Effects of salinity on bacterioplankton: field and microcosm experiments

Bacterial populations in a tidal estuary were monitored with the aim of investigating (i) the relationship between planktonic bacteria and salinity and (ii) the survival of allochthonous bacteria in microcosm experiments at different salinities. With the increase in salinity, bacterial numbers decreased in a curvilinear fashion, rather than monotonically. Maximal abundance for different bacterial groups was found between 7 and 22%₀ S. The salinity alone explained between 19 and 58% of bacterial variability. In microcosm experiments the adverse effect of salinity on the survival of freshwater bacteria and indicators of faecal contamination was particularly important. In freshwater and low salinity experiments grazing by protozoans had similar effects on the survival of allochthonous bacteria.