Acute and chronic toxicity of technical-grade pesticides to glochidia and juveniles of freshwater mussels (Unionidae)

Chemical contaminants are among many potential factors involved in the decline of freshwater mussel populations in North America, and the effects of pesticides on early life stages of unionid mussels are largely unknown. The objective of this study was to determine the toxicity of technical-grade current-use pesticides to glochidia and juvenile life stages of freshwater mussels. We performed acute toxicity tests with glochidia (five species) and juveniles (two species) exposed to a suite of current-use pesticides including herbicides (atrazine and pendimethalin), insecticides (fipronil and permethrin), and a reference toxicant (NaCl). Because of limited availability of test organisms, not all species were tested with all pesticides. Toxicity tests with fungicides (chlorothalonil, propiconazole, and pyraclostrobin) were performed with one species (Lampsilis siliquoidea). Lampsilis siliquoidea glochidia and juveniles were highly sensitive to the fungicides tested but the technical-grade herbicides and insecticides, at concentrations approaching water solubility, were not acutely toxic to this or the other unionid species. In a 21-d chronic test with four-month-old juvenile L. siliquoidea, the 21-d median effective concentration (EC50) with atrazine was 4.3 mg/L and in atrazine treatments >or=3.8 mg/L mussel growth was significantly less than controls. The relatively high sensitivity of L. siliquoidea to chlorothalonil, propiconazole, and pyraclostrobin is similar to that reported for other aquatic organisms commonly used for toxicity testing. The relative risk associated with acute exposure of early life stages of mussels to technical-grade atrazine, pendimethalin, fipronil, and permethrin is likely low; however, survival and growth results with juvenile L. siliquoidea indicate that chronic exposure to high concentrations (>/=3.8 mg/L) of atrazine may have the potential to impact mussel populations and warrants further investigation.     

Acute and chronic toxicity of glyphosate compounds to glochidia and juveniles of Lampsilis siliquoidea (Unionidae)

Native freshwater mussels (family Unionidae) are among the most imperiled faunal groups in the world. Factors contributing to the decline of mussel populations likely include pesticides and other aquatic contaminants; however, there is a paucity of data regarding the toxicity of even the most globally distributed pesticides, including glyphosate, to mussels. Therefore, the toxicity of several forms of glyphosate, its formulations, and a surfactant (MON 0818) used in several glyphosate formulations was determined for early life stages of Lampsilis siliquoidea, a native freshwater mussel. Acute and chronic toxicity tests were performed with a newly established American Society of Testing and Materials (ASTM) standard guide for conducting toxicity tests with freshwater mussels. Roundup, its active ingredient, the technical-grade isopropylamine (IPA) salt of glyphosate, IPA alone, and MON 0818 (the surfactant in Roundup formulations) were each acutely toxic to L. siliquoidea glochidia. MON 0818 was most toxic of the compounds tested and the 48-h median effective concentration (0.5 mg/L) for L. siliquoidea glochidia is the lowest reported for any aquatic organism tested to date. Juvenile L. siliquoidea were also acutely sensitive to MON 0818, Roundup, glyphosate IPA salt, and IPA alone. Technical-grade glyphosate and Aqua Star were not acutely toxic to glochidia or juveniles. Ranking of relative chronic toxicity of the glyphosate-related compounds to juvenile mussels was similar to the ranking of relative acute toxicity to juveniles. Growth data from chronic tests was largely inconclusive. In summary, these results indicate that L. siliquoidea, a representative of the nearly 300 freshwater mussel taxa in North America, is among the most sensitive aquatic organisms tested to date with glyphosate-based chemicals and the surfactant MON 0818.     

