Transport, respiration, and sequestration of organic carbon in the Canary current ecosystem : relevance within the global carbon cycle

Abstract

This thesis presents novel results focused on the dynamics of biogenic carbon within the eastern boundary of the North Atlantic subtropical gyre, a region of the Canary Current Ecosystem characterized by water mass entrainment, intermittent coastal upwelling, and episodical occurrence of mesoscales features such as eddies and filaments. First, a box model permits us to build an organic carbon budget through which we estimate a net respiration in the mesopelagic layer (i.e., 100 to 1500 m) of 1.6 mmol C m-2 d-1 in this particular region. The study shows that a major fraction of these rates is supported by dissolved organic carbon (DOC) partly exported to depth by the entrainment of central waters to intermediate levels as a consequence of the formation of Mediterranean Water (MW) in the North Atlantic. The computed estimates represent less to half the oxygen utilization rates (OUR) reported for the mesopelagic waters of the subtropical North Atlantic. These differences could be in part explained by mesoscale activity within the region. To quantify the contribution of mesoscale features we focus on the upwelling filament off Cape Ghir (~30.5º N) that lies within the box region. An observational study off Cape Ghir reveals that the offshore flux of the non-refractory total organic carbon is 2.1 x 109 kg C-1. Assuming this flux is representative of a typical year, the yearly offshore net transport of total organic carbon would represent at least 29% of the primary production associated with the coastal upwelling. Nonetheless, mesoscale processes at the periphery of the transition zone and the onshore fluxes might recirculate the exported organic matter. Hence, new approaches combining field data with fine-scale numerical solutions should be required for a better characterization of the filament-driven export and its variability. For that reason, a climatological physical solution of the Regional Ocean Modeling System (ROMS) is coupled to a biogeochemical model of the Pelagic Interactions Scheme for Carbon and ecosystem Studies (PISCES) to investigate the complex three-dimensional structure and dynamics of the organic carbon within the filament off Cape Ghir and, comparatively, the filament off Cape Juby (~27.0º N). The work shows that both filaments contributes significantly to export year round a major part of the productivity associated with the coastal upwelling to the open ocean. These results illustrate that, when considering the regional carbon budgets of eastern boundary regions, it is imperative to take into account the offshore transport of organic matter in the numerous and recurrent upwelling filaments.