Regulation of protist grazing on bacterioplankton by hydrological conditions in coastal waters

Abstract

A series of dilution experiments were conducted along an environmental gradient in coastal waters during April to September 2016, in order to examine the regulation of protist grazing on bacterioplankton by physical processes. Our results suggested that the density gradient in the frontal zone and the pycnocline played a significant role in regulating protist grazing on bacterioplankton in marine environments. The highest bacterial growth rate (1.33 d−1) and grazing rate (1.05 d−1) occurred in the frontal zone along the inshore-offshore transect and in the deep chlorophyll maximum (DCM) that always matched with the pycnocline in terms of the vertical profile in offshore waters. Spatial variability in bacterial growth rate and grazing rate was related to the presence of the density gradient in the front and the DCM. The density gradient aggregated phytoplankton and bacterial cells, enhancing their biomass. Meanwhile, high phytoplankton biomass provided more labile organic carbon to bacterioplankton in the front and the DCM, improving bacterial growth. Hence, the density gradient improved bacterial abundance, consequently enhancing the efficiency of protist grazing. Our findings implied that the density gradient improved bacterial carbon converted to high trophic levels and altered carbon flow in microbial food web in marine environments.