Acute and chronic toxicity of pesticide formulations (atrazine, chlorpyrifos, and permethrin) to glochidia and juveniles of Lampsilis siliquoidea

Freshwater mussels are among the most imperiled faunal groups in North America; approximately 67% of the nearly 300 native freshwater mussel species (family Unionidae) are listed as endangered, threatened, or of special concern. Despite evidence that glochidia and juvenile life stages are highly sensitive to some chemical contaminants, the effects of pesticides on early life stages of unionid mussels are largely unknown. In the United States, pesticide registration is based on toxicity data of the active ingredient, not formulations as they are sold and applied. Some pesticide formulations, however, are more toxic than their active ingredient (technical-grade pesticide) alone because of the presence of surfactants, adjuvants, or other ingredients in the formulation. The objective of the present study was to compare the toxicity of active ingredients of several current-use pesticides (atrazine, chlorpyrifos, and permethrin) to the toxicity of pesticide formulations to glochidia and juvenile life stages of a freshwater mussel (Lampsilis siliquoidea). The atrazine formulation (Aatrex) was more toxic than technical-grade atrazine in chronic tests with juvenile L. siliquoidea. For other pesticides, acute and chronic toxicity of technical-grade pesticides were similar to the toxicity of pesticide formulations. Median effective concentrations for chlorpyrifos were 0.43 mg/L for glochidia at 48 h, 0.25 mg/L for juveniles at 96 h, and 0.06 mg/L for juveniles at 21 d. Atrazine and permethrin as well as their formulations did not cause significant acute toxicity in glochidia or juveniles at exposure concentrations approaching water-solubility limits. Additional research is needed on other pesticides with different modes of action, on the role of different routes of exposure, and with other species of unionid mussels to evaluate similarities of toxic response.     

Acute effects of road salts and associated cyanide compounds on the early life stages of the unionid mussel Villosa iris

The toxicity of cyanide to the early life stages of freshwater mussels (order Unionida) has remained unexplored. Cyanide is known to be acutely toxic to other aquatic organisms. Cyanide-containing compounds, such as sodium ferrocyanide and ferric ferrocyanide, are commonly added to road deicing salts as anticaking agents. The purpose of the present study was to assess the acute toxicity of three cyanide compounds (sodium cyanide, sodium ferrocyanide, and ferric ferrocyanide), two road salts containing cyanide anticaking agents (Morton and Cargill brands), a brine deicing solution (Liquidow brand), and a reference salt (sodium chloride) on glochidia (larvae) and juveniles of the freshwater mussel Villosa iris. Sodium ferrocyanide and ferric ferrocyanide were not acutely toxic to glochidia and juvenile mussels at concentrations up to 1,000 mg/L and 100 mg/L, respectively. Lowest observed effect concentrations (LOECs) for these two chemicals ranged from 10 to >1,000 mg/L. Sodium cyanide was acutely toxic to juvenile mussels, with a 96-h median effective concentration (EC50) of 1.10 mg/L, although glochidia tolerated concentrations up to 10 mg/L. The EC50s for sodium chloride, Liquidow brine, Morton road salt, and Cargill road salt were not significantly different for tests within the same life stage and test duration (range, 1.66-4.92 g/L). These results indicate that cyanide-containing anticaking agents do not exacerbate the toxicity of road salts, but that the use of road salts and brine solutions for deicing or dust control on roads may warrant further investigation.     

Partial life-cycle and acute toxicity of perfluoroalkyl acids to freshwater mussels

Freshwater mussels are among the most sensitive aquatic organisms to many contaminants and have complex life-cycles that include several distinct life stages with unique contaminant exposure pathways. Standard acute (24-96 h) and chronic (28 d) toxicity tests with free larva (glochidia) and juvenile mussels are effective at generating data on contaminant effects at two discrete life stages but do not incorporate effects on brooded glochidia. We developed a novel partial life-cycle assay that incorporates exposures to brooding adult female mussels and used this method in combination with acute toxicity tests to assess adverse effects of perfluoroctanesulfonic acid (PFOS) and perfluoroctanoic acid (PFOA) on freshwater mussels. Fatmucket (Lampsilis siliquoidea) were exposed to PFOS at two life stages: brooding glochidia (in marsupia) for 36 d and free glochidia in water for 24 h. In standard acute tests with glochidia (24-48 h exposures) and juveniles (48-96 h exposures) of fatmucket and black sandshell (Ligumia recta), glochidia were 8 to 25 times more sensitive than juveniles. Perfluoroctanesulfonic acid significantly reduced the duration of glochidia viability and reduced probability of metamorphosis at concentrations 3,000 times lower than the most sensitive acute endpoint (24-h EC50). The partial life-cycle test is adaptable to a variety of endpoints and research objectives and is useful for identifying adverse effects at contaminant concentrations below those required for an acute lethal response.     

