Offshore export of shelf production in the Cape Blanc (Mauritania) giant filament as derived from coastal zone color scanner imagery

Abstract

Particulate organic carbon flux from the shelf to oceanic waters during upwelling between 19øN and 25øN off Mauritania has been analyzed using coastal zone color scanner (CZCS) imagery. Biomass in the giant filament has been estimated using surface pigment concentration registered by CZCS during time series in December 1983, March 1984, and October 1984. Evidence for active phytoplankton growth in offshore waters is presented. Formatron of the giant filament seems to be favored by the combination of a number of factors perhaps unique to the Mauritanian shelf. The large-scale circulation patterns promote offshore advection,• and the typically long period of wind forcing can move the upwelling center to the shelf break area, thus causing maximal phytoplankton growth rate to be achieved in oceanic waters. The relatively high pigment concentrations registered by CZCS in oceanic waters suggest that nutrients have not been depleted on the shelf and that in situ growth is occurring in deep waters. We have used a baroclinic, hydrodynamic model to estimate cross-slope flux of particulate organic carbon at the 200-m isobath and derive a mean value of about 1 x 105 g C s -1 during each of the CZCS time series. INTRODUCTION Biological export of carbon fixed during photosynthesis on continental shelves has been suggested as a major sink for the global carbon cycle [Deuser, 1979; Walsh et al., 1981; Walsh, 1983, 1989] although the general applicability of this hypothesis has been questioned [Rowe et al., 1986; Falkowski et al., 1988]. Compared with other continental shelf ecosystems, upwelling zones are distinguished by a high input of "new" nutrients and subsequent relatively high fertility. Because of this high fertility the quantity of organic material that can be exported from upwelling areas through loss to shelf sediments [D. J. ,Smith et al., 1983] or adjacent deep waters, or as a fishery yield, is 10 to 100 times higher than other ecosystems can support [Barber and Smith, 1981]. Major coastal upwelling zones occur along eastern oceanic boundaries where large-scale weather patterns produce consistent longshore wind stress inducing Ekman transport of surface waters offshore. The cooler, usually nutrient-rich oceanic waters which are upwelled onto the continental shelf to replace the surface waters promote high local primary production. In contrast to other coastal and oceanic ecosystems, which may be limited to a single major productivity event per year, the period and rate of primarYproduction are significantly greater in upwelling zones. Continental shelves where upwelling occurs are thus potentially important sites for conversion of dissolved inorganic carbon (in the newly upwelled waters as well as "old" shelf waters) to particulate organic form and export from the water column either to the shelf sediment, continental slope, or, by advection and settling processes, beneath the ocean thermocline [Wollast, 1990; McClain et al., 1986]. This last pathway, the so called biological pump, may be critical to the dynamics of...

Geochemical distribution patterns as indicators for productivity and terrigenous input off NW Africa

“…Large filaments such as those found at Cape Ghir, Cape Yubi, Cape Bojador, Cape Blanc and Mauritania (20.81N) are observable as colder jets and meanders extending several hundreds of kilometres offshore (Van Camp et al, 1991;Nykjaer and Van Camp, 1994;Barton et al, 1998;Davenport et al, 1999Davenport et al, , 2002Barton and Arístegui, 2004). Although upwelling occurs mostly on the shelf, these filaments play an important role in carbon cross-shelf transport and export to the deep ocean as has been described by Strub et al (1991), Gabric et al (1993) and Barton et al (1998). However, the zone between 1000 m and 2000 m water depth represents the locus of maximum concentrations of biogenic particulate matter in the surface sediments (Fütterer, 1983).…”

Off-shelf fluxes of labile materials by an upwelling filament in the NW Iberian Upwelling System

“…Filaments provide a mechanism whereby organic material produced in shelf waters can be transported hundreds of kilometres offshore into the ocean, and are features common to all the major coastal upwelling regions of the World Ocean (e.g. Brink & Cowles, 1991;Gabric et al, 1993). If the e-ratios determined here for the NW Iberian Upwelling System can be applied to other filaments, export fluxes have been dramatically underestimated because the contribution of DOM has not been included in the budgets.…”

“…Horizontal offshore transport of the export production is favoured, especially at sites where large upwelling filaments develop (e.g. Brink & Cowles, 1991;Gabric et al, 1993;Barton et al, 1998). However, even recent modelling approaches to horizontal carbon export by upwelling filaments (Gabric, Eifler, & Schrimpf, 1996;Moisan, Hofmann, & Haidvogel, 1996) have neglected the key contribution of fresh DOM.…”

Respiration in the mesopelagic and bathypelagic zones of the oceans

“…Mesoscale instabilities, such as filaments, may accelerate the transport of particulate and DOC from the coastal to the open ocean at upwelling regions (e.g. Gabric et al 1993;Barton et al 1998). Alvarez-Salgado et al (2001) estimated that up to 20% of new production in the NW Spain coastal upwelling region transforms into labile DOC, which is exported offshore to the adjacent open-ocean waters.…”