Toxicity Assessment of Diazinon in a Constructed Wetland Using Hyalella azteca

The goal of this study was to examine the use of a constructed wetland to mitigate the ecological impacts of simulated diazinon runoff from agricultural fields into receiving waters, via 48 h aqueous and sediment bioassays using Hyalella azteca. Aqueous animal 48 h survival varied temporally and spatially in conjunction with measured diazinon concentrations. Sediment H. azteca survival varied temporally and spatially in conjunction with measured diazinon concentrations, but less than aqueous exposures, confirming that sediment bound diazinon was less bioavailable than aqueous diazinon.     

Effect of metal and metalloid contaminated sediment on the spatial distribution of asiatic clams (Corbicula fluminea)

The purpose of this study was to determine the effect of metal and metalloid contaminated sediment on the movement and spatial distribution of asiatic clam (Corbicula fluminea) populations in the laboratory. One hundred clams were randomly placed in one meter diameter pools containing sediment collected from three sites of varying levels of metal contamination. The location of each clam was determined on days 0, 7, 14, 21 and 28. Ripley's L(t) function was used to measure the randomness or clustering of clams in experimental pools. There was no significant evidence of clustering of clam locations throughout the experiment. Further, sediment type had no significant effect on the distribution of clams in the pools, which suggests that clams were moving independently of one another. This information allowed us to estimate whether individual clams move sufficient distances to escape from patches of contaminated sediment of different sizes. Simulation results showed that after 150 weeks, clams can effectively escape from smaller contaminated patches of sediment (25-, 50-, and 100-cm radius), while less than 50% of clams could escape from a contaminated patch of 200-cm radius. These simulations suggest that an avoidance of contaminated sediment will not result in effective escape from moderately sized patches of contaminated sediment (>100-cm radius).This work was performed under the auspices of the U.S. Department of Energy.     

Bioaccumulation of polychlorinated biphenyls and metals from contaminated sediment by freshwater prawns,Macrobrachium rosenbergii and clams,Corbicula fluminea

Freshwater prawns,Macrobrachium rosenbergii, and clams,Corbicula fluminea, were exposed for 48 or 50 days to three concentrations of a river sediment that contained environmental contaminants such as polychlorinated biphenyls (PCBs) and metals. Sediment was obtained twice during 1980 from the same location. Prawns were exposed to the first batch, sediment 5–80, which contained higher concentrations of some metals and PCBs compared to sediment 11–80, used with the clams. The highest concentration of sediment 5–80 was toxic to the prawns; sediment 11–80 was not toxic to the clams. Tissue analyses of prawns for PCBs, as Aroclors® 1242 and 1254, demonstrated bioaccumulation. Maximum concentration of 1242 in prawns was achieved by day 7 while 1254 continued to accumulate in prawn tissues during the initial 40 days of the exposure period. The PCB sediment bioaccumulation factors (BAF) for prawns ranged from 0.11 to 0.90 for 1242 and 0.20 to 2.40 for 1254, and were highest for animals exposed to 10% sediment. Exposed clams also accumulated PCBs (1242 + 1254) from the sediment. Sediment BAFs for clams were 0.54 to 12.52 and were highest for animals exposed to 10% sediment. Analyses of clams for metals showed lead (Pb) in exposed animals at higher concentrations compared with controls. Bioaccumulation of Pb differed from PCB in that the Pb concentrations did not increase over time and concentrations were higher among animals exposed to 10% sediment compared to animals exposed to 100% sediment. Sediment 11–80 contained 99 mg/kg of Pb while exposed animals, at 48 days, contained approximately 2.2 mg/kg Pb. Analysis of clams for cadmium (Cd) showed exposed animals contained less Cd than controls.     

