The relative preference index (RPI) for phytoplankton nitrogen use is only weakly related to physiological preference

Abstract

In field studies where the uptake of different nitrogen compounds in phytoplankton is studied, a relative preference index (RPI) is often used which describes uptake relative to the abundance of a particular nitrogen compound, usually ammonium. This index can easily be misused when it is applied as a physiological preference indicator. In this study, it is shown that the index may be more dependent on nutrient concentrations than on the algal preference for ammonium under certain circumstances.