Sigmoid models for the uptake,concentration and effects of metals in consumer organisms

The uptake, concentration and effects of metals contamination in consumer organisms have been successfully described with the flexibly-shaped Richards sigmoid model. This model allows the simultaneous estimation of: (1) final asymptotic level of contaminant uptake or body growth, (2) the time required to approach asymptote, and (3) the particular path or trajectory taken by the function in approaching asymptote within the designated time. Analyses using the Richards model have shown that data for a variety of contaminants fail to conform to the shape of the classic monomolecular model and rather show an initial lag in the early uptake rate. In effects studies, the use of the Richards model to analyze body growth data from waterfowl exposed to dietary intake of Cd and other contaminants has shown that growth curve shape is more likely to change in response to contaminant intake than are either final asymptotic levels or the times required for asymptotes to be attained. Failure to consider such changes in growth curve shape could cause significant errors in the identification of growth responses to contaminant intake stress.