Assessment of Toxicity Reduction in Wastewater Effluent Flowing Through a Treatment Wetland Using Pimephales promelas, Ceriodaphnia dubia, and Vibrio fischeri

Effluent toxicity is regularly assessed with Ceriodaphnia dubia short-term chronic and Vibrio fischeri toxicity tests. Condition factor and hemotocrit of fish have recently been used to assess fish health following exposure to xenoestrogens in complex municipal effluents. To assess the ability of a treatment wetland to reduce or remove toxicity of a municipal effluent, we compared C. dubia and V. fischeri bioassays to Pimephales promelas responses in situ. Final whole effluent was diverted to a constructed wetland and effluent samples were taken daily from four sites, at incremental distances from the inflow, for a 3-week study. Overlapping 7-day C. dubia tests and V. fischeri assays were conducted with samples from each wetland site concurrent with a 3-week fish exposure. C. dubia survival and fecundity were significantly (< 0.0001) reduced at the inflow, but steadily improved with distance from the inflow. Fish condition and hematocrit were lower (α < 0.05) at wetland sites closer to the inflow than other wetland sites and laboratory controls. However, effluent toxicity was not detected by V. fischeri bioassays. Our findings indicate that 7-day C. dubia bioassays were most sensitive to effluent toxicity and suggest that other bioassays should be used concurrent with V. fischeri assays for municipal effluent toxicity testing. Received: 20 November 2000/Accepted: 18 June 2001     

Effects of Diazinon on Mummichog (<Emphasis Type="Italic">Fundulus heteroclitus</Emphasis>) Larvae Produced from Eggs Differentially Treated with PCB126

During their formation, fish eggs receive a load of contaminants including polychlorinated biphenyls (PCBs) from their mother and then, after spawning, are exposed to pesticides present in water. This is the first study investigating the interaction between PCBs and organophosphorous pesticides in fish. The effect of diazinon was evaluated in mummichog (Fundulus heteroclitus) larvae produced from eggs differentially treated with 3,3′,4,4′,5 pentachlorobiphenyl (PCB126). A few hours after fertilization, eggs were treated topically with a solution of PCB126 (100 pg/μl) in dimethyl sulfoxide (DMSO) (Group P), DMSO (Group D), or not treated (Group N). Newly hatched larvae from Groups P and D were exposed to diazinon (125–12,900 ng/L) in saltwater and Group N larvae to saltwater alone. Diazinon caused a dose-responsive inhibition of cholinesterase (ChE) activity at environmentally realistic concentrations (≥361 ng/L), with up to 85% inhibition at 12,900 ng/L. Body length was also inversely related to diazinon at concentrations ≥361 ng/L and was significantly reduced (by 4%) at 12,900 ng/L compared to controls. Mummichog larvae were highly sensitive to PCB126 with an eightfold induction of the activity of ethoxyresorufin-O-deethylase at a dose of 710 pg PCB126 or 3.6 pg TCDD-TEQ/g wet weight. Treatment with PCB126 also caused a slight reduction in body length but no effect on ChE activity. This study indicates that the effects of PCB126 and diazinon on body length are cumulative because no significant synergistic or antagonistic interactions were observed. Longer term studies with several doses of PCB126 are needed to fully assess the overall impact of joint exposure to diazinon and PCB126 on growth and survival of fish larvae.     

Sediment preference in the asiatic clam (Corbicula fluminea) and viviparid snail (Campeloma decisum) as a response to low-level metal and metalloid contamination

Sediment preference experiments were performed with the asiatic clam (Corbicula fluminea) and viviparid snail (Campeloma decisum) to determine the potential use of clam and snail behavior as a response to low-level metal and metalloid contamination. Three sediment types with varying levels of metal contamination were paired in various combinations. Clams and snails were placed in aquaria along the interface between the sediment types. Daily location and burial status were noted for two weeks. Clams spent significantly more days in the uncontaminated sediment when paired with one of the contaminated sediments. Snails spent more days in contaminated sediments when paired with the uncontaminated sediment, but none of these differences was statistically significant. Clams moved fewer days in tanks with the two most contaminated sediment types. Burrowing of snails was relatively unaffected by sediment treatments. The behavior of clams was more sensitive than the behavior of snails to sediment metal contamination. Consequently, clam behavior appears to be a better behavioral indicator of metal contamination.This work was performed under the auspices of the U.S. Department of Energy.     

