A relationship between gill silver accumulation and acute silver toxicity in the freshwater rainbow trout: support for the acute silver biotic ligand model

Rainbow trout were exposed to a range of silver concentrations (as AgNO3) in flowing synthetic soft water (0.05 mM Na+, 0.05 mM Cl-, 0.05 mM Ca2+, 0.02 mM Mg2+, 0.02 mM K+, pH 7.0, approximately 0.7 mg C/L dissolved organic carbon, 10 mg CaCO3/L, 10 +/- 2 degrees C) to investigate a possible relationship between short-term gill silver accumulation (3 h or 24 h) and acute silver toxicity (96-h mortality). We also investigated potential relationships between gill silver accumulation and inhibition of Na+ uptake plus inhibition of gill Na+K(+)-adenosine triphosphatase (ATPase) activity. The 96-h median lethal concentration (LC50) values were 13.3 microg total Ag L(-1) and 3.3 microg dissolved Ag L(-1). A relationship was demonstrated between 3-h and 24-h gill silver accumulation and 96-h mortality. A relationship also was demonstrated between gill silver accumulation and inhibition of Na+ uptake at 24 h of exposure. No relationship between gill silver accumulation and inhibition of gill Na+K(+)-ATPase activity was found. The 96-h median lethal gill accumulation (LA50) values of 129 (at 3 h) and 191 ng g(-1) (at 24 h) and a conditional equilibrium binding constant of 8.0 for Ag+ binding to the gills were calculated. These observations support use of the silver biotic ligand model (BLM) as a regulatory tool to predict acute silver toxicity.     

Influence of Dissolved Organic Matter on Acute Toxicity of Zinc to Larval Fathead Minnows (Pimephales promelas)

We conducted laboratory toxicity tests in support of the development of a biotic ligand model (BLM) to predict acute toxicity of zinc (Zn) to fathead minnows (Pimephales promelas). To test the effect of dissolved organic matter (DOM) on Zn toxicity, we exposed larval fathead minnows to Zn in water containing elevated concentrations of dissolved organic carbon (DOC) in 96-h static-renewal toxicity tests. We tested DOM isolated from four surface waters: Cypress Swamp, Delaware; Edisto River, South Carolina; Suwannee River, Georgia; and Wilmington, Delaware, wastewater treatment effluent. The DOM isolates from the Edisto River and Wilmington wastewater treatment effluent contained elevated concentrations of NaCl (20–110× control NaCl) due to the use of a Na⁺-exchange resin to remove Ca²⁺ and Mg²⁺ during the DOM isolation process. Therefore, we also performed Zn toxicity tests in which we added up to 20 mM NaCl to exposure solutions containing Cypress Swamp and Suwannee River DOM. A threshold concentration of 11 mg DOC/L was needed to decrease Zn toxicity, after which the 96 h Zn LC50 was positively correlated with DOC concentration. Elevated NaCl concentrations did not alter Zn toxicity in the presence of DOM. In conjunction with data from other studies with fish and invertebrates, results of this study were used to calibrate Version 2.1.1 of the Zn BLM. BLM-predicted LC50s for our exposure waters containing elevated DOM concentrations were within the range of acceptable deviation relative to the observed LC50s (i.e., 0.5–2× observed LC50s); however, BLM-predicted LC50s for our exposure waters containing < 1 mg DOC/L were 2–3× lower than the observed LC50s (i.e., the BLM over-predicted the toxicity). Therefore, the current composite-species BLM for Zn could be improved for fathead minnows if that species were modeled separately from the other species used to calibrate Version 2.1.1.     

Influence of natural organic matter source on copper toxicity to larval fathead minnows (Pimephales promelas): implications for the biotic ligand model

