On the Importance of Riverine Organic Matter for the Amazon Plume: A Modeling Study

The Amazon River is an important freshwater and nutrient source for the tropical Atlantic Ocean, sustaining large phytoplankton blooms that extend hundreds of kilometers from the river mouth. However, the main drivers of the productivity in the Amazon plume are still poorly understood. To address this question, a regional coupled ocean-biogeochemical configuration of the tropical Atlantic at 1/4° is used, as well as remote sensing observations of sea surface salinity and ocean color, which allow an improved model treatment of the organic matter of riverine origin. Results reveal that the offshore extension of the productive plume is mainly driven by the large fluxes of terrestrial dissolved organic matter (TDOM) supplied by the Amazon River. Two mechanisms by which TDOM influences plume extension have been identified. First, the colored fraction limits the light available for phytoplankton growth, thereby delaying nutrient consumption. Second, TDOM supplied by rivers has low lability and is therefore slowly remineralized, providing nutrients throughout the plume until far offshore. The inclusion of these two effects (shading and specific remineralization of TDOM) in the model allows for a better representation of Amazon plume productivity and offshore extension, something that is generally lacking in regional or global biogeochemical models.