Recent Developments in Species Sensitivity Distribution Modeling.

The species sensitivity distribution (SSD) is a statistical approach that is used to estimate either the concentration of a chemical that is hazardous to no more than x% of all species (the HCx) or the proportion of species potentially affected by a given concentration of a chemical. Despite a significant body of published research and critical reviews over the past 20 yr aimed at improving the methodology, the fundamentals remain unchanged. Although there have been some recent suggestions for improvements to SSD methods in the literature, in general, few of these suggestions have been formally adopted. Furthermore, critics of the approach can rightly point to the fact that differences in technical implementation can lead to marked differences in results, thereby undermining confidence in SSD approaches. Despite the limitations, SSDs remain a practical tool and, until a demonstrably better inferential framework is available, developments and enhancements to conventional SSD practice will and should continue. We therefore believe the time has come for the scientific community to decide how it wants SSD methods to evolve. The present study summarizes the current status of, and elaborates on several recent developments for, SSD methods, specifically, model averaging, multimodality, and software development. We also consider future directions with respect to the use of SSDs, with the ultimate aim of helping to facilitate greater international collaboration and, potentially, greater harmonization of SSD methods. Environ Toxicol Chem 2021;40:293-308. © 2020 SETAC.

Site-specific water quality guidelines: 1. Derivation approaches based on physicochemical, ecotoxicological and ecological data.

Generic water quality guidelines (WQGs) are developed by countries/regions as broad scale tools to assist with the protection of aquatic ecosystems from the impacts of toxicants. However, since generic WQGs cannot adequately account for the many environmental factors that may affect toxicity at a particular site, site-specific WQGs are often needed, especially for high environmental value ecosystems. The Australian and New Zealand Guidelines for Fresh and Marine Water Quality provide comprehensive guidance on methods for refining or deriving WQGs for site-specific purposes. This paper describes three such methods for deriving site-specific WQGs, namely: (1) using local reference water quality data, (2) using biological effects data from laboratory-based toxicity testing, and (3) using biological effects data from field surveys. Two case studies related to the assessment of impacts arising from mining operations in northern Australia are used to illustrate the application of these methods. Finally, the potential of several emerging methods designed to assess thresholds of ecological change from field data for deriving site-specific WQGs is discussed. Ideally, multiple lines of evidence approaches, integrating both laboratory and field data, are recommended for deriving site-specific WQGs.

Reanalysis of uranium toxicity data for selected freshwater organisms and the influence of dissolved organic carbon.

The present study reanalyzed 46 existing uranium (U) chronic toxicity datasets for four freshwater species to generate consistent toxicity measures and explore relationships between U toxicity and key physicochemical variables. Dissolved organic carbon (DOC) was consistently the best predictor of U toxicity based on 10% inhibitory concentration (IC10) and median inhibitory concentration (IC50) values, with water hardness also being a significant co-predictor of IC50 concentrations for one species. The influence of DOC on acute and chronic U toxicity was further characterized using existing data for five species, and was found to vary depending on species, DOC source, and exposure duration (acute vs chronic). The slopes of the relationships between DOC and (normalized) acute and chronic U toxicity were modeled using cumulative probability distributions. From these, slopes were selected for which to correct acute or chronic U toxicity values or hazard estimates based on the aquatic DOC concentration. The fifth percentiles of these cumulative probability distributions for acute and chronic exposure data were 0.064 and 0.090, respectively, corresponding to a 6.4 and 9.0% reduction in U toxicity relative to the toxicity at the base DOC concentration for each 1 mg/L increase in DOC concentration (over the DOC range 0-30 mg/L). Algorithms were developed to enable the adjustment of U toxicity values and U hazard estimates, depending on DOC concentrations. These algorithms will significantly enhance the environmental relevance of water quality/risk assessments for U in fresh surface waters.

Effects of alumina refinery wastewater and signature metal constituents at the upper thermal tolerance of: 1. The tropical diatom Nitzschia closterium.

Ecotoxicological studies, using the tropical marine diatom, Nitzschia closterium (72-h growth rate), were undertaken to assess potential issues relating to the discharge from an alumina refinery in northern Australia. The studies assessed: (i) the species' upper thermal tolerance; (ii) the effects of three signature metals, aluminium (Al), vanadium (V) and gallium (Ga) (at 32°C); and (iii) the effects of wastewater (at 27 and 32°C). The critical thermal maximum and median inhibition temperature for N. closterium were 32.7°C and 33.1°C, respectively. Single metal toxicity tests found that N. closterium was more sensitive to Al compared to Ga and V, with IC(50)s (95% confidence limits) of 190 (140-280), 19,640 (11,600-25,200) and 42,000 (32,770-56,000) µg L(-1), respectively. The undiluted wastewater samples were of low toxicity to N. closterium (IC(50)s>100% wastewater). Environmental chemistry data suggested that the key metals and discharge are a very low risk to this species.

