Influence of the Amazon and Pará Rivers on Water Composition and Phytoplankton Biomass on the Adjacent Shelf

In the context of the REVIZEE programme, an extensive sampling cruise was carried out in the northern section of the Brazilian Exclusive Economic Zone on board O.V. Antares (Operation North III). Several parameters were surveyed along the Amazon continental shelf during this cruise, such as horizontal and vertical distributions of temperature, salinity, dissolved oxygen, pH, dissolved inorganic nutrients, and phytoplankton biomass. Water with salinity below 33 psu and high silicate content was observed throughout the shelf, indicating the influence of the Amazon River plume up to a distance of 147 km from the coast; pH always remained alkaline. Water temperature was relatively homogeneous at the surface, with an average of 28°C. A permanent thermocline was detected in oceanic regions. Coastal waters were eutrophic, well oxygenated (maximum of 6.54 mL O2 L−1), and presented high chlorophyll a (maximum of 9.8 mg chlorophyll a m−3) and nutrient concentrations (up to 9.54 μmol L−1 NO3−, 0.89 μmol L−1 PO42−, and 108.59 μmol L−1 Si044−). Oceanic waters adjacent to the Amazon shelf presented high transparency (Secchi depth up to 35 m) and chlorophyll a (mean: 0.1 mg m−3) and low nutrient concentrations (maximum of 1.58 μmol L−1NO3−, 0.18 μmol L−1PO42−, and 5.92 μmol L−1Si044−). Cluster analysis performed with all data showed a clear separation between coastal and oceanic waters at the surface layer and at 1% light penetration layers. This separation into two groups of stations clearly represents areas with different degrees of influence from the discharges of the Amazon Basin. The Amazon River is very important to the nutrient budgets of the shelf.