Simple Graphical Methods for the Interpretation of Relationships Between Trophic State Variables

Graphical methods are presented that can identify relationships between the trophic state variables, total phosphorus, total nitrogen, chlorophyll, and Secchi depth. The graphical approach extends the use of empirical models beyond predictions based solely on local data. Instead, the authors suggest comparing the data against an established set of predictive equations. These equations serve as a “standard model” from which deviations can be observed and inter-preted. The goal of the use of deviations from a standard model is to enhance our ability to understand relationships between important nutrient, transparency, and biological variables and to adjust our predictive models and manage-ment decisions accordingly. Examples are given of interpretations of deviations caused by nitrogen limitation, non-algal turbidity, zooplankton grazing, and dissolved water color.