In situ monitoring of dredged material spoil sites using the oyster Crassostrea virginica

In situ and laboratory bioassays using the eastern oyster, Crassostrea virginica, were undertaken in the Wright River Estuary, South Carolina, to determine the toxic potential of effluent and sediment from recently dredged sediments. Current standards (ASTM, USEPA, and USACE) rely solely on laboratory-based bioassays to assess toxicity of dredge spoils prior to disposal. These bioassays do not necessarily replicate the natural physicochemical estuarine processes, limiting the environmental realism of this approach. In this study, oysters were collected from a site on Leadenwah Creek (SC) and deployed in plastic cages anchored above the sediment and within the intertidal zone for 90 days at four dredge spoil disposal areas (18 sites total, one bushel/site). Oysters were also deployed at a reference site (New River Estuary, SC) and the original collection site. Trace metals and polycyclic aromatic hydrocarbons (PAHs) in tissue, sediment, and effluent samples as well as the assessment of oyster health in adults (% mortality and % reduction in potential yield) and larvae (larval development) were measured. Results indicated high arsenic concentrations in surface water samples (<10 to 147 g/L), some of which exceeded the USEPA chronic marine water quality criteria and sediment concentrations (<1.0–82.2 mg/kg), which also exceeded the ERM (70 mg/kg) and the ERL (8.2 mg/kg) for arsenic, and which may have contributed to the toxic response seen in deployed oysters. A positive relationship was also seen between the in situ percent reduction in potential yield and laboratory-derived data from larval oyster development bioassays. The advantage of the combined in situ/laboratory approach used in this study is the ability to resolve probable factors influencing the toxicity of these effluents to oysters.     

Comparison of Bulk Sediment and Sediment Elutriate Toxicity Testing Methods

Numerous methods exist for assessing the potential toxicity of sediments in aquatic systems. In this study, the results from 10-day bulk sediment toxicity test methods using Hyalella azteca and Chironomus tentans were compared to results from 96-h Pimephales promelas and Ceriodaphnia dubia renewed acute toxicity tests conducted using elutriate samples prepared from the same sediments. The goal of the study was to determine if the results from the elutriate tests were comparable to those obtained from the bulk sediment tests. Of the 25 samples analyzed, 16 were found to be toxic to at least one of the species tested, in either elutriate or bulk sediment tests. The C. tentans 10-day bulk sediment test was the most sensitive, with 12 sediment samples exhibiting toxicity to this species, whereas the H. azteca bulk sediment test and C. dubia 96-h elutriate test were the least sensitive, exhibiting toxicity in only 7 of the 25 sediments tested. The P. promelas elutriate test found 8 of the 25 sediments to be toxic. Based on the total number of sites found to show toxicity, results from testing indicate 96-h elutriate tests show a level of sensitivity comparable to 10-day bulk sediment tests in assessing toxicity quantitatively. However, the methods did not always find toxicity at the same sites, suggesting that the ability of elutriate tests to predict toxicity (quantitatively) is not statistically correlated with bulk sediment methods. This would indicate that a suite of toxicity test methods would provide the most complete measure of site condition; however, in circumstances where bulk sediment testing is not feasible, elutriate tests can provide a practical and credible alternative for toxicity assessment.     

Field and Laboratory Assessment of a Coal Processing Effluent in the Leading Creek Watershed,Meigs County,Ohio

