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We assess the role of riverine inputs of N, Si, Fe,
organic and inorganic C in the tropical Atlantic Ocean using
a global ocean biogeochemistry model. We use a standard
model scenario and three sensitivity tests to investigate
the role of total river nutrient and carbon inputs, as
well as the western (South American) and eastern (African)
river inputs on the tropical Atlantic Ocean biogeochemistry,
between 20 S–20 N and 70W–20 E. Increased nutrient
availability from river inputs in this area (compared to a sensitivity
scenario without river nutrient inputs, NO_RIVER)
leads to an increase in primary production (PP) and export
production (EP), mainly in the coastal ocean area (modeled
ocean area with bathymetry<200 m). Model results suggest
an enhanced N-fixation by diazotrophs on the tropical Atlantic
mainly in open ocean areas. The increased rate of Nfixation
in the TODAY scenario is proportional to the increase
in PP and EP relative to the NO_RIVER scenario, and
may support up to 14% of the coastal ocean export production.
Inputs from South American rivers have an impact in
coastal PP and EP two times higher than those from African
rivers. On the other hand, results suggest that the contribution
of African and South American rivers to the total increase
in open ocean PP and EP is similar. Considering the
amount of delivered nutrients (2–3 times less nutrients and
carbon inputs by African rivers) one concludes that African
riverine inputs may have a larger impact on the whole tropical
Atlantic Ocean biogeochemistry. This is probably due
to a combination of nutrient trapping in upwelling areas off
the large rivers’ outflows and shallow mixed layers in the
eastern tropical Atlantic, concomitantly to the differences in
delivered nutrient ratios leading to alleviation in limitation
conditions, mainly for diatoms. When river inputs are added
to the model, we estimate a modest decrease in open ocean
sea-air CO2 fluxes (−5.2 TgC a−1) and an increase in coastal
ocean CO2 fluxes, mainly provoked by the remineralization
of riverine organic matter delivered by the South American
rivers.
