Effects of freshwater and seawater mixing on virio- and bacterioplankton in a tropical estuary

1. Estuaries are interesting sites to examine virus-bacteria interactions since they may vary gradually or abruptly in salinity, which presumably requires physiological, genetic and ecological adaptations. To explore such adjustments in tropical areas, freshwater and seawater samples were collected in the estuarine system of the River Senegal and experimentally subjected to abrupt mixing of seawater with freshwater and vice versa. 2. Production rates of freshwater bacteria (measured by the tritiated thymidine incorporation method) and of viruses (measured by the dilution method) sharply declined immediately after seawater addition, which was followed by a spectacular recovery of the surviving bacterial and viral populations within 24 and 48 h, respectively. 3. Conversely, neither marine bacteria nor viruses were significantly affected by mixing with freshwater. This suggests that the turbulent front between ascending tidal sea water and outwelling fluvial freshwater is a more favourable environment for marine bacterioplankton, which may take advantage of massive bursts of freshwater cells resulting from osmotic shock. 4. In both mixing experiments, the dynamics of virioplankton production followed that of the bacteria within 24 h, suggesting that viruses can rapidly respond to dramatic shifts in the abundance and community composition of bacterial hosts.