Acute Toxicity of Six Freshwater Mussel Species (Glochidia) to Six Chemicals: Implications for Daphnids and Utterbackia imbecillis as Surrogates for Protection of Freshwater Mussels (Unionidae)

Acute (24-h) toxicity tests were used in this study to compare lethality responses in early life stages (glochidia) of six freshwater mussel species, Leptodea fragilis, U. imbecillis, Lampsilis cardium, Lampsilis siliquoidea, Megalonaias nervosa, and Ligumia subrostrata, and two standard test organisms, Ceriodaphnia dubia and Daphnia magna. Concentrations of carbaryl, copper, 4-nonylphenol, pentachlorophenol, permethrin, and 2,4-D were used in acute exposures to represent different chemical classes and modes of action. The relative sensitivities of species were evaluated by ranking their LC₅₀ values for each chemical. We used these ranks to determine the extent to which U. imbecillis (one of the most commonly used unionids in toxicity tests) was representative of the tolerances of other mussels. We also calculated geometric mean LC₅₀s for the families Unionidae and Daphnidae. Rankings of these data were used to assess the extent to which Daphnidae can be used as surrogates for freshwater mussels relative to chemical sensitivity. While no single chemical elicited consistently high or low toxicity estimates, carbaryl and 2,4-D were generally the least toxic to all species tested. No species was always the most sensitive, and Daphnidae were generally protective of Unionidae. Utterbackia imbecillis, while often proposed as a standard unionid mussel test species, did not always qualify as a sufficient surrogate (i.e., a substitute organism that often elicits similar sensitivity responses to the same contaminant exposure) for other species of mussels, since it was usually one of the more tolerant species in our rankings. U. imbecillis should be used as a surrogate species only with this caution on its relative insensitivity.     

Persistence of selected organophosphate and carbamate insecticides in waters from a coastal watershed

Organophosphate and carbamate compounds are among the most widely used pesticides. Contamination of surface water by these compounds is of concern because of potential toxicity to aquatic organisms, especially those at lower trophic levels. In this study we evaluated the persistence of diazinon, chlorpyrifos, malathion, and carbaryl in waters from various sites in the Newport Bay-San Diego Creek watershed in southern California (USA). The persistence of diazinon and chlorpyrifos was much longer than that of malathion or carbaryl and was further prolonged in seawater. Microbial degradation contributed significantly to the dissipation of diazinon and chlorpyrifos in freshwater, but was inhibited in seawater, leading to increased persistence. In contrast, degradation of malathion and carbaryl was rapid and primarily abiotic. A greater temperature dependence was observed for carbaryl degradation in all waters and for diazinon degradation in freshwater. The interactions of pesticide persistence with water location, temperature, and type of pesticides suggest that site- and compound-specific information is needed when evaluating the overall ecotoxicological risks of pesticide pollution in a watershed. Because the persistence of diazinon and chlorpyrifos may increase significantly in seawater, mitigation should occur before the pesticides reach seawater. The relatively short persistence of these compounds in freshwater suggests that practices aimed at extending residence time (e.g., diversion to wetlands) may effectively reduce pesticide output to downstream water bodies.     

An evaluation of freshwater mussel toxicity data in the derivation of water quality guidance and standards for copper

The state of Oklahoma has designated several areas as freshwater mussel sanctuaries in an attempt to provide freshwater mussel species a degree of protection and to facilitate their reproduction. We evaluated the protection afforded freshwater mussels by the U.S. Environmental Protection Agency (U.S. EPA) hardness-based 1996 ambient copper water quality criteria, the 2007 U.S. EPA water quality criteria based on the biotic ligand model and the 2005 state of Oklahoma copper water quality standards. Both the criterion maximum concentration and criterion continuous concentration were evaluated. Published acute and chronic copper toxicity data that met American Society for Testing and Materials guidance for test acceptability were obtained for exposures conducted with glochidia or juvenile freshwater mussels. We tabulated toxicity data for glochidia and juveniles to calculate 20 species mean acute values for freshwater mussels. Generally, freshwater mussel species mean acute values were similar to those of the more sensitive species included in the U.S. EPA water quality derivation database. When added to the database of genus mean acute values used in deriving 1996 copper water quality criteria, 14 freshwater mussel genus mean acute values included 10 of the lowest 15 genus mean acute values, with three mussel species having the lowest values. Chronic exposure and sublethal effects freshwater mussel data available for four species and acute to chronic ratios were used to evaluate the criterion continuous concentration. On the basis of the freshwater mussel toxicity data used in this assessment, the hardness-based 1996 U.S. EPA water quality criteria, the 2005 Oklahoma water quality standards, and the 2007 U.S. EPA water quality criteria based on the biotic ligand model might need to be revised to afford protection to freshwater mussels.     

