Distribution of Bacteria, Picophytoplankton, and Flagellates in the Mekong Delta

— For the first time in the Mekong delta (Vietnam), the abundance and biomass of the main components of the planktonic microbial food web, and namely: bacteria, picophytoplankton, and heterotrophic and phototrophic flagellates were determined; the features of their spatial distribution and the species diversity of heterotrophic flagellates were studied. In this productive polluted tropical river ecosystem, heterotrophic bacteria were large in size and their biomass reached high values characteristic of hypertrophic waters. They made the main contribution (73.8%, on average) to the formation of the total biomass of the planktonic microbial community. The contribution of picophytoplankton and heterotrophic and phototrophic flagellates was 20.5, 3.9, and 1.9%, respectively. Twenty-nine species and forms of heterotrophic flagellates belonging to eight large taxa and a group of uncertain taxonomic position were identified.