Uranium toxicity and speciation during chronic exposure to the tropical freshwater fish, Mogurnda mogurnda.

The effects of chronic uranium (U) exposure on larval Northern trout gudgeon, Mogurnda mogurnda, were assessed in two experiments using a newly-developed 28d survival and growth toxicity test. Significant effects were observed in both tests, but toxicity was markedly higher in Test 2 than Test 1. The LC50s for Tests 1 and 2 were 2090microgL(-1) and 1070microgL(-1), respectively. Larval growth IC10s for Tests 1 and 2 were 860microgL(-1) and 660microgL(-1) (dry weight), and 1160microgL(-1) and 850microgL(-1) (length), respectively. Uranium speciation modelling showed that a lower pH in Test 2 (mean of 6.0) compared to Test 1 (mean of 6.7) resulted in a greater proportion of free uranyl ion (UO(2)(2+)), the predominant bioavailable form of U. A higher dissolved organic carbon concentration (DOC) in Test 2 (4.2mgL(-1)) compared to Test 1 (2.1mgL(-1)) resulted in a higher proportion of U-DOC in Test 2, but this was insufficient to counter the effect of pH on the proportion of UO(2)(2+). The difference in U toxicity between the two tests could be explained by normalising for UO(2)(2+); the concentrations of UO(2)(2+) at the LC50s for Tests 1 and 2 were calculated to be 13.3 and 13.7microgL(-1), respectively. Finally, the results of this study, and comparisons with other studies suggest that U toxicity to M. mogurnda appears to be as much, if not more, a function of exposure water quality and feeding regime, as exposure duration.

Mining in the Alligator Rivers Region, northern Australia: assessing potential and actual effects on ecosystem and human health.

This paper presents an overview of issues related to surface water contamination arising from uranium mining activities in the Alligator Rivers Region (ARR) of northern Australia, and a program of research and monitoring that must assess the potential and actual effects on ecosystem and human health. The program of assessing effects on aquatic ecosystems involves a four-tiered approach including the derivation of local water quality guideline trigger values, direct toxicity assessment of mine waters prior to their release, creekside or in situ monitoring for early warning of adverse effects during mine water release, and longer-term monitoring of macroinvertebrate and fish communities. Bioaccumulation in aquatic biota is also assessed, and is an issue of importance not only to ecosystem health, but also to the health of local Aboriginal people. The aquatic animals they consume represent potential sources of radiological dose, and as a result, a major component of the program to assess potential effects on human health is the prediction of doses to Aboriginal people living downstream of mining activities. Acknowledging the assumptions and uncertainties, the calculation of concentration factors for local aquatic (and other) food sources allows the prediction of potential radiological exposure to people following hypothetical mine water releases. The approaches described form the basis of best-practice protocols that are relevant at both regional and national levels.

A rapid response toxicity test based on the feeding rate of the tropical cladoceran Moinodaphnia macleayi.

Recent research has reported that the feeding rate of the cladoceran Daphnia magna (measured over 24 h) is a sensitive indicator of toxicity. Using a methodology similar to that used for D. magna, a feeding test for the Australian tropical cladoceran Moinodaphnia macleayi was developed and the sensitivity of the test was compared with the currently used reproduction test. The results of the study revealed that the feeding rate of M. macleayi was a more sensitive indicator of toxicity than reproduction for cadmium; a mean concentration of 1.4 microg/L caused significant inhibition of feeding (P < or = 0.05), while reproduction was significantly reduced at 3.1 microg/L (P < or = 0.05). No feeding inhibition was detected when M. macleayi were exposed to copper, despite significant mortality. The difference in feeding and reproductive responses of M. macleayi to cadmium and copper suggest differing modes of action for the two metals. The feeding test was validated on two samples of mine release water containing various metals. Results demonstrated that a cladoceran feeding test, performed over a 20-h period, was comparable in sensitivity to a reproduction test performed over a 5- to 6-day period.

Comparative sensitivity of three populations of the cladoceran Moinodaphnia macleayi to acute and chronic uranium exposure.

