Specific growth rates and grazing mortality of chroococcoid cyanobacteria Synechococcus spp. in pelagic surface waters in the sea

Abstract

Specific growth rates and grazing mortality of phycoerythrin-dominant (PE-dominant) Synechococcus spp. populations have been evaluated in oceanic surface waters off Japan. Diel changes in cell numbers and frequency (patterns) of dividing cells (FDC) of the Synechococcus population indicated that cell divisions occurred throughout the day and that a maximum is reached at dusk. The specific growth rate of the Synechococcus population determined by diffusion chamber cultures in which the micrograzers were removed by 1-μm Nuclepore filters, and by the FDC method, was as high as 0.10·h −1 which can double the population size in < 1 day. The grazing mortality by the small-sized (2–5 μmn) colorless flagellates and by the other large (> 10 μm) and less abundant heterotrophs (mostly ciliates) determined by diffusion chamber cultures was 0.04 and 0.06 · h −1, respectively. As a result, the growth rates and grazing mortality are expected to be balanced and the PE-dominant Synechococcus maintained its population size at a certain low level in the study areas.