Response of heterotrophic bacteria to the spring phytoplankton bloom in the Oyashio region

Temporal changes in abundance, growth and mortality of heterotrophic bacteria were investigated from frequent dilution experiments in the Oyashio region to evaluate their response to the spring phytoplankton bloom. Bacterial abundance fluctuated from 1.0 to 2.6×105 cells mL−1. Cell density was not different between the pre-bloom and bloom periods, and showed no clear temporal pattern. Potential growth (g′) and mortality (m) varied from 1.11 to 2.39 day−1 and from 1.21 to 2.16 day−1, respectively. Neither g′ nor m was correlated with water temperature or chlorophyll a. Apparent growth rate in unenriched 100% seawater incubations (go) was not limited by nutrients throughout the study period and was comparable to net growth rate (g′−m) estimates from dilution experiments. These results suggest that bacterial abundance is nearly steady-state in the pre-bloom and bloom period, because bacterial growth is nearly nutritionally saturated and balanced with high mortality.