Abstract
Abstract. A compilation of measurements of net community production (NCP) in the upper waters of the eastern subtropical North Atlantic had suggested net heterotrophic conditions, purportedly supported by the lateral export of organic carbon from the adjacent, highly productive Canary Upwelling System (CanUS). Here, we quantify and assess this lateral export using the Regional Ocean Modeling System (ROMS) coupled to a nutrient, phytoplankton, zooplankton, and detritus (NPZD) ecosystem model. We employ a new Atlantic telescopic grid with a strong refinement towards the northwestern African shelf to combine an eddy-resolving resolution in the CanUS with a full Atlantic basin perspective. Our climatologically forced simulation reveals an intense offshore flux of organic carbon that transports about 19 Tg C yr−1 away from the nearshore 100 km over the whole CanUS, amounting to more than a third of the NCP in this region. The offshore transport extends beyond 1500 km into the subtropical North Atlantic, adding organic carbon along the way to the upper 100 m at rates of between 8 and 34 % of the alongshore average NCP as a function of offshore distance. Although the divergence of this lateral export of organic carbon enhances local respiration, the upper 100 m layer in our model remains net autotrophic in the entire eastern subtropical North Atlantic. However, the vertical export of this organic carbon and its subsequent remineralization at depth makes the vertically integrated NCP strongly negative throughout this region, with the exception of a narrow band along the northwestern African shelf. The magnitude and efficiency of the lateral export varies substantially between the different subregions. In particular, the central coast near Cape Blanc is particularly efficient in collecting organic carbon on the shelf and subsequently transporting it offshore. In this central subregion, the offshore transport adds as much organic carbon as nearly 60 % of the local NCP to the upper 100 m, giving rise to a sharp peak of offshore respiration that extends to the middle of the gyre. Our modeled offshore transport of organic carbon is likely a lower-bound estimate due to our lack of full consideration of the contribution of dissolved organic carbon and that of particulate organic carbon stemming from the resuspension of sediments. But even in the absence of these contributions, our results emphasize the fundamental role of the lateral redistribution of the organic carbon for the maintenance of the heterotrophic activity in the open sea.
Highlights
Owing to the dominance of the sinking flux of particulate organic carbon (POC), the ocean’s biological pump is often simplified to a one-dimensional vertical process, consisting of the production of POC in the euphotic zone, its vertical export by gravitational sinking, and the remineralization of this organic carbon in the aphotic zone (e.g., Sarmiento and Gruber, 2006)
The simulation reveals in the long-term mean a strong onshore–offshore difference in the vertically integrated net community production (NCP), here NCP, i.e., primary production minus respiration and remineralization integrated from the bottom of the ocean up to the surface, including remineralization in the sediments (Fig. 6a)
The full water column NCP is negative across nearly the entire eastern subtropical North Atlantic, while only a narrow strip of less than 100 km along the northwestern African coast and a few offshore regions in the southern part of the domain have positive values
Summary
Owing to the dominance of the sinking flux of particulate organic carbon (POC), the ocean’s biological pump is often simplified to a one-dimensional vertical process, consisting of the production of POC in the euphotic zone, its vertical export by gravitational sinking, and the remineralization of this organic carbon in the aphotic zone (e.g., Sarmiento and Gruber, 2006). Reflecting this simplified view, most biogeochemical models currently used in the context of global climate models consider only the vertical export pathway for POC, neglecting its potential lateral transport by horizontal advection and diffusion (e.g., Aumont et al, 2003; Moore et al, 2004; Galbraith et al, 2010; Shigemitsu et al, 2012). As a consequence of these transport processes, the different organic carbon pools get redistributed laterally from regions of excess production to regions of intense remineralization and burial (Inthorn et al, 2006b; Hwang et al, 2008), giving rise to a complex three-dimensional pattern of organic carbon cycling
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