The influence of dissolved natural organic matter (NOM) source on copper toxicity was investigated with larval fathead minnows (Pimephales promelas) in reconstituted moderately hard water. Ninety-six-hour static renewal toxicity tests were conducted to investigate an assumption of the biotic ligand model (BLM) that NOM source does not need to be considered to adequately predict copper toxicity. The nine different NOM isolates used in these toxicity tests were chemically well-characterized substances that were obtained by reverse osmosis as part of an NOM typing project based in southern Norway. Three median lethal concentration (LC50) values were estimated for toxicity tests conducted with each NOM, at nominal dissolved organic carbon (DOC) concentrations of 2, 5, and 10 mg/L. Tests also were conducted in dilution waters in which no NOM was added. Regression analyses were conducted to compare NOM-specific (specific NOM source) LC50s versus DOC concentration relationships to each other, as well as to the overall LC50 versus DOC concentration relationship. Statistical differences were found regarding the effects of NOM source on copper toxicity. Similar analyses were conducted with humic acid (HA) concentrations and spectral absorbance, and differences in the effect of NOM source on copper toxicity were similarly concluded. These results do not support the assumption that copper toxicity can be adequately predicted by utilizing DOC concentration, regardless of NOM source. Evaluation of relationships between LC50 values and other NOM characteristics revealed that despite significant differences due to NOM source on copper toxicity, DOC and HA concentrations were the most effective parameters in explaining variability in LC50 values. When BLM-predicted LC50 values were compared to observed LC50 values, predicted values showed reasonable agreement with observed values, but some deviations occurred due to NOM source and DOC concentration.     

Influence of water quality and age on nickel toxicity to fathead minnows (Pimephales promelas)

This research characterized the effects of water quality and organism age on the toxicity of nickel (Ni) to fathead minnows (Pimephales promelas) to facilitate the accurate development of site-specific water-quality criteria. Nickel sulfate hexahydrate (NiSO4 x 6H2O) was used as the Ni source for performing acute toxicity tests (median lethal concentration after 96-h exposure [96-h LC50]) with < 1-d-old and 28-d-old P. promelas under varying regimes of hardness, pH, alkalinity, and natural organic matter (NOM). The toxicity of Ni was inversely related to water hardness between hardness values of 20 and 150 mg/L (as CaCO3). Below 30 mg/L alkalinity, Ni toxicity was related to alkalinity. The effect of pH was confounded by hardness and the presence of NOM. In the absence of NOM, the toxicity of Ni increased as pH increased at high hardness and alkalinity. In general, 28-d-old fish were less sensitive than < 1-d-old fish to Ni. This lower sensitivity ranged from 12-fold at low hardness and alkalinity (20 and 4 mg/L, respectively) to 5-fold at high hardness and alkalinity (100 and 400 mg/L, respectively). The presence of NOM (10 mg/L as dissolved organic carbon [DOC]) reduced Ni toxicity by up to 50%, but this effect appeared to be saturated above DOC at 5 mg/L. Incubating Ni with the NOM solution from 1 to 17 days had no effect on Ni toxicity. When using multivariate analysis, the 96-h LC50 for Ni was a function of fish age, alkalinity, hardness, and NOM (96-h LC50 = -0.642 + 0.270(fish age) + 0.005(alkalinity) + 0.018(hardness) + 0.138(DOC)). When using this model, we found a strong relationship between measured and predicted 96-h LC50 values (r2 = 0.94) throughout the treatment water qualities. The biotic ligand model (BLM) did not accurately predict Ni toxicity at high or low levels of alkalinity. Results of our research suggest that the BLM could be improved by considering NiCO3 to be bioavailable.     

The Effects of Salinity, pH, and Dissolved Organic Matter on Acute Copper Toxicity to the Rotifer, Brachionus plicatilis (“L” Strain)

This paper presents data from original research for use in the development of a marine biotic ligand model and, ultimately, copper criteria for the protection of estuarine and marine organisms and their uses. Ten 48-h static acute (unfed) copper toxicity tests using the euryhaline rotifer Brachionus plicatilis (“L” strain) were performed to assess the effects of salinity, pH, and dissolved organic matter (measured as dissolved organic carbon; DOC) on median lethal dissolved copper concentrations (LC50). Reconstituted and natural saltwater samples were tested at seven salinities (6, 11, 13, 15, 20, 24, and 29 g/L), over a pH range of 6.8–8.6 and a range of dissolved organic carbon of <0.5–4.1 mg C/L. Water chemistry analyses (alkalinity, calcium, chloride, DOC, hardness, magnesium, potassium, sodium, salinity, and temperature) are presented for input parameters to the biotic ligand model. In stepwise multiple regression analysis of experimental results where salinity, pH, and DOC concentrations varied, copper toxicity was significantly related only to the dissolved organic matter content (pH and salinity not statistically retained; α = 0.05). The relationship of the 48-h dissolved copper LC50 values and dissolved organic carbon concentrations was LC50 (μg Cu/L) = 27.1 × DOC (mg C/L)^1.25; r ² = 0.94.     

