Amazonian Contribution to Sargassum Enhancement in the Tropical Atlantic

The Amazon continental shelf (ACS) has a coastline of ~ 1000 km and is situated in a humid equatorial region with two distinct seasons, wet and dry, and two transitional climatic periods. This seasonality strongly affects the discharges of the Amazon and Pará rivers, with their plumes extending hundreds of kilometres into the Atlantic Ocean. A total of eight oceanographic cruises were conducted to investigate the dynamics of the inorganic nutrients dissolved in the surface layer of the water column of the ACS. An excess of nitrogen-N* (NH4+ = 0.01–9.30 µmol.l-1) was verified through the salinity gradient from the continent to the ACS, mainly during the period of maximum discharge of the Amazon and Tocantins-Pará rivers. The highest NH4+ concentrations supported the occurrence of macrophyte (Sargassum sp.) blooms in the mesohaline and oceanic regions. In the Sargassum belts we observed a N:P ratio = 60:1 in this region during wet period, while outside this region, the N:P ratio was 34:1. According to DIN vs. AOU ratio, we observe that heterotrophic processes prevail over productive processes, mainly in the mesohaline and low salinity regions