Most aquatic organisms are less sensitive to nickel exposure than to copper exposure, which is reflected by higher numerical values of the LC50s, but also of the measures of chronic toxicity. Toxicity of nickel in natural waters varies widely as a function of pH and hardness and - in particular -of DOC concentration.

Spencer (1980, ref. in IPCS, 1991) has reported that nickel at concentrations of 50 - 100 (o,g/l (as dissolved Ni) inhibits the growth of algae, though some species may be more tolerant. Especially green algae were indicated to be sensitive to nickel exposure at 100 ^g/l, while blue-greens appeared to be less sensitive.

Madoni (2000) measured the acute toxicity of nickel (NiCl2-6 H2O) to freshwater ciliates and found toxic concentrations ranging from 0.17 mg/l (Spirotomum teres) and 0.36 mg/l (Paramecium bursaria) to 7.7 mg/l (Euplotes patella).

In tests with Daphnia magna, a 64-h LC50 value of 320 ^g/l was recorded at a water hardness of about 100 mg/l CaCO3, while another species, D. hyalina, had a 48-h LC50 value of 1.9 mg/l at a lower water hardness, 58 mg/l (IPCS, 1991). Impairment of reproduction in Daphnia was recorded at nickel concentrations as low as 30 - 95 ^g/l in water with pH 7.74 and a hardness of about 45 mg/l CaCO3 (Biesinger and Christensen, 1972).

The sensitivity of fish to nickel varies considerably among species, but 96-h median lethal concentrations in soft freshwater generally fall within the range of 4 - 20 mg/l, i.e. the sensitivity of fish to acute effects of nickel is much lower than that of invertebrates and algae (IPCS, 1991). However, there are life stages in fish which show considerably higher sensitivity to nickel exposure; e.g. when eggs and larvae of rainbow trout were exposed at a water hardness of 100 mg/l CaCO3 (from fertilization to 4 days after hatching), the LC5o value was found to be 50 ^g/l (Birge and Black, 1980). In these experiments, it turned out that rainbow trout was significantly more sensitive to nickel exposure than any of the other about 10 different fish species tested (IPCS, 1991).

Recent work by Hoang et al. (2004) with fathead minnows of different age (<1-d old and 28-d old) under varying regimes of water hardness, pH, alkalinity and natural organic matter (NOM) demonstrated the following : The toxicity of Ni was inversly related to water hardness at CaCO3 levels between 20 and 150 mg/l. In the absence of NOM, Ni toxicity increased as pH increased at high hardness and alkalinity. The fry (<1-d old) were more sensitive to Ni than the 28-d old fish : the difference in sensitivity was 12fold at low hardness and alkalinity, but 5-fold at higher hardness and alkalinity (100 and 400 mg/l, respectively). The presence of NOM (10 mg DOC/l) reduced Ni toxicity by up to 50%, but this effect was saturated above DOC at 5 mg/l. The BLM that was developed gave results suggesting that the model could be improved by considering NiCO3 to be bioavailable.

As shown in Table 7.4 above, the relative sensitivity of fathead minnows to toxic effects of trace metals, expressed as critical concentration of metal bound to the biotic ligand in the gills, is much lower for nickel than for copper or silver. The LA50 value for nickel was given as 239 nmol/g ww, and for copper it is about 10 (5 - 12) nmol/g ww. Based on this data, the sensitivity of this fish species to nickel appears to be about 20 - 25 times lower than its sensitivity to copper.

In tests with three marine organisms, where both acute and chronic toxicity data were obtained, it turned out that a fish (Atherinopsis affinis) had much lower sensitivity to nickel (acute - 27 mg/l; chronic - 4.3 mg/l) than a mollusk (Haliotis rufescens - the red abalone), with acute and chronic values at 150 ^g/l and 26 ^g/l, and a crustacean (Mysidopsis intii), exhibiting an acute toxicity, i.e. 96-h LC50j of 150 ^g/l and a chronic toxicity of 22 ^g/l (Hunt et al., 2002). These chronic values, recorded in two marine invertebrates, appear to be among the lowest observed effect concentrations so far reported for nickel.

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