How much is enough for nutrients in microzooplankton dilution grazing experiments?

Abstract

The dilution technique is the most widely used method to estimate microzooplankton herbivory in aquatic systems. This technique relies on the assumption that during the incubation phytoplankton growth is independent of the dilution factor; otherwise, biased estimates of microzooplankton grazing are obtained. To fulfil the requirement, nutrients should be routinely added to the dilution set. Here, we question the extent to which the assumption of equal phytoplankton growth along the dilution series might be actually met in the published experiments, even when nutrients are added. We also provide evidence that the violation of such assumption results in severely inflated estimates of microzooplankton grazing. Finally, we propose a simple procedure, based on the Redfield relationship, to overcome the problem and guarantee accurate results of dilution grazing experiments.