The U.S. Environmental Protection Agency has not recommended water quality criteria (WQC) to protect aquatic life from elevated sodium and sulfate concentrations, such as those associated with the coal-processing effluent of Meigs County Mine #31. This discharge, received by a tributary of the Leading Creek Watershed (SE Ohio), had a mean specific conductivity (SC) of 8,109 (7,750–8,750) μS/cm and total metal concentrations below acute WQC. The mean 48-h LC₅₀ for Ceriodaphnia dubia in the effluent was 6,713 ± 99 μS/cm; mean 48-h survival was 44% for study sites downstream of the effluent. The best indicators of impairment used in this study were Ceriodaphnia fecundity, in situ Corbicula fluminea growth, EPT minus Hydropsychidae (richness and relative abundance), and relative Ephemeroptera abundance. Mayflies, reduced by more than 99% below the effluent, were absent from all but the furthest downstream study site. SC was strongly correlated with Corbicula growth (r = −0.9755, p = 0.0009) and EPT minus Hydropsychidae richness (r = −0.8756, p < 0.0001), suggesting the effluent was primarily responsible for biotic impairment. Our results indicated that SC levels, a measure of dissolved solids, in the Leading Creek Watershed that exceeded ∼3,700 μS/cm impaired sensitive aquatic fauna. Received: 17 March 2002/Accepted: 5 September 2002     

Toxicity of Sediment-Associated Pesticides to <Emphasis Type="Italic">Chironomus dilutus</Emphasis> and <Emphasis Type="Italic">Hyalella azteca</Emphasis>

Two hundred sediment samples were collected and their toxicity evaluated to aquatic species in a previous study in the agriculturally dominated Central Valley of California, United States. Pyrethroid insecticides were the main contributors to the observed toxicity. However, mortality in approximately one third of the toxic samples could not be explained solely by the presence of pyrethroids in the matrices. Hundreds of pesticides are currently used in the Central Valley of California, but only a few dozen are analyzed in standard environmental monitoring. A significant amount of unexplained sediment toxicity may be due to pesticides that are in widespread use that but have not been routinely monitored in the environment, and even if some of them were, the concentrations harmful to aquatic organisms are unknown. In this study, toxicity thresholds for nine sediment-associated pesticides including abamectin, diazinon, dicofol, fenpropathrin, indoxacarb, methyl parathion, oxyfluorfen, propargite, and pyraclostrobin were established for two aquatic species, the midge Chironomus dilutus and the amphipod Hyalella azteca. For midges, the median lethal concentration (LC₅₀) of the pesticides ranged from 0.18 to 964 μg/g organic carbon (OC), with abamectin being the most toxic and propargite being the least toxic pesticide. A sublethal growth endpoint using average individual ash-free dry mass was also measured for the midges. The no–observable effect concentration values for growth ranged from 0.10 to 633 μg/g OC for the nine pesticides. For the amphipods, fenpropathrin was the most toxic, with an LC₅₀ of 1–2 μg/g OC. Abamectin, diazinon, and methyl parathion were all moderately toxic (LC₅₀s 2.8–26 μg/g OC). Dicofol, indoxacarb, oxyfluorfen, propargite, and pyraclostrobin were all relatively nontoxic, with LC₅₀s greater than the highest concentrations tested. The toxicity information collected in the present study will be helpful in decreasing the frequency of unexplained sediment toxicity in agricultural waterways.     

Temporal patterns of brain cholinesterase activities of white-footed mice (Peromyscus leucopus) following dosing with diazinon or parathion

Brain cholinesterase (ChE) activities were determined for white-footed mice (Peromyscus leucopus) orally dosed with either diazinon (18.8 mg/kg body weight) or parathion (10 mg/kg body weight). Following treatment with diazinon, a latent period of approximately six hr elapsed during which time ChE activity was relatively unaffected. After the latent phase, ChE activity rapidly declined to a minimum 12 hr after dosing. After 48 hr, ChE activity recovered to levels only slightly below that of the controls. The response of both male and female white-footed mouse brain ChE to dosing with parathion was similar to that exhibited by animals dosed with diazinon. ChE activities declined rapidly at 6 hr and reached a minimum 24 hr after dosing. ChE activity of treated animals was comparable to that of controls 48 hr after treatment. Brain ChE activities of treated female white-footed mice were significantly lower (P<0.05) than those of treated males.     

