Abstract: The distribution of phytoplankton
assemblages in response to physicochemical variables was assessed using
Spearman’s correlation and canonical correspondence analysis in four highly
turbid estuaries of the Mekong River (MKR) system in Ben Tre Province, Vietnam. During two surveys
(September 2017 and April 2018) at 20 sampling sites, a total of 162 species
were recorded, with a dominance of diatoms (> 50%). Phytoplankton abundance
varied from 3.01 × 105 to 11.85 × 105 cells/L, with the highest cell densities
found at the BL2 station in the rainy season, mainly attributed to
cyanobacteria during the rainy period, whereas diatoms were dominant in the dry
season. Chlorophyll-a concentration in this area was low and decreased from the
mouth stations. Similarity analysis distinguished the freshwater and
oligohaline regions (characterized by lower phytoplankton abundance in the
oligohaline areas) from the mesohaline and polyhaline zones (characterized by
higher phytoplankton density in the polyhaline sections), based on the salinity
gradient, which mostly explained the spatio-temporal distribution of
phytoplankton. In which, freshwater and oligohaline sites were dominated by the
diatom Coscinodiscus, the euglenoids Lepocinclis, and the green alga Scenedesmus in the rainy season, while mesohaline and polyhaline
stations were dominated only by diatom species (Thalassionema and Skeletonema) in the dry season. High salinity and total dissolved solutes,
and low nutrients are the major factors affecting phytoplankton growth and lead
to low density and low primary biomass in turbid estuarine systems like the MKR
estuaries. Our results further emphasize the importance of taking into account
of salinity in understanding seasonal changes of phytoplankton communities,
which can provide a valuable baseline data for ecological management strategies
in tropical estuarine ecosystems.