Bioavailability and toxicity of nickel to freshwater organisms : a modeling approach

Water quality criteria setting for metals is hampered by the fact that the bioavailability and hence also the toxicity of many metals is largely influenced by physicochemical parameters such as water hardness, pH and dissolved organic matter. Consequently, a single water quality criterium may be too conservative for certain water bodies whereas it may not be protective enough for other water bodies. This metal-specific issue can be overcome with the development of mathematical models capable of predicting metal bioavailability and toxicity as a function of water chemistry. In the first part of this doctoral thesis it was investigated to what extent several physicochemical parameters (such as calcium, magnesium, pH, dissolved organic matter) affect the acute and chronic toxicity of nickel to freshwater organisms. For each testing organism (a unicellular green alga, an aquatic invertebrate and a fish) a bioavailability model was developed capable of accurately predicting nickel toxicity in both artificial and natural waters. An important point of concern was that bioavailability models such as those developed in the first part of this doctoral thesis may underestimate metal toxicity in waters with hardness far below the lower water hardness boundary for which these models were originally developed and validated using ‘standard’ test organisms. In the second part of this research it was therefore investigated whether this concern is scientifically justified for nickel. A series of toxicity tests with field-organisms (microalgae and water fleas) originating from soft and hard surface waters demonstrated that in most cases soft water organisms are equally sensitive to nickel as hard water organisms and that a single bioavailability model can be used to predict the water hardness-dependent toxicity of nickel to soft and hard water organisms. The data and models generated in this thesis were already used in the European risk assessment of nickel and nickel compounds. The pan-European water quality criterium for nickel will be based on the outcome of this risk assessment. Current developments at the international regulatory level feed the expectation that the application domain of this thesis will further expand

Nickel speciation and ecotoxicity in European natural surface waters: development, refinement and validation of bioavailability models

The accurate prediction of Ni ecotoxicity in natural surface water with bioavailability models such as the biotic ligand model (BLM) depends on how well these models can predict both the speciation of Ni (i.e. Ni2+ concentration), the toxicity of Ni2+ ions to an organism, and the effects of water chemistry parameters thereupon, such as dissolved organic carbon (DOC), pH, and water hardness. The overall aim of the study was to calibrate existing speciation models to Ni speciation in natural surface waters and to use these data to validate and/or refine bioavailability models for aquatic organisms from three trophic levels, i.e. algae, invertebrates (daphnids), and fish. The developed chronic Ni toxicity models for daphnids, fish and algae exhibit sufficiently high predictive capacities to yield a marked reduction of uncertainty associated with differences in chronic Ni bioavailability among different test waters. This is due to the fact that they can predict both Ni2+ concentrations as a function of dissolved Ni and water chemistry (mainly DOC, pH, Ca, Mg), as well as the toxicity of the Ni2+ ion as a function of water chemistry (mainly pH, Ca, Mg). The use of the models presented in the present study for normalizing Ni toxicity data will therefore decrease the overall uncertainty of the risk assessment, provided that the variability of bioavailability modifying parameters across different EU regions and water bodies is acknowledged

A novel method for predicting chronic nickel bioavailability and toxicity to Daphnia magna in artificial and natural waters

In the present study, the individual effects of Ca, Mg, and pH on the chronic toxicity of Ni to Daphnia magna were examined in a series of 21-d reproduction tests in synthetic test solutions. Based on the linear increase of 21-d median effective concentrations expressed as Ni2+ activity (21-d EC50(Ni2+)) with increasing activities of Ca2+ and Mg2+, the effects of Ca and Mg were modeled according to single-site competition with log K-CaBL = 3.53 and log K-MgBL = 3.57 (BL = biotic ligand). Because the increase of 21-d EC50(Ni2+) with increasing H+ activity was nonlinear, the effect of pH could not be described appropriately by single-site competition between Ni2+ and H+. Instead, the effect of pH was modeled based on an empirical linear relationship between pH and 21-d EC50(pNi2+)* (equal to -log [21-d EC50(Ni2+) corrected for the presence of Ca and Mg]) and was superimposed on the effects of Ca and Mg. For all test solutions used for model development, the developed model predicted the observed 21-d EC50 expressed as dissolved Ni concentration with an error of less than a factor of two. The importance of dissolved organic carbon in protecting D. magna against chronic Ni toxicity was demonstrated by conducting 21-d reproduction tests in a series of Ni-spiked natural waters. Because the model tended to systematically overestimate chronic Ni toxicity in these natural waters, it was further optimized to yield more accurate predictions in natural waters. Although some room still exists for improvement, the developed model is, to our knowledge, the first to present a useful tool for assessing the risk of Ni to aquatic invertebrates

A single bioavailability model can accurately predict Ni toxicity to green microalgae in soft and hard surface waters.

The major research questions addressed in this study were (i) whether green microalgae living in soft water operationally defined water hardness 25 mg CaCO3/L), and (ii) whether a single bioavailability model can be used to predict the effect of water hardness on the toxicity of Ni to green microalgae in both soft and hard water. Algal growth inhibition tests were conducted with clones of 10 different species collected in soft and hard water lakes in Sweden. Soft water algae were tested in a 'soft' and a 'moderately hard' test medium (nominal water hardness = 6.25 and 16.3 mg CaCO3/L, respectively), whereas hard water algae were tested in a 'moderately hard' and a 'hard' test medium (nominal water hardness = 16.3 and 43.4 mg CaCO3/L, respectively). The results from the growth inhibition tests in the 'moderately hard' test medium revealed no significant sensitivity differences between the soft and the hard water algae used in this study. Increasing water hardness significantly reduced Ni toxicity to both soft and hard water algae. Because it has previously been demonstrated that Ca does not significantly protect the unicellular green alga Pseudokirchnerielia subcapitata against Ni toxicity, it was assumed that the protective effect of water hardness can be ascribed to Mg alone. The log K-MgBL (=5.5) was calculated to be identical for the soft and the hard water algae used in this study. A single bioavailability model can therefore be used to predict Ni toxicity to green microalgae in soft and hard surface waters as a function of water hardness

A single bioavailability model can accurately predict Ni toxicity to green microalgae in soft and hard surface waters.

The major research questions addressed in this study were (i) whether green microalgae living in soft water operationally defined water hardness 25 mg CaCO3/L), and (ii) whether a single bioavailability model can be used to predict the effect of water hardness on the toxicity of Ni to green microalgae in both soft and hard water. Algal growth inhibition tests were conducted with clones of 10 different species collected in soft and hard water lakes in Sweden. Soft water algae were tested in a 'soft' and a 'moderately hard' test medium (nominal water hardness = 6.25 and 16.3 mg CaCO3/L, respectively), whereas hard water algae were tested in a 'moderately hard' and a 'hard' test medium (nominal water hardness = 16.3 and 43.4 mg CaCO3/L, respectively). The results from the growth inhibition tests in the 'moderately hard' test medium revealed no significant sensitivity differences between the soft and the hard water algae used in this study. Increasing water hardness significantly reduced Ni toxicity to both soft and hard water algae. Because it has previously been demonstrated that Ca does not significantly protect the unicellular green alga Pseudokirchnerielia subcapitata against Ni toxicity, it was assumed that the protective effect of water hardness can be ascribed to Mg alone. The log K-MgBL (=5.5) was calculated to be identical for the soft and the hard water algae used in this study. A single bioavailability model can therefore be used to predict Ni toxicity to green microalgae in soft and hard surface waters as a function of water hardness