Chronic toxicity of silver to the sea urchin (Arbacia punctulata)

The chronic toxicity of silver to the sea urchin (Arbacia punctulata) was determined in 30 per thousand salinity seawater during a three-part study: A fertilization test (1-h sperm exposure), a 48-h embryo test, and a 30-d adult test. Combined data from the three tests resulted in a lowest-observed-effect concentration of 19 microg/L, a no-observed-effect concentration of 8.6 microg/L, and a maximum acceptable toxicant concentration of 13 microg/L, based on measured concentrations of dissolved silver. The 96-h median effective concentration was 40 microg/L, and the acute to chronic toxicity ratio was 3.1. During the tests, measured concentrations of free ionic silver (Ag+) were only 0.0027 to 0.0046% of dissolved silver concentrations, as predicted by ion-speciation theory. Some measured Ag+ concentrations were lower than predicted, indicating the presence of other ligands in the seawater test media. These strong sulfide ligands were exuded by the exposed sea urchins into the seawater (where Ag-sulfide complexes formed) in amounts that increased in direct proportion to the silver concentration during the toxicity test. This suggests a toxicity-defense mechanism that functioned by modifying the chemistry of the surrounding external medium.     

Effects of dissolved organic carbon concentration and source, pH, and water hardness on chronic toxicity of copper to Daphnia magna

The effects of pH (5.3-8.7), water hardness (CaCO3 at 25-500 mg/L), dissolved organic carbon (DOC) concentration (1.6-18.4 mg/L), and DOC source on the chronic toxicity of copper to Daphnia magna were investigated by using a multifactorial, central composite test design. Natural dissolved organic matter (DOM) was collected at three sites in Belgium and The Netherlands by using reverse osmosis. For a total number of 35 toxicity tests performed, 21-d no-observed-effect concentrations (NOECs) of copper based on reproduction ranged from 29.4 to 228 microg/L and 21-d concentrations of copper causing 50% reduction of reproduction (EC50s) ranged from 41.5 to 316 microg/L. Statistical analysis revealed that DOC concentration and pH had a significant effect on copper toxicity but hardness (at the levels tested) did not. In general, an increase in pH or DOC resulted in a linear increase of 21-d NOEC and EC50 values. All DOMs (originating from three different sources) reduced copper toxicity to the same extent. Multiple linear regression analysis on the results of all 35 toxicity tests revealed that DOC concentration is the most important factor for chronic toxicity of copper to D. magna, explaining about 60% of the observed variability, whereas pH only explained about 15% of the observed variability. Regression models were developed (with DOC and pH as parameters) that were capable of predicting NOECs and EC50s within a factor of 1.9 from observed NOEC and EC50 values obtained with eight natural surface waters spiked with copper. Until future research further elucidates the mechanisms underpinning the observed bioavailability relations, these empirical regression models can become a first simple tool for regulatory applications.     

Acute silver toxicity in the euryhaline copepod Acartia tonsa: influence of salinity and food

The euryhaline copepod Acartia tonsa was exposed to silver (AgNO(3)) in either the absence or the presence of food (diatom Thalassiosira weissflogii; 2 x 10(4) cells/ml). Standard static-renewal toxicity tests that included a fixed photoperiod of 16: 8 h light:dark and temperature (20 degrees C) were run in three different salinities (5, 15, and 30 ppt) together with measurements of pH, ions (Na(+), Cl(-), K(+), SO(4)(2-), Mg(2+), and Ca(2+)), alkalinity, dissolved organic carbon, and total and dissolved (0.45 microm) silver concentrations in the experimental media. In the absence of food, the 48-h EC50 (concentration causing effect to 50% of the individuals tested) values based on total and dissolved silver concentrations were 11.6, 87.2, and 163.2 microg Ag/L and 7.1, 79.2, and 154.6 microg Ag/L at salinities 5, 15, and 30 ppt, respectively. In the presence of food, they were 62.1, 98.5, and 238.4 microg Ag/L and 48.4, 52.3, and 190.9 microg Ag/L, respectively. In all experimental conditions, most of the toxic silver fraction was in the dissolved phase, regardless of salinity or the presence of food in the water. In either the absence or the presence of food, acute silver toxicity was salinity dependent, decreasing as salinity increased. Data indicate that changes in water chemistry can account for the differences in acute silver toxicity in the absence of food, but not in the presence of food, suggesting that A. tonsa requires extra energy to cope with the stressful conditions imposed by acute silver exposure and ionoregulatory requirements in low salinities. These findings indicate the need for incorporation of both salinity and food (organic carbon) in a future biotic ligand model (BLM) version for estuarine and marine conditions, which could be validated and calibrated using the euryhaline copepod A. tonsa.     