Assessment of differences in the response of three different populations of the tropical cladoceran Moinodaphnia macleayi to uranium exposure was evaluated. The populations tested included a laboratory stock (maintained for 10 years), a wild population collected from Bowerbird Billabong (an uncontaminated environment), and a population collected from Djalkmara Billabong (a relatively contaminated environment with elevated levels of uranium), located on the Ranger uranium mine site, Jabiru East, NT, Australia. Chronic and acute toxicity of uranium was determined for all three populations. The no-observed-effect-concentration (NOEC; reproduction) and lowest observed-effect-concentration (LOEC; reproduction) for uranium ranged between 8-31 micrograms L-1 and 20-49 micrograms L-1, respectively, for all three populations. The 48 h EC50 (immobilization-lethality) for uranium ranged between 160-390 micrograms L-1 for all three populations. There was little difference in the response of the three populations of M. macleayi to acute and chronic uranium exposure, although the response of the laboratory population to chronic uranium exposure appeared more variable than the "wild" populations. There was no apparent tolerance in the population of M. macleayi obtained from Djalkmara Billabong when exposed to elevated levels of uranium. M. macleayi was significantly more sensitive to uranium exposure than other species previously tested. It was concluded that the sensitivity of the laboratory population (to uranium) is still representative of natural M. macleayi populations.

Effects of water hardness and alkalinity on the toxicity of uranium to a tropical freshwater hydra (Hydra viridissima).

In tropical Australian freshwaters, uranium (U) is of potential ecotoxicological concern, largely as a consequence of mining activities. Although the toxicity of uranium to Australian freshwater biota is comprehensive, by world standards, few data are available on the effects of physicochemical variables, such as hardness, alkalinity, pH and organic matter, on uranium speciation and bioavailability. This study determined the individual effects of water hardness (6.6, 165 and 330 mg l(-1) as CaCO3) and alkalinity (4.0 and 102 mg l(-1) as CaCO3), at a constant pH (6.0), on the toxicity (96 h population growth) of uranium to Hydra viridissima (green hydra). A 50-fold increase in hardness (Ca and Mg concentration) resulted in a 92% (two-fold) decrease in the toxicity of uranium to H. viridissima [i.e. an increase in the EC50 value and 95% confidence interval from 114 (107-121) to 219 (192-246) µg l(-1)]. Conversely, at a constant hardness (165 mg l-1 as CaCO3), the toxicity of uranium to H. viridissima was not significantly (P > 0.05) affected by a 25-fold increase in alkalinity (carbonate concentration) [i.e. EC50 values of 177 (166-188) and 171 (150-192) µg l(-1) at 4.0 and 102 mg l(-1) as CaCO3, respectively]. A knowledge of the relationship between water chemistry variables, including hardness and alkalinity, and uranium toxicity is useful for predicting the potential ecological detriment in aquatic systems, and can be used to relax national water quality guidelines on a site-specific basis.

Chronic toxicity of diethylenetriamine pentaacetic acid to crimson-spotted rainbowfish (Melanotaenia fluviatilis): effects on reproduction, condition, and ethoxyresorufin O-deethylase activity.

The chronic effects of the chelating agent diethylenetriamine pentaacetic acid (DTPA) on reproduction, condition factor, liver somatic index (LSI), gonad somatic index (GSI), and ethoxyresorufin O-deethylase (EROD) activity of adult Australian crimson-spotted rainbowfish (Melanotaenia fluviatilis) were assessed. Breeding groups of three females and two males were exposed to 0, 1, 10, or 100 mg/liter DTPA (nominal) in a 28-day "static-renewal" experiment. Overall, the toxicity of DTPA to adult crimson-spotted rainbowfish was relatively low. Reproduction was not affected at concentrations up to 100 mg/liter DTPA, although an early effect on hatchability was potentially attributed to direct toxicity to rainbowfish eggs. DTPA also had little effect on the condition of adult rainbowfish, with condition factor and GSI being unaffected at concentrations up to 100 mg/liter, the latter finding supporting the reproduction results. However, LSI in male rainbowfish exposed to 100 mg/liter was significantly lower than in those exposed to 1 mg/liter DTPA (P

Comparative acute and chronic toxicity of diethylenetriamine pentaacetic acid (DTPA) and ferric-complexed DTPA to Daphnia carinata.

