Viral lysis and nanoflagellate grazing as factors controlling diel variations of Synechococcus spp. summer abundance in coastal waters of Taiwan

Abstract

Diel variations in the protozoan grazing and viral-mediated mortality of Synechococcus spp. were simultaneously estimated using a modified dilution method in the coastal waters off Taiwan during 4 cruises in summer 2011. During the study period, the abundance of Synechococcus spp. showed a clear diel cycle and ranged from 1.2 x 10(4) cells ml(-1) at 08:00 h to 9.8 x 10(4) cells ml(-1) at midnight. The frequency of dividing cell (FDC) values were highest between afternoon and dusk and decreased to <10% between 20:00 and 08:00 h. Synechococcus spp. specific growth rates varied during the experimental period from 0.025 to 0.033 h(-1) and 0.050 to 0.085 h(-1) at daytime and nighttime, respectively. Rates of Synechococcus spp. mortality due to nanoflagellate grazing averaged 0.031 h(-1) during daytime and 0.041 h(-1) at night. The estimated mortality due to viral lysis was between 0.011 and 0.019 h(-1) during daytime and between 0.026 and 0.065 h(-1) at night. Nanoflagellate grazing was the dominant cause of Synechococcus spp. mortality during daytime; however, mortality from viral lysis and nanoflagellate grazing was more balanced at night.