Distribution and sources of particulate organic matter in a mesoscale eddy dipole in the Mozambique Channel (south-western Indian Ocean): Insight from C and N stable isotopes

This study investigates eddy induced transport processes that affect the sources and sinks of organic matter in the Mozambique Channel (MC). Eddies generated in the MC interact with the continental shelf and have been hypothesized to act as conveyor belts that entrain coastal production offshore. A combination of natural isotope tracers, elemental composition and physico-chemical variables allowed distinguishing two sources of organic matter within the eddy dipole. Near the surface, coastal POM was entrained at the anti-cyclonic boundary and further transferred into the eddy. Anti-cyclonic surface POM containing coastal POM was downwelled into deeper layers where decomposition greatly affected its composition. In contrast, cyclonic water promoted new production, by upwelling nutrient-rich deep waters into the euphotic zone. This production circulated through the mixed layer of the cyclone and was possibly advected into the boundary. Boundary F(max) and surface waters shared characteristics with the cyclone and anti-cyclone respectively as the result of both import of primary production of cyclonic origin occurring at F(max) and entrainment of coastal biological material at the surface. In the overall low productivity environment of the MC, it is likely that the continuous migration of mesoscale eddies along the shelf plays an important ecological role both in enhancing pelagic production and transporting coastal production offshore.

► We study eddy processes affecting sources of POM in the Mozambique Channel.
► Coastal POM is entrained at the surface of boundary and transferred to the anti-cyclone.
► POM is affected by decomposition during downwelling of anti-cyclone surface waters.
► Cyclone promotes new primary production through upwelling of nutrient-rich deep water.
► Continuous chains of eddies along the shelf produce outburst of pelagic production.