Acute and chronic toxicity of lead in water and diet to the amphipod Hyalella azteca

We evaluated the influence of waterborne and dietary lead (Pb) exposure on the acute and chronic toxicity of Pb to the amphipod Hyalella azteca. Test solutions were generated by a modified diluter with an extended (24-h) equilibration period. Acute (96-h) toxicity of Pb varied with water hardness in the range of 71 to 275 mg/L as CaCO3, despite similar dissolved Pb concentrations. Acute toxicity was greatest in soft test water, with less than 50% survival at the lowest dissolved Pb concentration (151 microg/L). Survival also was significantly reduced in medium-hardness water but not in hard test water. In chronic (42-d) studies, amphipods were exposed to waterborne Pb and fed either a control diet or a diet equilibrated with waterborne Pb levels. For animals fed the control diet, the median lethal concentration (LC50) for Pb was 24 degrees g/L (as dissolved Pb), and significant reductions in survival occurred at 16 microg/L. Exposure to Pb-treated diets significantly increased toxicity across a wide range of dissolved Pb concentrations, with a LC50 of 16 microg/L and significant reductions in growth and reproduction at 3.5 microg/L. Significant effects on growth and reproduction occurred at dissolved Pb concentrations close to the current U.S. chronic water-quality criterion. Our results suggest that both aqueous- and dietary-exposure pathways contribute significantly to chronic Pb exposure and toxic effects in aquatic biota.     

Dissolved fraction of standard laboratory cladoceran food alters toxicity of waterborne silver to Ceriodaphnia dubia

The biotic ligand model (BLM) for the acute toxicity of cationic metals to aquatic organisms incorporates the toxicity-modifying effects of dissolved organic matter (DOM), but the default parameterization (i.e., assuming 10% of DOM is humic acid) does not differentiate DOM from different sources. We exposed a cladoceran (Ceriodaphnia dubia) to Ag in the presence of DOM from filtered YCT (standard yeast-Cerophyll(R)-trout chow food recommended by the U.S. Environmental Protection Agency [EPA] for cladocerans), from the Suwannee River (GA, USA; relatively little anthropogenic input), and from the Desjardins Canal in Hamilton (ON, Canada; receives treated municipal wastewater effluent). In all three treatments, the dissolved organic carbon (DOC) concentration was 2 mg/L (the concentration following addition of YCT slurry at the U.S. EPA-recommended volume ratio). The average 48-h median effects concentration (EC50) ratios for dissolved Ag in the presence and absence of DOM [i.e., (EC50 with DOM)/(EC50 without DOM)] were as follows: Suwannee River, 1.6; Desjardins Canal, 2.2; and YCT filtrate, 26.8. Therefore, YCT filtrate provided much more protection against Ag toxicity than that provided by DOM from the surface waters. The major spectral characteristic that differentiated YCT filtrate from the other two types of DOM was a strong tryptophan peak in the excitation- emission matrix for YCT. These results have important implications for interpreting Ag toxicity tests in which organisms are fed YCT, and they suggest BLM-calculated toxicity predictions might be improved by incorporating specific chemical constituents or surrogate indices of DOM. Another component of the protective effect against Ag toxicity, however, might be that the dissolved fraction of YCT served as an energy and/or nutrient source for C. dubia.     

Influence of flow-through and renewal exposures on the toxicity of copper to rainbow trout

We examined changes in water chemistry and copper (Cu) toxicity in three paired renewal and flow-through acute bioassays with rainbow trout (Oncorhynchus mykiss). Test exposure methodology influenced both exposure water chemistry and measured Cu toxicity. Ammonia and organic carbon concentrations were higher and the fraction of dissolved Cu lower in renewal tests than in paired flow-through tests. Cu toxicity was also lower in renewal tests; 96 h dissolved Cu LC(50) values were 7-60% higher than LC(50)s from matching flow-through tests. LC(50) values in both types of tests were related to dissolved organic carbon (DOC) concentrations in exposure tanks. Increases in organic carbon concentrations in renewal tests were associated with reduced Cu toxicity, likely as a result of the lower bioavailability of Cu-organic carbon complexes. The biotic ligand model of acute Cu toxicity tended to underpredict toxicity in the presence of DOC. Model fits between predicted and observed toxicity were improved by assuming that only 50% of the measured DOC was reactive, and that this reactive fraction was present as fulvic acid.     

