Variability in plankton community structure, metabolism, and vertical carbon fluxes along an upwelling filament (Cape Juby, NW Africa)

The variability in dissolved and particulate organic matter, plankton biomass, community structure and metabolism, and vertical carbon fluxes were studied at four stations (D ₁-D ₄), placed along a coastal-offshore gradient of an upwelling filament developed near Cape Juby (NW Africa). The filament was revealed as a complex and variable system in terms of its hydrological structure and distribution of biological properties. An offshore shift from large to small phytoplankton cells, as well as from higher to lower autotrophic biomass, was not paralleled by a similar gradient in particulate (POC) or dissolved (DOC) organic carbon. Rather, stations in the central part of the filament (D ₂ and D ₃) presented the highest organic matter concentrations. Autotrophic carbon (POC _Chl) accounted for 53% (onshore station, D ₁) to 27% (offshore station, D ₄) of total POC (assuming a carbon to chlorophyll ratio of 50), from which nano- and pico-phytoplankton biomasses (POC _{A < 10 μm}) represented 14% (D ₁) to 79% (D ₄) of POC _Chl. The biomass of small hetrotrophs (POC _{H < 10 μm}) was equivalent to POC _{A < 10 μm}, except at D ₁, where small autotrophs were less abundant. Dark community respiration (R _d) in the euphotic zone was in general high, almost equivalent to gross production (P _g), but decreasing offshore (D ₁-D ₄, from 108 to 41 mmol C m ^-2 d ^-1). POC sedimentation rates (POC _sed) below the euphotic zone ranged from 17 to 6 mmol C m ^-2 d ^-1. Only at D ₄ was a positive carbon balance observed: P _g-(R _d + POC _sed) = 42 mmol C m ^-2 d ^-1. Compared to other filament studies from the NE Atlantic coast, the Cape Juby filament presented lower sedimentation rates and higher respiration rates with respect to gross production. We suggest that this is caused by the recirculation of the filament water, induced by the presence of an associated cyclonic eddy, acting as a trapping mechanism for organic matter. The export capacity of the Cape Juby filament therefore would be constrained to the frequency of the interactions of the filament with island-induced eddies.