On the long-range offshore transport of organic carbon from the Canary Upwelling System to the open North Atlantic
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 (Ca...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-07-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/14/3337/2017/bg-14-3337-2017.pdf |
| Summary: | 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<sup>−1</sup> 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. |
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| ISSN: | 1726-4170 1726-4189 |