Acute and chronic toxicity of mercury to early life stages of the rainbow mussel, Villosa iris (Bivalvia: Unionidae)

Mercury (Hg) contamination is receiving increased attention globally because of human health and environmental concerns. Few laboratory studies have examined the toxicity of Hg on early life stages of freshwater mussels, despite evidence that glochidia and juvenile life stages are more sensitive to contaminants than adults. Three bioassays (72-h acute glochidia, 96-h acute juvenile, and 21-d chronic juvenile toxicity tests) were conducted by exposing Villosa iris to mercuric chloride salt (HgCl2). Glochidia were more sensitive to acute exposure than were juvenile mussels, as 24-, 48-, and 72-h median lethal concentration values (LC50) for glochidia were >107, 39, and 14 microg Hg/L, respectively. The 24-, 48-, 72-, and 96-h values for juveniles were 162, 135, 114, and 99 microg Hg/L, respectively. In the chronic test, juveniles exposed to Hg treatments > or = 8 microg/L grew significantly less than did control organisms. The substantial difference in juvenile test endpoints emphasizes the importance of assessing chronic exposure and sublethal effects. Overall, our study supports the use of glochidia as a surrogate life stage for juveniles in acute toxicity tests. However, as glochidia may be used only in short-term tests, it is imperative that an integrated approach be taken when assessing risk to freshwater mussels, as their unique life history is atypical of standard test organisms. Therefore, we strongly advocate the use of both glochidia and juvenile life stages for risk assessment.     

Assessing the biological relevance of exposing freshwater organisms to atrazine and molinate in environmentally realistic exposure test systems

Assessing the toxicity of chemicals in treated laboratory water may not accurately represent the toxicity of chemicals in natural aquatic systems. In natural water, dissolved organic matter, suspended particulate matter, and sediment play key roles in the sorption of contaminants from the water. Our previously published series of papers illustrated that the presence of sediment in aquatic toxicity testing systems significantly (p < 0.05) reduced the bioavailability of the herbicides atrazine and molinate to five Australian freshwater organisms. It is not clear whether the reduced bioavailability means that the trigger values (TVs) in the current Australian and New Zealand water quality guidelines, which are calculated using toxicity data from water-only toxicity tests, provide appropriate environmental protection. Several new sets of TVs were derived in the present study and were compared to each other and to the current Australian and New Zealand TVs for atrazine and molinate. The current Australian and New Zealand TVs for atrazine and molinate provided appropriate protection to Australian freshwater species. Australian freshwater species have a sensitivity distribution similar to those of overseas species to atrazine and molinate.     

Acute toxicity of copper, ammonia, and chlorine to glochidia and juveniles of freshwater mussels (Unionidae)

