THE RELATIVE PREFERENCE INDEX (RPI) FOR PHYTOPLANKTON NITROGEN USE ISONLY WEAKLY RELATED TO PHYSIOLOGICAL PREFERENCE

In field studies where the uptake of different nitrogen compounds in p hytoplankton is studied, a relative preference index (RPI) is often us ed which describes uptake relative to the abundance of a particular ni trogen compound, usually ammonium. This index can easily be misused wh en it is applied as a physiological preference indicator. In this stud y, it is shown that the index may be more dependent on nutrient concen trations than on the algal preference for ammonium under certain circu mstances.