The Influence of Salinity and Dissolved Organic Carbon on the Toxicity of Copper to the Estuarine Copepod, <Emphasis Type="Italic">Eurytemora affinis</Emphasis>

The objectives of this study were to determine the influence of salinity (2.5, 5, 15, and 25 ppt) at dissolved organic carbon (DOC) concentrations of 1.3–3.3 mg/L and DOC concentrations of 2, 4, 6, and 8 mg/L at a fixed salinity of 10 ppt on the acute toxicity (96-h LC50s) of copper to the sensitive estuarine copepod, Eurytemora affinis. For both salinity and DOC experiments, various other chemical constituents such as DOC, Ca²⁺, Cl^-, Mg²⁺, Na⁺, K⁺, SO₄^2-, hardness, alkalinity, salinity, pH, temperature, and dissolved oxygen were measured at selected copper concentrations at test initiation and test termination. Dissolved copper, copper speciation, and organic copper complexation were measured at various test conditions during the salinity and DOC experiments. Ninety-six-hour dissolved copper LC50 values for the four salinities ranged from 58 μg/L (25 ppt) to 104 μg/L (5 ppt) with intermediate values of 71 μg/L (2.5 ppt) and 68 μg/L (15 ppt). The 58, 68, and 71 μg/L LC50 values were not significantly different. Copper LC50 values at 5 ppt were higher than at both 15 and 25 ppt. The isosmotic salinity of E. affinis is approximately 5–10 ppt, which was a likely factor for why the LC50 value increased for copper at 5 ppt. The dissolved copper 96-h LC50s for E. affinis increased from 76 to 166 μg/L as DOC increased from 2 to 8 mg/L. This result is not surprising and is consistent with reported values for other saltwater species.     

Effects of ligand-bound silver on Ceriodaphnia dubia

In aqueous media, ionic silver concentrations are low and transport occurs in the colloidal phase. In the aquatic environment, silver forms 1:1 complexes with thiol-containing compounds such as cysteine and glutathione. In order to quantitatively characterize the risk associated with silver in aquatic ecosystems, the bioavailabilities and toxicities of silver cysteinate and silver glutathionate were characterized. Static renewal bioassays were conducted with Ceriodaphnia dubia to estimate chronic toxicity, using mortality and reproduction as endpoints. Silver nitrate was the most lethal compound, with a median lethal concentration (8-d LC50) of 0.32 microg Ag/L (95% confidence interval [CI] = 0.19-0.54). The 48-h LC50 for AgNO3 was 0.5 microg/L and did not change significantly through 8 d. The presence of food in the bioassay did not change the 48-h LC50 for AgNO3. Silver glutathionate (AgGSH) and silver cysteinate (AgCys) induced less mortality during the 8-d bioassay. Silver cysteinate appeared to have the greatest effect on fecundity, with a no-observable-effect concentration (NOEC) less than 0.001 microg/L. Silver nitrate and AgGSH had lowest-observable-effect concentration (LOEC) values (nominal concentrations) of 0.01 and 0.6 microg/L, respectively. Results indicate that the ligand-bound silver in these laboratory studies is bioavailable and impairs reproduction of C. dubia at low aqueous concentrations.     

Cross-phylum comparison of a chronic biotic ligand model to predict chronic toxicity of copper to a freshwater rotifer, Brachionus calyciflorus (Pallas)

Short chronic 48-h toxicity tests with the freshwater rotifer Brachionus calyciflorus (Pallas) were conducted to assess the modifying effects of pH and natural dissolved organic carbon (DOC) concentration on reproductive toxicity of Cu. Toxicity tests were carried out in four test waters according to a 2 x 2 design, in which pH (6 and 7.8) and DOC (5 and 15 mg C/L) were the test variables. Concentrations of dissolved Cu with no observed effect at 48 h (NOEC) varied 12-fold between 8.2 and 103 microg/L. Higher DOC and higher pH resulted in a reduction of toxicity, which is in line with the concepts of the biotic ligand model (BLM). A chronic Cu-BLM, originally developed for the cladoceran Daphnia magna, was calibrated to the rotifer dataset and was found to be able to predict all rotifer NOECs with an error factor of less than 1.6. This finding may be of great interest for risk assessment and the establishment of water quality criteria, as it suggests that chronic Cu-BLMs are comparable across phyla (i.e., arthropoda to rotifera).     

