The internal distribution of nickel and thallium in two freshwater invertebrates and its relevance to trophic transfer.

Although nickel and thallium are present at potentially harmful concentrations in some lakes, there is little information on their bioaccumulation and transfer up aquatic food webs. To measure the propensity of animals for accumulating and transferring these contaminants along food chains, we exposed two common types of invertebrates, an insect (Chironomus riparius) and a worm (Tubifex tubifex), to these metals spiked into sediment. We then measured the subcellular distribution of Ni and Tl in these invertebrates to estimate the likelihood that these metals will have toxic effects on these prey or be transferred to higher trophic levels. In both species, at least half of their Ni and TI was present in fractions that are purportedly detoxified (granules and metal-binding proteins). Furthermore, based on information in the literature concerning prey subcellular fractions that are likely to be trophically available (TAM), we estimate that much of the Ni and TI in these animals (43-84%) is available for transfer to a predator. To test this prediction, we fed these invertebrates to the alderfly Sialis velata, and measured the efficiency with which this predator assimilated Ni and Tl from each prey type. The majority of both trace metals (58-83%) was assimilated by the predator, which suggests that these contaminants would be easily transferred along aquatic food chains and that models describing Ni and Tl accumulation by aquatic animals should consider food as a source of these metals. The proportion of metal that could potentially be taken up by a consumer (% TAM) and the actual percentage assimilated by S. velata fell on or reasonably close to a 1:1 line for the 4 prey-metal combinations.