Possible transport pathways of diazotrophic Trichodesmium with Agulhas rings from the Indian Ocean into the Atlantic Ocean

Abstract Diazotrophic cyanobacteria such as Trichodesmium play a crucial role in the nitrogen budget of the oceans due to their capability to bind atmospheric nitrogen. Little is known about their interoceanic transport pathways and their distribution in upwelling regions. Trichodesmium has been detected using a Video Plankton Recorder (VPR) mounted on a remotely operated towed vehicle (TRIAXUS) in the southern and northern Benguela Upwelling System (BUS) in austral autumn, Feb/Mar 2019. The TRIAXUS, equipped with a CTD as well as fluorescence and nitrogen sensors, was towed at a speed of 8 kn on two onshore–offshore transects undulating between 5 and 200 m over distances of 249 km and 374 km, respectively. Trichodesmium was not detected near the coast in areas of fresh upwelling but was found in higher abundances offshore, mainly in subsurface water layers down to 80 m depth with elevated salinities on both transects. These salinity lenses can be related to northward moving eddies that have been detached from the warm and salty Agulhas Current. We provide the first indications that Trichodesmium can be transported with Agulhas rings from the Indian into the Atlantic Ocean.