Growth Rates and Carrying Capacities

The growth rates (μ) of four physiological groups (LNA, HNA, Live and CTC+ cells) were calculated as the slope of the natural logarithm of bacterial abundances vs. time during the corresponding exponential growth phases. Total heterotrophic bacterial growth rates correspond to the changes in abundance of total bacteria [LNA + HNA (reversed order for consistency)]. Viruses were unlikely to affect bacterial growth rates due to the short-time of the exponential growth periods (1–2 days, Guixa-Boixareu et al., 1999). Therefore, we will use the terms “specific growth rate” for the Filtered treatment and “net growth rate” for the Community treatment, containing the entire microbial food web (i.e., with protistan grazers). Carrying capacity is defined here as the maximum abundance recorded for each bacterial group at the plateau stage of the incubations. The ratio between the Filtered and the Community treatment carrying capacities was used as a proxy of top–down control.