Influence of the Amazon River discharge on the biogeography of phytoplankton communities in the western tropical north Atlantic

- The distribution of phytoplankton communities within the Amazon River plume during peak river discharge are described.
- Three distinct communities of phytoplankton were identified which separated largely on the basis of salinity gradients across the plume.
- Although salintiy has a substantial influence on phytoplankton communities in the plume, the biomass and composition of each community was controlled by the availability of inorganic nitrate and nitrite.

An Advanced Laser Fluorometer (ALF) capable of discriminating several phytoplankton pigment types was utilized in conjunction with microscopic data to map the distribution of phytoplankton communities in the Amazon River plume in May-June-2010, when discharge from the river was at its peak. Cluster analysis and Non-metric Multi-Dimensional Scaling (NMDS) helped distinguish three distinct biological communities that separated largely on the basis of salinity gradients across the plume. These three communities included an “estuarine type” comprised of a high biomass mixed population of diatoms, cryptophytes and green-water Synechococcus spp. located upstream of the plume, a “mesohaline type” made up largely of communities of Diatom-Diazotroph Associations (DDAs) and located in the northwestern region of the plume and an “oceanic type” in the oligotrophic waters outside of the plume made up of Trichodesmium and Synechococcus spp. Although salinity appeared to have a substantial influence on the distribution of different phytoplankton groups, ALF and microscopic measurements examined in the context of the hydro-chemical environment of the river plume, helped establish that the phytoplankton community structure and distribution were strongly controlled by inorganic nitrate plus nitrite (NO3 + NO2) availability whose concentrations were low throughout the plume. Towards the southern, low-salinity region of the plume, NO3 + NO2 supplied by the onshore flow of subsurface (∼80 m depth) water, ensured the continuous sustenance of the mixed phytoplankton bloom. The large drawdown of SiO3 and PO4 associated with this “estuarine type” mixed bloom at a magnitude comparable to that observed for DDAs in the mesohaline waters, leads us to contend that, diatoms, cryptophytes and Synechococcus spp., fueled by the offshore influx of nutrients also play an important role in the cycling of nutrients in the Amazon River plume.