Transport of organic-walled dinoflagellate cysts in nepheloid layers off Cape Blanc (N-W Africa)

Abstract

Many aspects of to the vertical and lateral transport of organic matter particles produced in the upper ocean are still not clear. This study documents the production, vertical and lateral displacement of the particulate organic matter fraction formed by dinoflagellate cysts in nepheloid layers present off Cape Blanc in November 2015. The position and extent of cyst production in the upper water column has been assessed by collecting the export rain of cysts with floating sediment traps. This has been compared to cyst associations in different intermediate nepheloid layers in the water column along two onshore-offshore transects.We document that highest cyst export production took place at the rim of a newly formed upwelling eddy/filament. We observed lateral transport up to 130 km off the shelf break into the deeper ocean in the bottom nepheloid layers (BNL) and in intermediate nepheloid layer (INL). The depth of the INL gradually increased from 600 m closest to the shelf break to 1300 m at the most offshore station. Transport in the BNL was observed in the more onshore part between 750 and 2300 m water depth. Cyst associations in both layers were comparable, indicating a common source. The large abundance of living cysts in these layers suggest that this source had a recent origin rather than being the result of resuspension of older sediment. The cyst associations of these samples differed from those of a second INL deeper in the water column as well as from samples from the more offshore part of the BNL. This indicates that the particles in this deeper INL and the offshore part of the BNL had a different origin. This origin could not be identified therefore no conclusions were drawn with respect to the extent to which these particles have been transported vertically or laterally.The cyst associations in the surface sediments differed from those of the nepholoid layers and the upper water column and did not reflect the effects of lateral transport as registered from the water column. The surface sediments contained a higher proportion of cysts that are highly resistant to aerobic degradation than the samples from the water column. This indicates that the former were modified predominantly by species-specific post depositional degradation. Selective degradation altering the cyst associations was not observed for the nepheloid layers.