We monitored the change in the isotope composition of sulfur, carbon, and nitrogen in broad whitefish (Coregonus nasus) tissues in response to a change in the isotope composition of their food. One of two batches of 2.5-yr-old fish raised in the laboratory were given a new food source with different δ34S, δ13C, and δ15N, which were monitored in muscle and liver tissue for 1 yr. A model including change due to tissue accumulation (growth) and metabolic replacement was developed. For all three isotopes, most of the change could be attributed to growth. Metabolic replacement expressed as a turnover rate was only 0.1 −0.2%∙d−1 and was similar for the three isotopes. Although liver tissue was −4.4 and −4.1‰, respectively, for δ34S and δ13C relative to muscle tissue, the response over time to the new food was the same as for muscle. We expect that the complete change in the isotope composition of fish tissue in response to a change in food could take years in slow-growing wild populations. The stable isotope composition would represent a long-term average of the food. In fast-growing fish the rate of change would directly reflect the growth rate.

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A sampler for defining compositional changes over distances on the order of 1 cm in aqueous systems operates by equilibration of water contained by a dialysis membrane with the surrounding water. Depth profiles of methane and phosphate in Hudson Estuary sediments are presented as examples.

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