Role of Vegetation in a Constructed Wetland on Nutrient–Pesticide Mixture Toxicity to <Emphasis Type="Italic">Hyalella azteca</Emphasis>

The toxicity of a nutrient–pesticide mixture in nonvegetated and vegetated sections of a constructed wetland (882 m² each) was assessed using Hyalella azteca 48-h aqueous whole-effluent toxicity bioassays. Both sections were amended with a mixture of sodium nitrate, triple superphosphate, diazinon, and permethrin simulating storm-event agricultural runoff. Aqueous samples were collected at inflow, middle, and outflow points within each section 5 h, 24 h, 72 h, 7 days, 14 days, and 21 days postamendment. Nutrients and pesticides were detected throughout both wetland sections with concentrations longitudinally decreasing more in vegetated than nonvegetated section within 24 h. Survival effluent dilution point estimates—NOECs, LOECs, and LC₅₀s—indicated greatest differences in toxicity between nonvegetated and vegetated sections at 5 h. Associations of nutrient and pesticide concentrations with NOECs indicated that earlier toxicity (5–72 h) was from permethrin and diazinon, whereas later toxicity (7–21 days) was primarily from diazinon. Nutrient–pesticide mixture concentration–response assessment using toxic unit models indicated that H. azteca toxicity was due primarily to the pesticides diazinon and permethrin. Results show that the effects of vegetation versus no vegetation on nutrient–pesticide mixture toxicity are not evident after 5 h and a 21-day retention time is necessary to improve H. azteca survival to ≥90% in constructed wetlands of this size.     

Relative Importance of Burrow Sediment and Porewater to the Accumulation of Trace Metals in the Clam Amiantis umbonella

The objectives of this study were to evaluate the infaunal, facultative deposit-feeding clam Amiantis umbonella as a bioindicator of trace-metal contamination and the relative importance of clam burrow sediment and porewater to total accumulation in an urban/industrial coastal environment. Concentrations of eight trace metals (cadmium [Cd], chromium, copper, mercury [Hg], nickel, lead [Pb], vanadium, and zinc) were measured in the soft tissues of clams and in sediment and porewater from clam burrows along a 5-km transect from desalination/power plant discharges in inner Kuwait Bay. All metals had significantly greater concentrations in clams collected near the desalination/power plant discharges than from the reference site and exhibited decreasing trends with distance from the point source in clam soft tissues and burrow sediment and porewater. Concentrations of Hg (1–9 ppm [dry weight]) and the highest concentrations of Pb (3 ppm) and Cd (7 ppm) in clams from contaminated sites in Kuwait Bay were greater than human consumption limits. Metal concentrations in clams were correlated with those in burrow sediment and porewater across all sites and at sites closest to the point source but not within the reference site. The concentrations of all metals, except Pb, in clams from the contaminated sites were more highly correlated with those in clam burrow sediment than porewater. Concentrations of Pb in clam soft tissues were more strongly correlated with those in burrow porewater than sediment. These results indicate that A. umbonella is an excellent bioindicator of trace metal contamination and that sediment is an important source of contaminant metals to this infaunal clam; however, the source of each metal must be evaluated separately.     

Effects of Diazinon Exposure on Cholinesterase Activity in Different Tissues of European Eel (Anguilla anguilla)

Cholinesterase (ChE) activity was measured in brain, plasma, and whole eye ofAnguilla anguillaexperimentally exposed to a sublethal concentration of 0.042 mg/liter (0.50 of the 96-hr LC₅₀) of the organophosphorous pesticide diazinon. Whole eye was the tissue which revealed higher values of ChE activity (8.17 μmol/min/g) in nonexposed animals. Brain, plasma, and whole eye ChE activity ofA. anguillawas inhibited at 6, 24, 48, 72, and 96 hr of diazinon exposure. Pesticide induced significant inhibitory effects on the ChE activity of this species ranging from >70% inhibition in brain tissue to >90% in plasma samples. Brain and plasma presented technical difficulties in their collection. Due to its ease of collection, high ChE activity levels, and sensitivity to ChE pesticide inhibition, the use of whole eye is recommended for post-mortem ChE analysis inA. anguilla.     

