Riverine influence on the tropical Atlantic Ocean biogeochemistry

L C Da Cunha,E T Buitenhuis

doi:10.5194/bg-10-6357-2013

Abstract

Abstract. 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 70° W–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 N-fixation 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 Tg C 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.

Highlights

The tropical Atlantic Ocean directly receives the discharge of the three largest rivers in the world: the Amazon River (0◦, 50◦ W), the Congo River (6◦ S, 12◦ E), and the Orinoco River (10◦ N, 63◦ W) (Fig. 1)
In the AFRICA scenario, like in the TODAY scenario, our results suggest an increase in primary production along the African coast, including the Gulf of Guinea and the outflow of the Congo River, with no changes occurring in the Western tropical Atlantic
For lack of data to the contrary, we have no reason to assume the export in an ecosystem that includes diatom-diazotroph association (DDA) would respond differently than one that does not. We warn against this shortcoming in a general way by drawing attention to the ecological resolution as a limiting factor of our model, even though PlankTOM10 is currently one of the most complex global biogeochemical models available. We conclude from this sensitivity modeling study that South American river nutrient inputs may have a larger impact on tropical Atlantic coastal primary and export production than African rivers

Summary

Introduction

The tropical Atlantic Ocean directly receives (i.e. not filtered by regional seas or enclosed coastal areas) the discharge of the three largest rivers in the world: the Amazon River (0◦, 50◦ W), the Congo River (6◦ S, 12◦ E), and the Orinoco River (10◦ N, 63◦ W) (Fig. 1). In this region, a weak Coriolis forcing leads to extended river plumes off the coast, creating nutrient-rich areas of optimal production in sites far removed from the coast (Corredor et al, 2003). The Amazon River discharge generates a plume that covers up to 2 × 106 km and is characterized by low salinity and low Published by Copernicus Publications on behalf of the European Geosciences Union

Methods

Results

Conclusion