Possible Amazonian contribution to Sargassum enhancement on the Amazon Continental Shelf

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. 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 and their associations with Sargassum blooms. 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 coincided with the occurrence of macrophyte (Sargassum sp.) blooms in the mesohaline and oceanic regions. During this period the high values of NH₄⁺ accompany the low values of Sea Surface Salinity (SSS), indicating the advection of the Amazonian plume towards the Caribbean Sea. In the Sargassum belts we observed a N:P ratio = 60:1 during wet period, while outside this region, the N:P ratio was 34:1. According to DIN vs. AOU ratio, we observed that heterotrophic processes prevail over productive processes, mainly in the mesohaline and low salinity regions. Nitrogen concentrations from runoff, mainly from hydrographic basins, showed significant increases in the last decade, mainly due to anthropogenic factors such as agriculture, deforestation, livestock, urban sewage (domestic and industrial), and demographic growth of the population in the Tocantins-Pará Basin. Additionally, associations of diatoms and cyanobacteria provide more nitrogen in the mesohaline and oceanic regions that nourishes Sargassum blooms. Seasonal comparisons (October and April) of Sargassum belts with other studies, indicated that currents play a fundamental role in the transport of these macrophytes from the east edge (Africa) to the west edge (Brazil and French Guiana) towards the Caribbean Sea in the tropical Atlantic.