Regional variability in the abundance, type and activity of large particles in the Mesopelagic ("Twilight") Zone of the North Atlantic and its link to epipelagic biogeochemical provinces (M60/5)

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The Mesopelagic ( Twilight ) zone, ranging from below the upper mixed layer to ca. 1500 m depth, is one of the least explored regions of the water column. Considerable respiration, repackaging and remineralisation point to the role of an active biological community that is responsible for utilising over 80% of the organic material sedimenting from the epipelagic zone to the deep sea. Circumstantial evidence suggests that regional variations in the efficiency of remineralisation in the twilight zone is positively correlated with surface export production, which makes this an important region for net export by the biological pump. During the Meteor expedition M60/5, to the tropical and subtropical North Atlantic investigations of particles in the mesopelagic layers below regions of varying productivity and trophic status. Additionally, studies on regional differences in the diversity and distribution of filamentous and unicellular cyanobacteria, conducted by others on the cruise offer the unique opportunity to investigate direct export of diazotrophs in sinking particles. Deployments of a marine snow catcher and collection of mesopelagic particles was done with the major aims of determining regional variability in particle abundance, type and activity and relating these to differences in pelagic export systems as well as investigating the export of nitrogen fixing cyanobacteria in sedimenting aggregates.