Investigation of time series stations along a productivity gradient north of the Canary Islands

Three stations along a productivity gradient north of the Canary Islands were investigated for surface-water properties, particle flux, and composition (biogenic and lithogenic components, and stable nitrogen isotope composition, delta15N) and export production. Investigation sites along the east-west transect off the NW African upwelling margin included the European Station for Time-Series in the Ocean, Canary Islands (ESTOC), one location contiguous to the NW African upwelling zone in the Eastern Boundary Current (EBC) and one station north of the island La Palma (LP). The seasonality of surface-water properties along the transect was mainly influenced by the winter cooling and simultaneous phytoplankton maximum and, in addition at EBC, by nearby upwelling. Accordingly, particle flux and composition along the transect were closely linked to the winter bloom sedimentation and upwelling related enhanced plankton biomass stemming from the primary upwelling and the Cape Yubi filament at EBC. During all seasons, particle flux was highest at EBC and had the highest contribution of biogenic opal and lithogenic components, and the lowest delta15N compared to the offshore stations. But contrary to what would be expected from the productivity gradient, particle flux did not decrease from ESTOC to LP. Below the upper several hundred meters, particle flux was enhanced by additional particle input along the entire transect, manifested by an increase of flux with depth and lower delta15N values. We offer a scenario in which intermediate nepheloid layers originating from the primary upwelling as well as particle dispersion from upwelling filaments, mainly the Cape Ghir filament, impact on the trap stations as far as 700 km into the open ocean. This study contributes to our understanding of the poorly resolved biogeochemical transition between the productive shelf and subtropical gyre provinces.

Concentration of particulate matter and its dispersion factor in surface waters of the Canary upweling region, East Atlantic

Particulate matter concentration and water temperature at 5 m depth level are compared in the Canary upwelling region to the east of the Cape Blanc. It was found that accumulation of particulate matter was timed to hydrofrontal zones. Particle size distributions for particulate matter obtained using the Coulter counter agree with the hyperbolic law (of the Junge type) with double values for the size parameter, which changes for particle diameters of 5-6 microns. Average values for the size parameter in the region of the upwelling are significantly lower than in the open ocean. Specific surface of particulate matter associated with reactivity differs significantly on different sides of the upwelling front and increases beyond the upwelling.