2.1 Data compilation
The data on the toxicity of nickel to marine organisms were compiled from three main sources: open literature, internationally recognized databases (e.g., Science Direct, Web of Science), and industry-sponsored research programs. A large dataset on the chronic ecotoxicity of nickel to marine organisms was compiled. Estuarine species were not covered in this assessment. All gathered data were further screened using the criteria as outlined in Section 2.2.
2.2 Data quality screening
Each individual ecotoxicity data point was screened for quality before incorporation in the nickel ecotoxicity database based on the following criteriai:
- data were retained for the following groups of organisms: micro- and macro-algae, invertebrates, and fish;
- data covered the following relevant endpoints: survival, development, growth and/or reproduction;
- Ni-only exposure data were considered relevant (studies were rejected if indications of impurities or other substances might have an effect on the toxic properties of nickel);
- the results reported measured pH and salinity;
- the toxicity tests were performed in artificial or natural seawater at a salinity varying between 28 and 39 ppt;
- the data were from studies conducted according to approved international standard test guidelines (however, data from non-standardized tests were also assessed);
- only long-term or chronic toxicity data were used;
- the tests were performed according standard operational procedures, with a detailed description of the methods employed during toxicity testing;
- preference was clearly given on the use of measured nickel concentrations in the test concentrations;
- a clear concentration-response was observed;
- toxicity threshold values calculated as L(E)C10 (the concentration that causes 10% effect during a specified time interval) values were preferred; however, NOEC values (No Observed Effect Concentration) were also seen as equivalent;
- the toxicity tests were performed with soluble nickel salts (e.g., NiCl2 and NiSO4);
- the toxicity test results reflected dissolved nickel concentrations and were expressed as µg Ni/L; and
- ecotoxicity threshold values were derived using the proper statistical methods.
Only the identified ecotoxicity data fulfilling the above mentioned criteria were used for the marine aquatic PNEC derivation.
2.3 Database development
Applying the above mentioned quality screening criteria to the identified ecotoxicity data resulted in the selection of an extensive high quality database on the ecotoxicity of nickel to marine organisms. Indeed, the database comprised 15 different “species means” for 14 different families from 25 individual high quality L(E)C10/NOEC values (9 individual NOEC for micro- and macro-algae, 14 for invertebrates, 2 for fish).
An overview of all accepted individual high quality chronic ecotoxicity data is presented in the Environmental Risk Assessment of Nickel and Nickel Compounds (see Section 5).
2.4 Data normalization
Most of the physico-chemical characteristics known to affect nickel toxicity in the marine environment (i.e., pH, cation concentration, salinity) are fairly uniform in coastal marine waters. One parameter [i.e., dissolved organic carbon (DOC)] can vary substantially in marine waters. However, relationships between nickel toxicity and DOC for marine organisms are unknown. Therefore, normalization of the toxicity data has not been applied to the effect concentrations [NOEC/L(E)C10] compiled in the accepted high quality ecotoxicity database. All of the marine ecotoxicity tests have been performed at low DOC, which would be expected to maximize bioavailability. Therefore, the approach followed represents a reasonable worst case PNEC value.
2.5 Data aggregation
High quality ecotoxicity data are grouped/aggregated in order to avoid over representation of ecotoxicological data from one particular species. The following major rules were used to aggregate data:
- If several chronic NOEC/L(E)C10 values based on the same toxicological endpoint were available for a given species, the values were averaged by calculating the geometric mean, resulting in the “species mean” NOEC/L(E)C10.
- If several (geometric mean) chronic NOEC/L(E)C10 values based on different toxicological endpoints were available for a given species, the lowest (geometric value) value was selected.
After the data aggregation step, only one ecotoxicity value (i.e., the geometric mean for the most sensitive endpoint) was assigned to a particular species.
2.6 Calculation of PNEC using statistical extrapolation methods
Estimation of the HC5 from the species sensitivity distribution
When a large data set for different taxonomic groups is available, the PNEC can be calculated using a statistical extrapolation method. In this approach, the ecotoxicity data are ranked from low (most sensitive species) to high (least sensitive species) and a species sensitivity distribution (SSD) is then constructed by applying an appropriate curve fitting distribution (usually a log-normal distribution) to the high quality aggregated chronic toxicity data (Aldenberg & Jaworska, 2000). However, because of the bad fit of this distribution curve, alternative distributions were used for the fitting of the marine toxicity data. From each statistically relevant SSD, an individual 5th percentile value (at the median confidence interval) is calculated and the final selected median HC5 value is calculated as the mean value of the individual median 5th percentile.
Selection of appropriate assessment factor and derivation of the PNEC
To account for uncertainty, an assessment factor (AF) may be applied to the median HC5. In general, such AFs vary between 1 and 5 and are determined on a case-by-case basis. The marine aquatic PNEC would therefore be calculated as follows:
marine aquatic PNEC = median HC5/AF
Based on the available chronic NOEC/L(E)C10 data, the following points were considered when determining the AF:
- The overall quality of the database and the endpoints covered (e.g., are all the compiled data representative of “true” chronic exposure?)
- The diversity of the taxonomic groups covered by the database (e.g., do the databases contains all of the major groups of marine organisms?)
- The number of species (e.g., does the SSD cover at least 10 different L(E)C10/NOECs and preferably more than 15?)
- Statistical extrapolation (e.g., how well does the SSD fit the toxicity data?
- Comparisons between field and mesocosm studies and the PNEC (e.g., is the PNEC value protective for the effects observed in mesocosm/field studies?)
In the Nickel EU RA, no marine mesocosm/field data are available that allow derivation of threshold concentrations of nickel in marine waters in the field. In addition, not all marine taxonomic groups are covered in the marine toxicity database. On the other hand, the ecotoxicity testing have been performed under conditions that tend to maximize bioavailability, and the estimated PNEC value using an AF of 2 is well below the lowest available measured toxicity value of the database. Therefore, based on weight of evidence, it was proposed to use an AF of 2.
i The application of the quality screening criteria would also apply in case additional or new ecotoxicity data would be considered.