Assessment of the biological impact of point source discharges employing Asiatic clams

Three thousandCorbicula fluminea were transplanted for six months along a stress gradient produced by the thermal discharge of a power plant to ascertain whether Asiatic clams could be employed to monitor the biological impact of point source discharges. Water temperature at the warmest site (station 1) was between 25 and 35°C, 7–13°C above that in the river. Almost complete clam mortality was recorded here within 60 days. Water temperature at the next cooler site (station 2) was elevated 0.5–2.0°C above ambient river temperature (stations 3 and 4). Adult and juvenile clam shell and tissue growth and condition index was less here (P < 0.05) than at sites 3 and 4 where all three parameters were similar (P > 0.1). A one time measurement of juvenile clam scope for growth (SFG) also demonstrated a decreasing trend along the thermal gradient. SFG was positively correlated with juvenile clam shell and tissue growth (P < 0.01 and P < 0.05) but not with condition index (0.1 > P > 0.05).     

Toxicity of the Insecticide Terbufos,its Oxidation Metabolites,and the Herbicide Atrazine in Binary Mixtures to <Emphasis Type="Italic">Ceriodaphnia</Emphasis> cf <Emphasis Type="Italic">dubia</Emphasis>

The acute toxicity of terbufos and its major metabolites, tested alone, in binary mixtures or in combination with atrazine were evaluated using neonates of the cladoceran Ceriodaphnia cf dubia. Terbufos, terbufos sulfoxide, and terbufos sulfone tested individually were highly toxic to C. cf dubia, with mean 96-h EC₅₀ values of 0.08, 0.36, and 0.19 μg/l, respectively. The addition of atrazine (10 μg/l) significantly increased the toxicity of terbufos. The toxicity of terbufos sulfone was unaffected by atrazine, whereas the results for terbufos sulfoxide were equivocal. Equitoxic mixtures of the metabolites showed additive toxicity to C. cf dubia. The high toxicities of terbufos and its environmentally persistent oxidative metabolites suggest that contamination of aquatic systems with this insecticide mixture and the coapplied herbicide atrazine might pose a greater hazard to some biota than their individual toxicities.     

Acute Toxicity, Toxicokinetics, and Tissue Target of Lead and Uranium in the Clam Corbicula fluminea and the Worm Eisenia fetida: Comparison with the Fish Brachydanio rerio

The general objective of our work was to propose new reference material for chemical toxicity testing and new sentinel organisms for environmental quality survey programs (freshwater or soils). We also wanted to provide basic toxicological data on the environmental effects of uranium. Thus, we conducted a comparative study to establish the acute toxicity and toxicokinetics of lead (Pb) and uranium (U) to the bivalve mollusc Corbicula fluminea and the terrestrial annelid Eisenia fetida andrei and to compare these findings with those of the well-known teleost fish Brachydanio rerio. We then measured the concentration of these metals in various tissues of the clam and the worm after two periods of exposure (4 and 11 days) to identify the affinities of these tissues for Pb and U. Our results have shown that Pb and U are very toxic to Eisenia and relatively nontoxic to Corbicula. By comparison, Pb was relatively nontoxic and U appeared to be very toxic to the fish. The toxicokinetic studies indicated that the three species are able to accumulate Pb and U, the rate and level of accumulation depending both on the species and the metal. We also found that fish and clams depurate the two metals. Data collected for the worm were conflicting: Pb was not depurated whereas tissue concentrations of U declined after the eighth day of exposure. Our study has also shown that the tissue distribution of Pb in the mollusc and in the earthworm differs significantly from that of U, both after 4 and 11 days exposure. In conclusion, these three species showed potential as bioindicators of environmental contamination by metals. Indeed, they could be used in conjunction to test different compartments of an ecosystem: worms for soils, fish for the water column, and clams for the water/sediment interface. Received: 30 October 1996/Accepted: 1 September 1998     

Lead in clams and fish of dietary importance from Coatzacoalcos estuary (Gulf of Mexico), an industrialized tropical region

With the aim of knowing seasonal variability of lead in fish and bivalve species from Coatzacoalcos estuary, biota collected during three seasons was examined. In muscle tissue of fish, the highest level (5.4 μg g⁻¹) was found in the longnose gar Lepisosteus osseus from San Francisco stream (a highly impacted site); the lowest value (0.2 μg g⁻¹) was registered in yellowfin mojarra Gerres cinereus from Ostión lagoon (control site). In bivalves, concentrations in soft tissue ranged from 1.5 μg g⁻¹ in Polymesoda caroliniana from Calzadas river, to 0.1 μg g⁻¹ in Corbicula fluminea from Hidalgotitlán (control site).     

