Applicability of BLMS

Applicability in Surface Waters

The ranges of Ca2+, Mg2+, and pH used to develop and validate the BLMs represent the physico-chemical boundaries of the models. Some freshwater systems and geographic regions will be outside of these boundaries, and the use of the BLMs in these situations may be over- or under-protective. An overview of the range of physico-chemical parameters for which the chronic Ni BLMs were validated is provided in Table 1.

Applicability to Different Species

As mentioned above, BLMs have been developed and validated for only a limited number of species, and they therefore do not cover all freshwater species included in the chronic Ni database.  The chronic Ni aquatic toxicity database contains data for 31 different species (see Fact Sheet 1) while fully validated chronic Ni BLMs are available for only 4 species (i.e., C. dubia, D. magna, O. mykiss and P. subcapitata). Thus, extrapolation of the BLMs to other species is necessary to develop site-specific PNECs.  Much debate has centered on the validity of cross species extrapolation of BLMs.  Conceptually, the extrapolation of BLMs developed for one species (e.g., the invertebrate D. magna) to other taxonomically similar species (e.g., other invertebrates, including crustaceans, molluscs, insects, etc.) should be justified as long as the mechanisms of toxicity are similar among the broader taxonomic group.  With empirical evidence showing that mechanisms of Ni toxicity are similar among and between broader taxonomic groups, the following approach for cross-species extrapolation can be used:

  • the D. magna/C. dubia BLMs can be used to normalize the chronic toxicity to other invertebrates;
  • the P. subcapitata BLM can be used to normalize the chronic toxicity to other algae; and
  • the O. mykiss BLM can be used to normalize the chronic toxicity to fish and amphibians.

While empirical evidence is important, legitimate uncertainties on the mechanisms of Ni toxicity exist, and these uncertainties raise questions about the validity of cross-species extrapolation of BLMs.  To resolve these uncertainties, an agreement was reached during the EU Risk Assessment process on what information and criteria was needed to support employment of the full cross species BLM extrapolation. Sufficient information was available to convincingly demonstrate similarity in Ni toxicity mechanisms among different fish species and among different algae species, but not among different invertebrate or vascular plant species.  Therefore, a “spot-check” study was undertaken to test whether or not the invertebrate and plant BLMs were able to predict chronic Ni toxicity to organisms for which no BLM had been developed. 

Table 1: Physico-Chemical Ranges for the Chronic Nickel BLMs

Physico-Chemical Parameter

P. subcapitata

D. magna

C. dubia

O. mykiss






Ca2+ (mg/L)





Mg2+ (mg/L)





In the spot check study, four non-BLM organisms were tested.  Three invertebrates were tested, including the insect Chironomus tentans, the rotifer Brachionus calyciflorus, and the snail Lymnaea stagnalis.  One plant species, Lemna minor, was also tested. Toxicity tests were performed with each species in five natural waters that represent the range of bioavailability found within typical European freshwater systems.  Results showed that the BLMs were able to accurately predict Ni toxicity to the spot-check species (Schlekat et al., 2010), which gave quantitative support to the cross-species extrapolation approach.

Based on the results from the spot-check exercise and other weight-of-evidence arguments (i.e., the ecological relevance of the BLMs, accuracy of the BLMs, and the conservatism of the proposed cross-species approach), the following normalization approach was determined to be appropriate for the normalization of toxicity data:

  • for algae, the P. subcapitata BLM can be used;
  • for higher aquatic plants, the D. magna (best fitting BLM) BLMs can be used;
  • for cladocerans, insects, and amphipods, the most stringent of the D. magna and C. dubia BLM can be used;
  • for rotifers, the D. magna BLM can be used;
  • for molluscs and hydra, the C. dubia (best fitting BLM) BLMs can be used; and
  • for fish and amphibians, the O. mykiss BLM can be used.