ANNUAL STUDY OF BACTERIOPLANKTON COMMUNITY DYNAMICS

Bacterial growth, abundance, and grazing were measured simultaneously during a full seasonal cycle in a temperate brackish water estuary, covering diel and seasonal variability. Growth of bacteria was balanced by bacterivory on an annual basis, and observed net change in bacterial numbers matched the net change calculated by a simple Lotka-Volterra equation lacking immigration or emigration terms. Bacterial growth was dependent on water temperature below 6-degrees-C, but other factors dominated at higher temperatures. The community clearance rate of the bacterivores was stringently regulated at 0.6 d-1, showing a linear functional response within the range of natural bacterial abundances recorded. Furthermore, bacterivores showed a threshold at 7 x 10(5) bacteria ml-1 when grazing ceased, suggesting that pelagic bacteria have a numerical refuge promoting coexistence with bacterivores in aquatic environments. The observed lack of timing between bacterial growth and grazing in spring allowed accumulation of bacterial cells, but grazing exceeded growth in winter. We inferred that the specific growth rate of the bacterial community and the community clearance rate of bacterivores together determined change in and abundance of bacterioplankton in the waters examined.