Possible transport pathway of diazotrophic Trichodesmium by Agulhas Leakage from the Indian into the Atlantic Ocean

TL;DR

The study suggests that Trichodesmium, a nitrogen-fixing cyanobacterium, may be transported from the Indian Ocean to the Atlantic Ocean via Agulhas Rings.

Contribution

The paper presents new evidence for a transport pathway of Trichodesmium via Agulhas Leakage from the Indian to the Atlantic Ocean.

Findings

Trichodesmium was found in offshore subsurface waters with elevated salinity, not in coastal upwelled waters.

Salinity lenses associated with northward-moving eddies suggest transport via Agulhas Rings.

Significant associations between Trichodesmium and salinity indicate a potential transport mechanism.

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 372 km, respectively. Trichodesmium was not detected near the coast in areas of freshly upwelled waters but was found in higher abundances offshore on both transects, mainly in subsurface water layers down to 80 m depth with elevated salinities. These salinity lenses can be related to northward moving eddies that most probably have been detached from the warm and salty Agulhas Current. Testing for interaction and species-habitat associations of Trichodesmium colonies with salinity yielded significant results, indicating that Trichodesmium may be transported with Agulhas Rings from the Indian Ocean into the Atlantic Ocean.

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