An annual cycle of the offshore transport of organic matter and chlorophyll by an upwelling filament off Cape Ghir (NW Africa)

Abstract

The bi-monthly variability (from December 2008 to October 2009) of the net coastal-ocean export of particulate organic carbon (POC), dissolved organic carbon (DOC) and chlorophyll a (Chl a) has been studied in the Cape Ghir filament, a recurrent feature located in the NW African upwelling system. The estimated flux of excess total organic carbon (the non-refractory pools of DOC and POC) ranged from 4.6 x108 to 5x109 kg C y−1. DOC represented 65% of the excess organic carbon in February 2009 increasing up to a 80% during an upwelling event that took place in April 2009. Assuming these fluxes are representative of the range in a typical year, the yearly offshore transport of total organic carbon would represent at least 9% of the primary production in this area, reaching up to 57% in periods of intense upwelling events. Since the Cape Ghir filament may extend hundreds of km offshore, its seaward flux of organic carbon would contribute to the high microbial respiration rates reported from the nearby oligotrophic open ocean region. Our results highlight the importance of considering the offshore transport of organic matter channeled by filaments in regional carbon budgets, particularly in eastern boundary regions where upwelling filaments are numerous and recurrent throughout the year.