Toxicity of selenomethionine- and seleno-contaminated sediment to the amphipod Corophium sp

The acute toxicity of four chemical species of selenium to juvenile amphipods (Corophium sp.) was assessed in water-only tests. The seleno-amino acid compounds seleno-L-methionine and seleno-DL-cystine were found to be more toxic (96-h LC(50) values of 1.5 and 12.7 microg Se/L) than the inorganic selenite and selenate (96-h NOEC values of 58 and 116 microg Se/L). New marine sediment testing procedures were developed using juvenile and adult Corophium sp. Both life stages were highly sensitive to seleno-L-methionine-spiked sediment. The juveniles were approximately five times more sensitive, with a 10-day LC(50) of 1.6 microg Se/g (dry weight) compared to 7.6 microg Se/g (dry weight) for the adults. Sediment collected from three sites in Lake Macquarie, a marine barrier lagoon with elevated concentrations of total selenium, had no effect on the survival of adult Corophium over 10 days. The toxicity of seleno-L-methionine to other amphipod species occurring in Lake Macquarie was assessed in water-only tests, with Paracalliope australis being highly sensitive (96-h LC(50) 2.58 microg Se/L).     

Sensitivity of juvenile freshwater mussels (Lampsilis fasciola, Villosa iris) to total and un-ionized ammonia

This study evaluated the sensitivity of juveniles of two freshwater unionid mussel species (Villosa iris [Lea] and Lampsilis fasciola [Rafinesquel) to un-ionized and total ammonia. Five concentrations of ammonium chloride were tested using 96-h static-renewal toxicity tests at 12 and 20 degrees C. Based on their respective mean 96-h lethal concentration to 50% (LC50s), V. iris (0.11 mg/ L NH3-N) was more sensitive than L. fasciola (0.26 mg/L NH3-N). At 96 h, significant differences in sensitivity to un-ionized ammonia between the two temperatures were not observed for either species. Comparison of LC50s reported for other aquatic organisms to the 96-h LC50s calculated for juvenile L. fasciola and V. iris shows these two mussel species to be among the most sensitive to un-ionized ammonia. Based on reported levels of un-ionized ammonia in the aquatic environment from anthropogenic sources, un-ionized ammonia may be an important limiting toxicological factor to freshwater mussel populations.     

Multigenerational acclimation of Daphnia magna to mercury: relationships between biokinetics and toxicity

We examined the effects of multigenerational exposure of mercury (Hg) on Hg toxicity and biokinetics in a population of Daphnia magna. After chronic Hg exposure at 3.8 microg Hg/L, the first generation (F0) adults had an elevated 24-h median lethal concentration (LC50) of Hg (76 microg/L) when compared to the control adults (56 microg/L). The dissolved influx rate of Hg was depressed significantly in the Hg-treated adults, which was accompanied by a reduced ingestion rate and enhanced induction of metallothionein-like proteins (MTLP). The second-generation (F1) juveniles originating from the control and exposed lines had no major differences in these parameters (except the dietary assimilation efficiency). Recovery from Hg stress enhanced the vulnerability of F1 adults to Hg toxicity, with a reduced 48-h LC50 (44 microg/L) and a decreased concentration of MTLP (80% of control). Nevertheless, Hg-treated F1 adults had similar tolerance (in terms of LC50s) as the control line, indicating that D. magna acclimated to Hg stress after the first generation of exposure. No major difference occurred in the Hg biokinetics and toxicity among different groups of F2 daphnids. However, the F2 neonates produced by the Hg-treated F1 adults had much higher 48-h LC50 (149 microg/L) and MTLP concentration (148% of control) when there was continuous Hg exposure after birth. We concluded that acclimation to Hg stress occurred quickly in D. magna, though animals recovering from Hg stress were more vulnerable to Hg toxicity. Both ingestion rate and MTLP may not be good biomarkers of Hg stress in the field, because acclimation can be achieved through multigenerational exposure to elevated Hg concentrations.     