Enantioselective Chronic Toxicity of Fipronil to <Emphasis Type="Italic">Ceriodaphnia dubia</Emphasis>

Fipronil is a phenylpyrazole pesticide that has greatly increased in popularity in recent years. As a chiral molecule, fipronil is released into the environment as a 1:1 mixture (called a racemate) of its two enantiomers. Previous toxicity work has indicated that the enantiomers of fipronil exhibit significantly different levels of acute toxicity to the nontarget organism Ceriodaphnia dubia. In this work we examine the chronic effects of the pure enantiomers and racemate on the survival, development, mobility, and reproduction of C. dubia adults and the survival and mobility of their offspring. Based on 8-day trials, the (+) enantiomer of fipronil showed a significantly greater reduction in the number of offspring (LOEC = 2 μg/L) than either the racemate (LOEC = 15 μg/L) or the (−) enantiomer (LOEC = 30 μg/L). The (+) enantiomer was also shown to be significantly more toxic to neonates born during the course of the experiment (LC50₂₄ = 18.1 μg/L, LC50₄₈ = 10.3 μg/L) than the racemate (LC50₂₄ = 33.3 μg/L, LC50₄₈ = 30.3 μg/L), but only after 48 h. Both the (+) enantiomer and the racemate were significantly more toxic to C. dubia than the (−) enantiomer (LC50₂₄ = 65.2 μg/L, LC50₄₈ = 50.1 μg/L) at both time points. Qualitative mobility data followed a similar trend, with the (+) enantiomer causing greater impairment in mobility at low concentrations. These data imply that the (−) enantiomer has less impact on the reproductive success of C. dubia than either the (+) enantiomer or the racemate. Enantiomerically pure or enriched formulations of (−) fipronil may reduce impacts to the nontarget organism C. dubia.     

Acute Toxicity of Methyl-Parathion in Wetland Mesocosms: Assessing the Influence of Aquatic Plants Using Laboratory Testing with <Emphasis Type="Italic">Hyalella azteca</Emphasis>

Methyl-parathion (MeP) was introduced into constructed wetlands for the purpose of assessing the importance of distance from the source of contamination and the role of emergent vegetation on the acute toxicity to Hyalella azteca (Crustacea: Amphipoda). A vegetated (90% cover: mainly Juncus effuses) and a nonvegetated wetland (each with a water body of 50 × 5.5 × 0.2 m) were each exposed to a simulated MeP storm runoff event. H. azteca was exposed for 48 h in the laboratory to water samples taken from the wetlands at a distance of 5, 10, 20, and 40 m from the pesticide inlet 3 h, 24 h, 96 h, and 10 days following application. Methyl-parathion was detected throughout the nonvegetated wetland, whereas the pesticide was only transported halfway through the vegetated wetland. A repeated-measure three-way analysis of variance (ANOVA) using time, location, and vegetation indicated significantly lower toxicity in the vegetated wetland. Furthermore, the mortality decreased significantly with both increasing distance from the inlet and time (48-h LC₅₀ ± 95% CI: 9.0 ± 0.3 g/L). A significant three-way interaction of time × vegetation × location confirmed higher toxicity at the inlet area of the nonvegetated wetland immediately after contamination. Significant linear regressions of maximum mortality (independent of time) versus distance from the pesticide inlet indicated that 44 m of vegetated and 111 m of nonvegetated wetland would reduce H. azteca mortality to 5%. These results suggest that vegetation contributes to reduced MeP effects in constructed wetlands.