Influence of an aquatic humic acid on the bioconcentration of selected compounds in Daphnia magna

Several chemicals covering a wide range of octanol-water partition coefficients were assayed for their bioconcentration in Daphnia magna in the presence of an aquatic humic acid. Assuming the properties of the humic acid used were similar to those known for organic matter from soils, the influence of the aquatic humic acid on physicochemical moderation of bioconcentration could not be demonstrated experimentally in a log Kow range of the chemicals between 2 and 6. Thus, the lipophilic character of the aquatic humic acid and the related effect of solubilizatio- n of the chemicals in the aqueous phase are much less than expected. This result is confirmed by determining the octanol-water partition coefficient of the aquatic humic acid. Comparing this value with log Kow values of humic acids of terrestrial origin, it can be clearly demonstrated that the lipophilicity of the aquatic humic acid is lower by a factor of 50.