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Keywords:

  • suspended POC;
  • lateral transport;
  • subtropical northeast Atlantic

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

[1] We have estimated the lateral transport and consumption, from surface to 3000 m, of suspended particulate organic carbon (POC), through a box model approach, in the Canary Current region (subtropical northeast Atlantic). Our results show that lateral POC fluxes are up to 3 orders of magnitude higher than vertical fluxes. In the mesopelagic ocean, the central waters (100–700 m) presented a net carbon consumption of 8.51 × 108 mol C d−1 with the highest POC entering through the more coastal section. This lateral flux accounted for 28–59% of the total mesopelagic respiration (R), on the basis of lower and upper case scenarios of vertical POC inputs and dissolved organic carbon contribution to R. We suggest that boundary currents may support higher lateral export of coastally produced POC than previously assumed. A large fraction of this POC would, however, be remineralized in the upper 1000 m instead of being transported to the ocean interior.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

[2] One of the interests of boundary currents in the global ocean context is the role they may play in the transport and remineralization of organic matter produced in coastal waters of continental margins. Several studies have suggested that margins may export significant amounts of organic matter to the ocean interior, which are not considered in global ocean biogeochemical models [e.g., Walsh, 1991; Falkowski et al., 1994; Santschi et al., 1999; Liu et al., 2000; Wollast and Chou, 2001; Arístegui et al., 2005a; Ducklow and McCallister, 2005; Inthorn et al., 2006]. However, the overall exchange rate of organic matter between the coast and the open ocean remains a matter of speculation since the lateral transport of organic matter, rather than being directly measured, has being estimated by mass balance approaches, frequently solely on the basis of sinking particles [Walsh, 1991; Liu et al., 2000; Wollast and Chou, 2001; Ducklow and McCallister, 2005]. Bauer and Druffel [1998], comparing the natural radiocarbon abundance in two coastal and open ocean profiles in the water column, found that continental slope and rise waters of the North American coasts contained both dissolved (DOC) and suspended particulate (POCsusp) organic carbon concurrently older and in higher concentrations that in the adjacent subtropical gyres of the Atlantic and Pacific Oceans. These results led the authors to conclude from their study that the POCsusp inputs from ocean margins to the ocean interior could be more than an order of magnitude greater than inputs of recently produced organic carbon derived from the surface ocean. In spite of their observations, and the general knowledge that the suspended POM pool is quantitatively far larger than the sinking pool [McCave, 1984; Kepkay, 2000; Verdugo et al., 2004], the construction of ocean carbon budgets is still largely based on vertical fluxes of sinking POM (POMsink) collected with sediment traps. However, according to the abundance of suspended material, one would expect that a significant amount of excess suspended or low-sinking-rate particles, not remineralized on the continental margins, could be exported to the open ocean. Indeed, the analysis of POMsink collected with sediment traps deployed across the path of the Canary Current, revealed that most of the particulate material collected in the deeper traps proceeded from the NW Africa coastal upwelling system [Neuer et al., 2002a; Abrantes et al., 2002]. These particles traveled as far as 700 km off the coast, where the furthermost trap was deployed, suggesting a significant lateral transport of particles with low sinking rates from the continental margin to the open ocean.

[3] The exchange of material between margins and the open ocean would presumably be particularly intense along eastern boundary currents, because of the high productivity of the upwelling regions and the high mesoscale variability of their coastal transition zones (CTZ), which help enhance the exchange of shelf waters with the open ocean. During the past two decades intense effort has been focused on complex multidisciplinary programs along the CTZ of eastern boundary regions, like the California Current [Brink and Cowles, 1991] and the Canary Current [Barton et al., 1998; Barton and Arístegui, 2004] in the northern hemisphere. However, most of this research was restricted to the near-surface waters, looking at fluxes [e.g., Álvarez-Salgado et al., 2007] and variability [e.g., Basterretxea and Arístegui, 2000; Arístegui et al., 2005a] at the mesoscale level, but ignoring the deep-water transport of organic matter to the ocean interior.

[4] In this study we have estimated the horizontal transport and consumption, from surface to 3000 m depth, of particulate organic carbon (POC) collected with oceanographic bottles, assumed to be suspended in the water column or having very low sedimentation rates. We aimed to evaluate if the Canary Current and its underlying intermediate and deep waters act as links or sinks of organic matter transported from the NW African coast to the subtropical Gyre. The analysis was performed through a box model approach, with physical boundaries extending from 20° to 29°10′N and 20°35′ to 26°W (1000 × 600 Km) in the Canary Current region (subtropical northeast Atlantic Ocean), during a low-productivity period in the year. To our knowledge this is the first effort to directly estimate the lateral transport of POMsusp across an eastern boundary current toward the ocean interior.

