

Assessment of contaminant impacts to federally identified endangered, threatened and candidate, and state-identified endangered species (collectively referred to as "listed" species) requires understanding of a species' sensitivities to particular chemicals. The most direct approach would be to determine the sensitivity of a listed species to a particular contaminant or perturbation. An indirect approach for aquatic species would be application of toxicity data obtained from standard test procedures and species commonly used in laboratory toxicity tests. Common test species (fathead minnow, Pimephales promelas; sheepshead minnow, Cyprinodon variegatus; and rainbow trout, Oncorhynchus mykiss) and 17 listed or closely related species were tested in acute 96-hour water exposures with five chemicals (carbaryl, copper, 4-nonylphenol, pentachlorophenol, and permethrin) representing a broad range of toxic modes of action. No single species was the most sensitive to all chemicals. For the three standard test species evaluated, the rainbow trout was more sensitive than either the fathead minnow or sheepshead minnow and was equal to or more sensitive than listed and related species 81% of the time. To estimate an LC50 for a listed species, a factor of 0.63 can be applied to the geometric mean LC50 of rainbow trout toxicity data, and more conservative factors can be determined using variance estimates (0.46 based on 1 SD of the mean and 0.33 based on 2 SD of the mean). Additionally, a low- or no-acute effect concentration can be estimated by multiplying the respective LC50 by a factor of approximately 0.56, which supports the United States Environmental Protection Agency approach of multiplying the final acute value by 0.5 (division by 2). When captive or locally abundant populations of listed fish are available, consideration should be given to direct testing. When direct toxicity testing cannot be performed, approaches for developing protective measures using common test species toxicity data are available.

Fate and effects of the triazinone herbicide metribuzin in experimental pond mesocosms.

Metribuzin is a triazinone herbicide that is widely used for the control of grasses and broad-leaved weeds in soybeans, sugarcane, and numerous other crops. Metribuzin is highly toxic to freshwater macrophytes and algae under laboratory conditions (median plant EC(50) = 31 microg/L; n = 11 species) but has not been studied under controlled outdoor conditions. We conducted a 6-week study to examine the aquatic fate and effects of metribuzin in 0.1-ha outdoor aquatic mesocosms. Mesocosms (n = 2 per treatment) were treated with metribuzin at one of five concentrations: 0, 9, 19, 38, or 75 microg/L. Concentrations were selected to bracket known laboratory effect concentrations and to reflect calculated edge-of-field concentrations. The dissipation half-life of metribuzin in water was 5 days. Metribuzin had no statistically significant effects on water quality, periphyton biomass, macrophyte biomass, macrophyte species composition, fish survival, or fish growth at treatment levels ranging up to and including 75 microg/L. Although metribuzin is highly toxic to freshwater macrophytes and algae under laboratory conditions, it poses little risk to nontarget aquatic plants due to the short aqueous dissipation half-life. The findings also demonstrate that current herbicide risk assessment procedures used in the registration process could benefit from empirical assessments of the fate of chemicals under realistic environmental conditions.

Contaminant sensitivity of threatened and endangered fishes compared to standard surrogate species.

Standard environmental assessment procedures are designed to protect terrestrial and aquatic species. However, it is not known if endangered species are adequately protected by these procedures. At present, toxicological data obtained from studies with surrogate test fishes are assumed to be applicable to endangered fish species, but this assumption has not been validated. Static acute toxicity tests were used to compare the sensitivity of rainbow trout, fathead minnows, and sheepshead minnows to several federally listed fishes (Apache trout, Lahontan cutthroat trout, greenback cutthroat trout, bonytail chub, Colorado pikeminnow, razorback sucker, Leon Springs pupfish, and desert pupfish). Chemicals tested included carbaryl, copper, 4-nonylphenol, pentachlorophenol, and permethrin. Results indicated that the surrogates and listed species were of similar sensitivity. In two cases, a listed species had a 96-h LC50 (lethal concentration to 50% of the population) that was less than one half of its corresponding surrogate. In all other cases, differences between listed and surrogate species were less than twofold. A safety factor of two would provide a conservative estimate for listed cold-water, warm-water, and euryhaline fish species.

Effects of carbaryl, permethrin, 4-nonylphenol, and copper on muscarinic cholinergic receptors in brain of surrogate and listed fish species.

We investigated the regulation of the muscarinic cholinergic receptor (MChR) in brain from seven species of fish, two surrogates and five threatened or endangered species exposed to a series of chemicals as a measure of compensatory response among species. Fish were classified as either cold water (rainbow trout-surrogate, apache trout, lahanton trout) or warm water (fathead minnow-surrogate, razorback sucker, bonytail chub, colorado squawfish) and were exposed to chemicals shown to affect cholinergic pathways (carbaryl and permethrin) and two chemicals whose relationships to the cholinergic system is less clear (4-nonylphenol and copper). Downregulation of MChR occurred in all warm water species, except colorado squawfish, and at carbaryl concentrations similar to those causing downregulation observed in rainbow trout. Permethrin exposure resulted in downregulation in fathead minnow and razorback sucker, but the concentrations required for observation of this phenomenon were much greater than observed in cold water species. Copper exposure caused a decrease in brain MChR in rainbow trout and apache trout, whereas 4-nonylphenol exposure resulted in a decrease in brain MChR in all three cold water species. Our results indicate that surrogates are useful in assessing sublethal physiological responses to chemicals with a known mechanism of action such as carbaryl and support use of surrogates for assessing physiological responses to chemicals with diverse, less clear mechanisms of action.