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. In marine environments, nitrogen is available for phytoplankton in different forms. Ammonium is the most reduced form and preferentially used by phytoplankton. Nitrogen recycling within the photic zone is via ammonium or organic nitrogen. Nitrate, the most oxidized form, is considered to be formed below the photic zone. In stratified waters, the use of nitrate for phytoplankton growth relative to the total nitrogen uptake is often used to determine the new production (Dugdale and Goering, 1967), i.e. the import of nitrogen into the photic zone. The relative preference index (RPI), originally introduced by McCarthy et al. (1977) was meant to compare the utilization of a nitrogen compound (originally ammonium) relative to availability [equation (1)].