Regulation of bacterial abundance and production by substrate supply and bacterivory: A mesocosm study

Daily bacterial abundance and production, heterotrophic nanoflagellates (HNAN) abundance, chlorophyll, and NH4 (+) concentrations were measured in four indoor 400-liter tanks over 13 days to study the role of heterotrophic bacterioplankton in NH4 (-) cycling and to identify the succession of top-down and bottom-up processes in regulating bacterial biomass and production. Ammonium (NH4 (+)) was added to these four tanks daily whenever its concentration in tanks was < 4 μM. Tanks 3 and 4 (treatment tanks) also received 4 μM of glucose daily till the end of experiment. Lower NH4 (-) concentrations and higher bacterial specific growth rate and production observed in the treatment tanks indicated that bacteria might take up NH4+ with the addition of labile organic carbon. Bacterial biomass was controlled by substrate supply and HNAN grazing from day 7 to day 13, when phytoplankton declined. Bacterial size distribution patterns were determined primarily by substrate supply, with HNAN grazing playing a less important role. Certain variabilities existed between the control (and the treatment) tanks. These inconsistencies could be due to differences in time of expression of given variables. However, the total amounts of bacterial biomass accumulated in the four tanks were very similar. The inconsistency in timing of expression of variables was probably due to different initial conditions in each tank. The ecological meanings of the inconsistency in timing and overall consistency were discussed.