The acute and chronic toxicity of diethylenetriamine pentaacetic acid (DTPA) and ferric complexed DTPA (Fe[III]-DTPA) to Daphnia carinata were compared, while the effects of DTPA exposure prior to and/or during 1st brood embryogenesis were also assessed. For chronic exposures, daphnids were exposed to DTPA at high or low food levels, or to Fe(III)-DTPA at high food level until the 6th reproductive instar. The 48 h LC50S of DTPA and Fe(III)-DTPA to D. carinata were 245 mg/L and > 1,000 mg/L, respectively. Chronic exposure to 10 mg/L DTPA resulted in a significant reduction in all individual brood sizes, while it increased the age at each reproductive instar. Ten mg/L DTPA also significantly decreased the cumulative number of offspring per adult at high and low food level from 161.3 +/- 14.6 to 11.3 +/- 4.9 offspring, and 56.4 +/- 1.8 to 0 +/- 0 offspring, respectively, while a similar effect was observed for the number of offspring per adult per day. Both the 3rd and 5th brood sizes were also significantly reduced at 1 mg/L DTPA, but only at high food level, from 39.0 +/- 2.9 to 27.6 +/- 3.8 offspring, and 49.3 +/- 5.0 to 39.9 +/- 4.2 offspring, respectively. Chronic exposure to Fe(III)-DTPA had little effect on D. carinata, but there was a significant negative relationship between Fe(III)-DTPA and the number of offspring per adult per day (y = -0.024x + 14.048, r2 = 0.20, n = 26, P < 0.02). This was due to a 25% reduction in reproduction at 134 mg/L Fe(III)-DTPA, the highest test concentration, compared to controls. The no-observed-effect concentrations (NOEC) and lowest-observed-effect concentrations (LOEC) for DTPA and Fe(III)-DTPA following chronic exposure to D. carinata were 1.0 and 10 mg/L, and 67 and 134 mg/L, respectively, although the possibility of effects occurring below 10 mg/L DTPA could not be discounted. Exposure to DTPA prior to 1st brood embryogenesis significantly decreased the 1st brood size but did not affect the 2nd brood size, while exposure during 1st brood embryogenesis significantly decreased the 2nd brood size, but did not affect the 1st brood size, indicating the reproductive impairment was due to maternally-mediated factors and not direct toxicity to the eggs. The decrease in DTPA toxicity when complexed with Fe(III) was attributed to preferential binding with that metal, thereby limiting any further chelating ability. Concentrations of DTPA in receiving waters are unlikely to be toxic to D. carinata.

Toxicity of DTPA to Daphnia carinata as modified by oxygen stress and food limitation.

First-instar Daphnia carinata were exposed to one of four or five sublethal concentrations of the industrial chelating agent diethylenetriamine pentaacetic acid (DTPA) either alone, or in conjunction with, high (90-100%) or low (10-25%) oxygen saturation and high (2 x 10(5) cells/ml) or low (2 x 10(4) cells/ml) food conditions for 6 to 7 days, in a series of three experiments. Survival, growth, reproduction, and hemoglobin (Hb) content were assessed. Mortality increased significantly from 6.5 +/- 4.2 to 38.9 +/- 5.2%, and mean length was significantly reduced from 2.73 +/- 0.02 to 1.37 +/- 0.01 mm at 100 mg/liter DTPA in experiment 1. Mean length was also significantly reduced from 2.64 +/- 0.12 to 1.9 +/- 0.1 mm at 50 mg/liter DTPA in experiment 3. This was attributed to an indirect effect via the food supply in the third experiment. There was a significant decrease in the mean number of first-brood eggs at 10 mg/liter DTPA in all three experiments. Hemoglobin concentration was significantly increased under low oxygen conditions from 27.6 +/- 1.7 to 65.5 +/- 4.6 mg Hb/g Daphnia dry wt, and 23.0 +/- 1.8 to 49.4 +/- 3.5 mg Hb/g Daphnia dry wt in experiments 2 and 3, respectively. However, DTPA had no effect on hemoglobin concentration in any experiment. DTPA toxicity to D. carinata was not significantly altered by oxygen stress or food limitation and could not be attributed to an inhibition of Hb synthesis. Increased exposure times may result in further reproductive effects and also an indirect effect on hemoglobin concentration via the gradual depletion of iron stores. The no-observed effect concentration and the lowest observed effect concentration for D. carinata in this study were 1.0 and 10 mg/liter DTPA, respectively, based on reproduction, giving an estimated threshold concentration of 3.2 mg/liter DTPA.