PHYTOPLANKTON BIOMASS AND PRODUCTIVITY IN THE AMAZON RIVER PLUME - CORRELATION WITH SEASONAL RIVER DISCHARGE

Phytoplankton biomass and primary productivity were assessed on the co ntinental shelf in the plume of the Amazon River during a series of cr uises conducted within periods of minimum, maximum, rising and falling river discharge. Chlorophyll concentrations were greatest (up to 25.5 mu g l(-1)) in a zone located outside the turbid, high nutrient, low salinity riverine waters but shoreward of the clear, high salinity, lo w nutrient waters. Vertical distributions of chlorophyll further delin eated the influences of these environmental regimes, with maximum chlo rophyll concentrations occurring in the upper 5 m of water columns cha racterized by reduced salinities and elevated nutrients at the surface and low nutrient, high salinity water below. Fluorescence was elevate d in the transition zone as a result of the phytoplankton standing sto cks, and was also elevated in low-salinity waters influenced by the Am azon outflow. The residual fluorescence was coupled to salinity but no t to chlorophyll, which suggested that it was related to dissolved org anic matter which originated in the Amazon. Primary productivity on th e continental shelf was greatest in the transition zone and occasional ly exceeded 8 g C m(-2) d(-1). Productivity in the turbid, nutrient-ri ch waters and the clear, offshore regions averaged 2.18 and 0.81 g C m (-2) d(-1), respectively. Phytoplankton photosynthesis in waters influ enced by the Amazon River appeared to be limited by low levels of avai lable irradiance inshore, whereas offshore it was nutrient-limited. Th e narrow zone of high production was supported by the riverine input o f nutrients and the dynamics of sediment flocculation. Removal of the inorganic sediment load was necessary to allow for adequate irradiance penetration to support photosynthesis.