Effect of water hardness on the toxicity of cadmium to the alga Pseudokirchneriella subcapitata
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- NIVA-rapport 
The growth inhibiting effect of cadmium on the green alga Pseudokirchneriella subcapitata has been investigated in an artificial medium and three natural soft lake waters. The hardness of the media was manipulated by addition of calcium. Increasing hardness resulted in a lower slope of the concentration/response curve in all media. The EC50 increased with hardness (reduced toxicity), but the results for EC10 were less consistent. Slopes in the hardness/toxicity relationship were calculated. The maximum slope of EC50 versus hardness in the hardness range 3.4-15 mg CaCO3/l was 5.44 µg Cd/mg CaCO3/l. The corresponding slope for EC10 was 1.59 µg Cd/mg CaCO3/l. At the higher hardness range (15-45 mg CaCO3) the maximum slopes were lower. The highest growth inhibiting effect of Cd was found in Lake Byglandsfjorden with hardness 3.5 mg CaCO3 and total organic carbon 1.8 mg/l. When tested without addition of artificial chelators the EC50 for inhibition of growth rate in this lake was 9.4 µg Cd/l and EC10 was 2.8 µg Cd/l.