Effects of using synthetic sea salts when measuring and modeling copper toxicity in saltwater toxicity tests

Synthetic sea salts are often used to adjust the salinity of effluent, ambient, and laboratory water samples to perform toxicity tests with marine and estuarine species. The U.S. Environmental Protection Agency (U.S. EPA) provides guidance on salinity adjustment in its saltwater test guidelines. The U.S. EPA suggests using commercial sea salt brands, such as Forty Fathoms (now named Crystal Sea Marinemix, Bioassay Grade), HW Marinemix, or equivalent salts to adjust sample salinity. Toxicity testing laboratories in Canada and the United States were surveyed to determine synthetic sea salt brand preference. The laboratories (n = 27) reported using four brands: Crystal Sea Marinemix (56%), HW Marinemix (22%), Instant Ocean (11%), and Tropic Marin (11%). Saline solutions (30 g/L) of seven synthetic sea salts were analyzed for dissolved copper and dissolved organic carbon (DOC) content. Brands included those listed above plus modified general-purpose salt (modified GP2), Kent Marine, and Red Sea Salt. The synthetic sea salts added from < 0.1 to 1.2 microg Cu/L to the solution. Solutions of Crystal Sea Marinemix had significantly elevated concentrations of DOC (range = 5.4-6.4 mg C/L, analysis of variance, Tukey, alpha = 0.05, p < 0.001) while other brands generally contained < 1.0 mg C/L. The elevated DOC in Crystal Sea Marinemix was expected to reduce copper toxicity. However, the measured dissolved copper effective concentration 50% (EC50) for Crystal Sea Marinemix was 9.7 microg Cu/L, similar to other tested sea salts. Analysis indicates that the organic matter in Crystal Sea Marinemix differs considerably from that of natural organic matter. On the basis of consistently adding little DOC and little dissolved copper, GP2 and Kent Marine are the best salts to use.     

Sensitivity of juvenile freshwater crayfish Cherax destructor (Decapoda: Parastacidae) to trace metals

Juvenile Cherax destructor was investigated as a potential test species for toxicity testing of trace metals in Australian freshwater systems. Adult male and female C. destructor were bred in the laboratory to obtain 4-week old juveniles, which were used in the toxicity tests. Animals were exposed to a range of concentrations of the trace metals ions copper (377-1275 microg/L), cadmium (377-1275 microg/L), nickel (300-1013 mg/L) and iron (36-168 mg/L) in static-renewal 96-h bioassays. The 96-h LC50 value for cadmium was 379, 494 microg/L for copper, 50 mg/L for iron and 327 mg/L for nickel demonstrating a decreasing toxicity of these metals to C. destructor. Comparison of LC50 values for metals for this species with those for other aquatic organisms reveals that C. destructor is less sensitive to trace metals than most other tested species.     

Acutei and chronic toxicity of nickel to marine organisms: implications for water quality criteria

Acute and chronic toxicity tests were conducted to determine the effects of nickel on three U.S. west coast marine species: a fish (the topsmelt, Atherinops affinis), a mollusk (the red abalone, Haliotis rufescens), and a crustacean (the mysid, Mysidopsis intii). The 96-h median lethal concentration (LC50) for topsmelt was 26,560 microg/L, and the chronic value for the most sensitive endpoint in a 40-d exposure was 4,270 microg/L. The median effective concentration (EC50) for 48-h abalone larval development was 145.5 microg/L, and the chronic value for juvenile growth in a 22-d exposure through larval metamorphosis was 26.43 microgAL. The mysid 96-h LC50 was 148.6 microg/L, and the chronic value for the most sensitive endpoint in a 28-d, whole life-cycle exposure was 22.09 microg/L. The abalone and mysid acute values were lower than other values available in the literature. Acute-to-chronic ratios for nickel toxicity to the three species were 6.220, 5.505, and 6.727, respectively, which were similar to the only other available saltwater value of 5.478 (for Americamysis [Mysidopsis] bahia) and significantly lower than the existing values of 35.58 and 29.86 for freshwater organisms. Incorporation of data from the present study into calculations for water quality criteria would lower the criterion maximum concentration and raise the criterion continuous concentration for nickel.