The objective of the present study was to determine acute toxicity of copper, ammonia, or chlorine to larval (glochidia) and juvenile mussels using the recently published American Society for Testing and Materials (ASTM) Standard guide for conducting laboratory toxicity tests with freshwater mussels. Toxicity tests were conducted with glochidia (24- to 48-h exposures) and juveniles (96-h exposures) of up to 11 mussel species in reconstituted ASTM hard water using copper, ammonia, or chlorine as a toxicant. Copper and ammonia tests also were conducted with five commonly tested species, including cladocerans (Daphnia magna and Ceriodaphnia dubia; 48-h exposures), amphipod (Hyalella azteca; 48-h exposures), rainbow trout (Oncorhynchus mykiss; 96-h exposures), and fathead minnow (Pimephales promelas; 96-h exposures). Median effective concentrations (EC50s) for commonly tested species were >58 microg Cu/L (except 15 microg Cu/L for C. dubia) and >13 mg total ammonia N/L, whereas the EC50s for mussels in most cases were <45 microg Cu/L or <12 mg N/L and were often at or below the final acute values (FAVs) used to derive the U.S. Environmental Protection Agency 1996 acute water quality criterion (WQC) for copper and 1999 acute WQC for ammonia. However, the chlorine EC50s for mussels generally were >40 microg/L and above the FAV in the WQC for chlorine. The results indicate that the early life stages of mussels generally were more sensitive to copper and ammonia than other organisms and that, including mussel toxicity data in a revision to the WQC, would lower the WQC for copper or ammonia. Furthermore, including additional mussel data in 2007 WQC for copper based on biotic ligand model would further lower the WQC.     

Determination and aquatic risk assessment of pesticide residues in riparian drainage canals in northeastern Greece

An approach combining monitoring and ecotoxicological data has been undertaken to assess pesticide loading in the drainage canals of two transboundary rivers of northeastern Greece near the Greek/Bulgarian/Turkish borders as well as the subsequent risk to non-target aquatic organisms. Aquatic risk assessment was based on the Risk Quotient (RQ=MEC/PNEC) regarding three trophic levels, algae, aquatic invertebrates and fish. Alachlor, atrazine, carbaryl, carbofuran, cypermethrin, DEA, DIA, diazinon, dimethoate, endosulfan, metolachlor, monilate, prometryn and trifluralin were the compounds detected at the highest concentrations on a regular basis. Extreme concentrations were observed just after high rainfall events during the month of pesticide application. Aquatic risk assessment revealed non-acceptable risk for 10 compounds when median concentrations were used as ΜEC values. However, should extreme concentrations be taken into account, 15 compounds were considered as likely to pose a threat to aquatic organisms. Conformity to EC environmental quality standards is also discussed.     

Sublethal genotoxicity and cell alterations by organophosphorus pesticides in MCF-7 cells: implications for environmentally relevant concentrations

Organophosphorus pesticide (OPP) toxicity is believed to be mediated through inhibition of acetylcholinesterase (AChE). Given their widespread distribution in aquatic systems and their ability to undergo chemical transformation, their environmental impacts at sublethal concentrations in nontarget organisms have become an important question. We conducted a number of mammalian-cell genotoxic and gene expression assays and examined cellular biochemical changes that followed low-dose exposure of MCF-7 cells to fenitrothion, diazinon, and the aqueous degradate of diazinon, 2-isopropyl-6-methyl-4-pyrimidinol (IMP). After exposure to the OPPs at low concentrations (10(-12) M to 10(-8) M), greater than twofold elevations in micronucleus formation were noted in MCF-7 cell cultures that went on to exhibit greater than 75% clonogenic survival; these levels of chromosomal damage were comparable to those induced by 10(-6) M benzo[a]pyrene, a known genotoxic agent. At this low concentration range, a fenitrothion-induced twofold elevation in B-cell leukemia/lymphoma-2 (BCL-2) and cytochrome P450 isoenzyme (CYP1A1) gene expressions was observed. Principal component analysis-linear discriminant analysis (PCA-LDA) of derived infrared (IR) spectra of vehicle control (nonexposed) and OPP-exposed cells highlighted that both fenitrothion and diazinon induced marked biochemical alterations in the lipid, protein, and DNA/RNA absorbance regions. Our findings demonstrate that the two OPP parent chemicals and IMP degradate can mediate a number of toxic effects or cellular alterations at very low concentrations. These are independent of just selective inhibition of AChE, with potential consequences for nontarget organisms exposed at environmentally relevant concentrations. Further assays on relevant aquatic organism cell lines are now recommended to understand the mechanistic low-dose toxicity of these chemicals present in aquatic systems.     

