The impact of latitude and longitude on isotopic ratios (δ15N and δ13C) on the three morphotypes of Sargassum were analyzed from data collected during two oceanographic Atlantic cruises in 2017 and supplemented by coastal sampling at West Indies. Longitude and morphotypes have an influence on the δ13C content. The δ13C is lower in S. fluitans III (-17.5‰) than in S. natans I and VIII (-15.2‰). The variation of δ13C varies according to the state of the sea and the presence of large rafts which influence the absorption of CO2 and HCO3-. The δ15N levels are very negative and do not differ from east to west between the morphotypes. This show the importance of diazotrophs in nitrogen fixation. The δ15N levels are higher towards the coasts or in the up-welling zones. Ammonium from excretion by metazoans is al so a source of nitrogen for Sargassum off Africa. Whatever the position of the Sargassum in the ocean, the nitrogen is not limiting and comes from different sources, diazotrophy, terrigenous inputs, upwellings and contribution by metazoans associated with rafts.

Primary Presenter: Thierry THIBAUT, Aix-Marseille University (thierry.thibaut@univ-amu.fr)

Authors:

Thierry Thibaut,   ()

Baptiste Le Bourg,   ()

François Le Loch',   ()

Aurélie Blanfuné,   ()

Sandrine Ruitton,   ()

Valérie Stiger-Pouvreau,   ()

Solene Connan,   ()

Frédéric Menard,   ()