Multiple nutrient limitations in ecological models

Ecological processes are often limited by more than one nutrient, for example, nitrogen and phosphorus. Therefore, mathematical representation of simultaneous limitations becomes important in many modeling and data analysis problems. This article reviews eight functional forms that have been proposed in the literature and presents a general theoretical framework that illustrates the commonalities and differences among the functions. Based on the general framework, three additional functional forms are derived. We then evaluate the functions based on their ability to fit experimental data. A total of eleven functions are fitted to eleven data sets by nonlinear least-squares. A new functional form, the Additive model, is selected as the most general function based on available data.