Sensitivity of Lamprey Ammocoetes to Six Chemicals

As part of the ecological risk assessment for Portland Harbor Superfund site, a study was conducted to address the question of whether the use of surrogate species in the risk assessment would be protective of lamprey ammocoetes. The study evaluated the acute toxicity of six chemicals: pentachlorophenol, copper, diazinon, aniline, naphthalene, and lindane; these chemicals represent the toxic modes of action of oxidative phosphorylation uncoupler, gill dysfunction, acetylcholinesterase inhibitor, polar narcosis, narcosis, and central nervous system interference, respectively. Field-collected lamprey ammocoetes were exposed to each of the six chemicals in a definitive 96-h flow-through acute water-only toxicity test. LC₅₀s were calculated for pentachlorophenol at 31 μg/l, copper at 46 μg/l, diazinon at 8.9 mg/l, and aniline at 430 mg/l. Species sensitivity distributions based on LC₅₀s for aquatic organisms indicated that lamprey ammocoetes were relatively sensitive to pentachlorophenol (15th percentile). The sensitivity of lamprey ammocoetes to copper approximated the average of aquatic species tested (46th percentile). Lamprey ammocoetes were relatively insensitive to diazinon and aniline (72th and 90th percentile, respectively). The 96-h LC₅₀ for naphthalene was estimated at 10 mg/l, based on 50% mortality in the highest concentration. Based on a comparison with LC₅₀s for four other fish species, ranging from 2.0 to 6.6 mg/l, lamprey ammocoetes were relatively insensitive to naphthalene. A 96-h LC₅₀ could not be derived for lindane, with 12.5% mortality in the highest test concentration of 2.68 mg/l. LC₅₀s for numerous other fish species ranged from 0.001 to 0.24 mg/l, indicating that lamprey ammocoetes were relatively insensitive to lindane. The study concluded that the use of surrogate species in the ecological risk assessment for Portland Harbor would be protective of lamprey ammocoetes.     

Increased toxicity to Ceriodaphnia dubia in mixtures of atrazine and diazinon at environmentally realistic concentrations

The acute toxicity of diazinon in combination with atrazine concentrations of 5, 10, 20, and 40 microg/L was evaluated using Ceriodaphnia dubia. Atrazine concentrations as low as 5 microg/L in combination with diazinon significantly increased toxicity to C. dubia compared to diazinon alone. Atrazine and diazinon residues within water samples collected from 65 subbasins throughout Denton, Texas, USA were used to assess the environmental relevance of pesticide concentrations. A geographical information system was used to examine the relationship between subbasin land uses and pesticide concentrations. Significant correlations were observed between in situ atrazine and diazinon concentrations and some subbasin land uses. Atrazine was significantly (P < 0.05) correlated to diazinon during some months. Of the 276 samples collected, 39% exceeded our experimentally derived diazinon LC(50) value, and 39% exceeded our minimum atrazine concentration of 5.0 microg/L. Results indicate the potential for increased toxicity from mixtures of compounds at environmentally realistic concentrations.     

Survival and precopulatory guarding behavior of Hyalella azteca (Amphipoda) exposed to nitrate in the presence of atrazine

Nitrate is one of the most commonly detected contaminants found in aquatic systems with other pesticides such as atrazine. The current study examined potential combined effects of nitrate and atrazine on adults of the freshwater amphipod Hyalella azteca, using survival and precopulatory guarding behavior as toxic endpoints. Although significant differences in acute toxicity with nitrate alone and in binary combination with atrazine (200 μg/L) in water-only tests were not consistently observed for each time point, potential biologically relevant trends in the data were observed. Posttest growth and behavioral observations (10-day period) conducted after 96-hour exposure suggested that atrazine and nitrate at these concentrations did not result in delayed effects on H. azteca. However, when test conditions were modified from standard toxicity tests by feeding amphipods, nitrate was found to be more toxic, with a reduction in median lethal concentration (LC50) values of approximately 80%. We also demonstrated that nitrate exhibits a dose-response effect on precopulatory guarding behavior of H. azteca, suggesting that reproductive effects may occur at environmentally relevant concentrations.     

