GRAZING RATES ON BACTERIA - THE SIGNIFICANCE OF METHODOLOGY AND ECOLOGICAL FACTORS

Protists and viruses are recognized as the main predators on bacteria in a variety of environments. Several factors may potentially influenc e the grazing rates of protists on bacteria: temperature, bacterial ab undance and production, and protist abundance. We inspected the relati ve importance of these variables in determining the specific grazing r ates of flagellates (GRF), and total community grazing rates (GT) by a nalyzing a data set extracted from the literature. We included freshwa ter and marine environments, ranging from oligo- to eutrophic systems. As different methods to measure grazing rates (GRF and GT) were used by different authors, the data were also analyzed for possible differe nces among methods. As there were significant differences among certai n methods, we classified them into 2 groups: (1) M1 methods, those tha t measured grazing rates (GRF and GT) through the uptake of fluorescen t particles; and (2) M2 methods, those determining community grazing r ates (GT) by dilution, inhibition, filtration, or the uptake of geneti cally marked bacteria. GT measured with M2 methods were systematically higher than those measured by M1. For studies with M1 methods, both G RF and GT were positively correlated to temperature, bacterial abundan ce and production. GT were also correlated to heterotrophic nanoflagel late abundance. GT and GRF showed a different pattern of change with t emperature, GRF increased with temperature within the range 0 to 30-de grees-C, while GT increased only up to 18-degrees-C. We did not find a ny satisfactory multiple regression model that explained the variation in the GT measurements with M2 methods. For M1 methods, flagellate ab undance and temperatures below 18-degrees-C were the main factors affe cting GT (the model explained 78 % of the variance). While bacterial a nd flagellate abundances were the main factors at higher temperatures, the model in this case explained only 19% of the variance. Grazing ra tes could be affected by other sources of variability not considered h ere (e.g. prey and predator size, chlorophyll). These effects could be come much more conspicuous above certain temperatures (18-degrees-C in our study).