Both carbon and nitrogen appear in nature as different stable isotopes. Many natural processes affect the relative proportions of heavy and light isotopes in predictable ways. The isotopic composition of organisms and other organic materials can therefore serve as records of the processes involved in their formation. Materials with origin in different production systems like land, lakes, rivers and oceans differ in this respect. Inferences about the origin of different materials can therefore be made from their isotopic composition. The application of stable isotope methods in ecology turns nature into a vast tracer experiment in which we are free to participate, observe, deduce and learn.

Heavy isotopes tend to accumulate in tissue of organisms. The isotopic composition thereby reflects the trophic positions of organisms in a food web. Many environmental contaminants are also accu-mulated by organisms from their food and thereby come to appear in higher concentrations in consumers. Environmental contaminants, found in low levels in primary producers may, by consecutive food chain transfers, be boosted to concentrations that are detrimental, or even fatal, to the top consumers. A method is presented by which the food web position of organisms (estimated by heavy nitrogen isotope content) can be used to quantitatively estimate the food web accumulation of persistent organic contaminants. The method can be used as an <missing material!>