Abstract
The Tropical Atlantic is facing a massive proliferation of Sargassum since 2011, with severe environmental and socioeconomic impacts. As a contribution to this proliferation, an increase in nutrient inputs from the tropical rivers, in response to climate and land use changes or increasing urbanization, has been often suggested and widely reported in the scientific and public literature. Here we discuss whether changes in river nutrient inputs could contribute to Sargassum proliferation in the recent years or drive its seasonal cycle. Using long-term in situ and satellite measurements of discharge, dissolved and particulate nutrients of the three world largest rivers (Amazon, Orinoco, Congo), we do not find clear evidences that nutrient fluxes may have massively increased over the last 15 years. Moreover, focusing on year 2017, we estimate that along the year only 10% of the Sargassum biomass occurred in regions under river plume influence. While deforestation and pollution are a reality of great concern, our results corroborate recent findings that hydrological changes are not the first order drivers of Sargassum proliferation. Besides, satellite observations suggest that the major Atlantic river plumes suffered a decrease of phytoplankton biomass in the last two decades. Reconciling these observations requires a better understanding of the nutrient sources that sustain Sargassum and phytoplankton growth in the region.
Highlights
The documents may come from teaching and research institutions in France or abroad, or from public or private research centers
The large-scale seasonal distribution of Sargassum for year 2017 is confronted to numerical experiments of river plume dispersal. We focused on this year because basin scale Sargassum fractional coverage observations from MODIS were available (Berline et al 2020), with concurrent observations carried out during two cruises in the Tropical Atlantic (Ody et al 2019)
The importance of the particulate P is in line with observations by Berner and Rao (1994) who conclude that the solubilization of P from bacterial decomposition of river-transported organic matter and desorption from ferric oxide/hydroxide may result in an effective flux of reactive P
Summary
Jean-Michel Martinez, et al. Ménard, Jean-Michel Martinez, et al To cite this version: Julien Jouanno, Jean-Sébastien Moquet, Léo Berline, Marie-Hélène Radenac, William Santini, et al. Evolution of the riverine nutrient export to the Tropical Atlantic over the last 15 years: is there a link with Sargassum proliferation?. The Tropical Atlantic receives the fresh and nutrient rich waters of the three largest rivers on the planet - in terms of flow (Amazon, 209 000 m3 s-1, Congo, 42 000 m3 s-1 and Orinoco, 35 000 m3 s-1), which alone represent 21% of the total global riverine flow (Milliman and Farnsworth, 2011) Their low-saline and productive plumes extend thousands of kilometers far offshore (Muller-Karger et al 1988, Signorini et al 1999). Year 2017 was the third most important year of the decade in terms of quantity of Sargassum (as inferred from time series in Wang et al 2019), with a seasonal pattern that closely mirrors the averaged seasonal pattern from Wang et al (2019)
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