Prediction of contaminant accumulation by free‐living organisms: Applications of a sigmoidal model

The accumulation of contaminants by free‐living organisms has traditionally been determined with permutations of the deterministic model: C t = C e (1 – e − kt ). However, studies utilizing a variety of species and exposure scenarios now suggest that significant deviations may occur from this classic form. In many cases noted to date, these deviations have involved a sigmoidal pattern of accumulation. While there may be no one single causal mechanism responsible for the expression of such a pattern in all cases studied, the application of a flexibly shaped Richards sigmoidal model can improve goodness of fit to the data relative to the classic model form. Several examples are presented for use of the Richards model: accumulation of 137 Cs by free‐living American coots ( Fulica americana ) and yellow‐bellied turtles ( Pseudemys scripta ), and Hg accumulation by mosquitofish ( Gambusia affinis ).