Summer hydrography, nutrient stoichiometry, and phytoplankton distributions off Southeast Viet nam

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.