Abstract:
Waters off southeast Viet Nam experience strong monsoonal wind and
riverine buoyancy forcing during the summer months, which regulate
patterns in ocean circulation, nutrient regime, and planktonic biomass
and composition. Intensive phytoplankton blooms have been detected
along the southeast coast of Viet Nam with increasing frequency,
temporally coincident with the southwest monsoon in summer; however,
the processes that control phytoplankton growth and the changes in
assemblage composition remain uncertain. Hydrological profiles,
dissolved nutrients, particulate matter, and phytoplankton pigments were
measured off the coast of southeast Viet Nam during summer months in
2009 and 2010 to assess the temporal variations in oceanographic and
biological conditions, as well as the causative mechanisms in the control
of phytoplankton biomass and distributions. Seasonal climatologies of
forcing and responding variables were also determined from satellite
measurements. The results demonstrated that there was a correlation
between strong monsoonal winds and elevated pigment concentrations
during monsoon seasons, and that freshwater input played an important
role in generating stratification in the surface waters. Significant
differences in phytoplankton biomass and hydrological conditions were
consistently observed between July 2009 and August 2010, suggesting a
high level of temporal heterogeneity in both. Changes in nutrient regime,
especially in the reduction in NO3:NH4 ratio, were considered to be a
dominant control on the succession from diatom- to Phaeocystisdominated
assemblage off southeast Viet Nam.