A modified dilution method reveals higher protozoan growth rates than the size fractionation method

Protozoan growth rates are typically estimated with the size fractionation method in which the organisms studied are separated from the larger grazers. This method was compared with a specially modified dilution method which removed not only the grazing pressure of larger grazers but also that of predators of the same size as the organisms studied. Five comparisons were performed under natural conditions with the following marine protozoa: Balanion comatum (3 sets of data), Ebria tripartita, and Strombidium sp. The dilution method revealed significantly higher growth rates (Wilcoxon's signed-rank test, p = 0.04), which underscores the importance of predation between organisms of the same size. It directly demonstrated that the size fractionation method may significantly underestimate protozoan growth rates.