Acute toxicity of atrazine, endosulfan sulphate and chlorpyrifos to Vibrio fischeri, Thamnocephalus platyurus and Daphnia magna, relative to their concentrations in surface waters from the Alentejo region of Portugal

Ecotoxicological effects of the herbicide atrazine and the insecticides endosulfan sulphate and chlorpyrifos were evaluated using a test battery comprising aquatic organisms from different trophic levels. According to the categories established in the EU legislation, atrazine can be considered non-harmful for the species tested, while the insecticides can be considered very toxic for the crustaceans. The results of acute toxicity tests showed that the sensitivity of organisms were as follows: Thamnocephalus platyurus > Daphnia magna > Vibrio fischeri. Chlorpyrifos may act as a toxic compound in the aquatic environment of Guadiana River, as it may be detected in water at levels that promote toxic effects.     

Studies on the environmental fate of carbaryl as a function of pH

The effects of pH upon the environmental fate of carbaryl were determined in acute toxicity tests, microcosm analyses, and abiotic water stability studies. The toxicity of carbaryl varied significantly with pH in 24-hr toxicity tests withChironomus riparius. Toxicity was greatest at pH 4; the chemical was equitoxic at pH's 6 and 8. Greater amounts of carbaryl were detected in water at pH 4, both in the microcosm and abiotic studies, than at pH's 6 and 8. In spite of the marked persistence of carbaryl in water at pH 4, only minor differences were seen in the amount of parent compound in the microcosm organisms as a function of pH because of the facility with which carbaryl was degraded. The hazard associated with aquatic contamination by carbaryl is affected by pH, but is most significant when contamination of the water exceeds the capacity of aquatic biota to metabolize the chemical. These data underscore the need to consider physical factors which affect environmental fate, particularly in environments in which biotic degradation is minimal.     

Intra- and interlaboratory variability in acute toxicity tests with glochidia and juveniles of freshwater mussels (Unionidae)

The present study evaluated the performance and variability in acute toxicity tests with glochidia and newly transformed juvenile mussels using the standard methods outlined in American Society for Testing and Materials (ASTM). Multiple 48-h toxicity tests with glochidia and 96-h tests with juvenile mussels were conducted within a single laboratory and among five laboratories. All tests met the test acceptability requirements (e.g., >or=90% control survival). Intralaboratory tests were conducted over two consecutive mussel-spawning seasons with mucket (Actinonaias ligamentina) or fatmucket (Lampsilis siliquoidea) using copper, ammonia, or chlorine as a toxicant. For the glochidia of both species, the variability of intralaboratory median effective concentrations (EC50s) for the three toxicants, expressed as the coefficient of variation (CV), ranged from 14 to 27% in 24-h exposures and from 13 to 36% in 48-h exposures. The intralaboratory CV of copper EC50s for juvenile fatmucket was 24% in 48-h exposures and 13% in 96-h exposures. Interlaboratory tests were conducted with fatmucket glochidia and juveniles by five laboratories using copper as a toxicant. The interlaboratory CV of copper EC50s for glochidia was 13% in 24-h exposures and 24% in 48-h exposures, and the interlaboratory CV for juveniles was 22% in 48-h exposures and 42% in 96-h exposures. The high completion success and the overall low variability in test results indicate that the test methods have acceptable precision and can be performed routinely.     

Effects of metal and organophosphate mixtures on Ceriodaphnia dubia survival and reproduction

The toxicity of mixtures of copper, zinc, and diazinon were determined for Ceriodaphnia dubia using 7-d survival and reproduction tests. Fifteen treatments, including combinations of the chemicals at 0, 25, 50, 75, and 100% of their individual median lethal concentrations, adding up to one toxic unit (TU) were tested. The TU was then used to classify each mixture response as additive, greater than additive, or less than additive. For survival, additive responses occurred in the 75% zinc plus 25% diazinon and the 50% copper plus 25% zinc plus 25% diazinon treatments. For reproduction, additive responses occurred in the 75% copper plus 25% zinc, 75% copper plus 25% diazinon, and 75% zinc plus 25% diazinon treatments. Copper and zinc played a greater role in toxicity than diazinon did. Less-than-additive interactions were found in all remaining mixtures, perhaps because of differences in mode of action between diazinon and metals. Consideration of dose-response curves can help to explain inconsistencies regarding toxic response in treatments with different ratios of the same chemicals. As TU percentages changed, mixture components were taken from different locations on differently shaped dose-response curves. Because most responses were less than additive, however, water-quality criteria based on individual concentrations probably are protective for most metal-organophosphate mixtures.