2. Data and Methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

[5] In September 2003, the R/V Thalassa carried out a high-density hydrographic survey along the path of the Canary Current (CORICA cruise). It consisted of four sections shaping a box with a total of 51 hydrographic (30 of them biogeochemical) stations (Figure 1). Conductivity, temperature and depth were recorded with a SeaBird 911 + CTD. The temperature and pressure sensors were calibrated at the SeaBird factory before the cruise. Salinity calibrations were carried out on board with a Guildline AUTOSAL model 8400 B salinometer (see Hernández-Guerra et al. [2005] for further details).

image

Figure 1. Hydrographic box: black dots indicate the position of CTD stations. Locations of biogeochemical stations are circled. The full box was sampled between 7 and 29 September 2003.

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[6] At the biogeochemical stations (Figure 1), discrete samples for particulate organic matter (POM) were obtained at selected depths from surface to 3000 m (5, 150, 300, 500, 700, 1000, 1200, 1500, 2000, 2500 and 3000 m), by means of a rosette sampler equipped with 24 10 L Niskin bottles. Water samples (2–6 L) for particulate organic carbon and nitrogen (POC and PON, respectively) were collected and filtered onto precombusted (450°C, 12 h) 25 mm Whatman GF/F filters. The filters were wrapped in precombusted aluminum foil and frozen at −20°C until processed. In the laboratory, the filters were thawed and dried overnight at 55°C, then placed overnight in a desiccator saturated with HCl fumes, dried again with silica gel and packed in nickel sleeves. The carbon analyses were performed with a Perkin-Elmer 2400 CHN elemental analyzer [UNESCO, 1994]. The DOC adsorption onto GF/F filters was subtracted from samples to avoid the overestimation of POC [Turnewitsch et al., 2007]. DOC adsorption onto the filters ranged from 0.6 to 2 μmol C per 25 mm diameter GF/F filter (about 3.2 cm2 of exposed filter), being similar to the blanks reported by Moran et al. [1999] and Turnewitsch et al. [2007].

[7] In order to quantify the transport of organic matter to the open ocean, and to investigate the relative importance of the remineralization processes versus the overall transport in the Canary Current, we used a box model approach. For this purpose we selected a grid box across the path of the Canary Current, which was spaced from the continental shelf about 250–450 miles at its eastern section. The reason for this distancing was to avoid the large mesoscale variability, in the form of eddies and filaments close to the upwelling jet, which would impede to estimate with accuracy the mass transport fluxes. Hence we assume that a large fraction of organic matter exported from the coast could be already remineralized before entering in our box.

[8] The geostrophic velocities were obtained integrating the thermal wind equations, considering the neutral density level γn = 28.072 Kg m−3 as the reference level of no motion. This level generally occurs at the 3000 m isobath, along the interface separating Middle North Atlantic Deep Water (MNADW) and Lower North Atlantic Deep Water (LNADW). The choice of the reference level of no motion at γn = 28.072 Kg m−3 follows the study of Ganachaud [2003] for the North Atlantic. The water column was divided into a number of layers on the basis of the neutral density that roughly separates different water masses. The upper four layers coincide with the thermocline waters, the next ones with intermediate waters, and the lowest layers with the NADW (MNADW and LNADW). The Ekman transport was added to the shallowest layer (see Hernández-Guerra et al. [2005] for further details). Lateral transports were calculated as Tij = ρij AijVij, where i, j stand for each pair station and layer, respectively, ρij is the density, Aij is the area and Vij is the geostrophic velocity. Since ∑Ti = 0, for each layer, it was not necessary to apply an inverse box model to estimate the geostrophic flow.

[9] POC concentrations were linearly interpolated in neutral density layers and used along with the mass transport to obtain the perpendicular POC flux.

3. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

3.1. POM Concentrations

[10] In the north transect (29°10′N) particulate organic carbon (POC) concentrations at stations to the west of 22°W decreased from about 4 μmol L−1 at surface to 1 μmol L−1 below 1000 m, whereas the eastern stations presented relatively higher and more constant concentrations (3 μmol L−1) throughout the water column (Figure 2a). This resulted in a marked east-west zonal distribution, particularly noticeably below 500 m depth. At 25°W an extremely high POC concentration (15.1 μmol L−1) was found in the interface layer between the North Atlantic Central Water (NACW) and the Antarctic Intermediate Waters (AAIW) and Mediterranean Intermediate Waters (MW). This value coincided with a sharp change in the density gradient at about 700 m (data not shown), suggesting passive accumulation of refractory material (C/N = 25). The C/N molar ratios show an opposite zonal trend than the POC, with lower average ratio (10.5 ± 2.3; n = 55) between 20°W and 22°W and higher (13.5 ± 3.5; n = 56) west of this longitude. It is noteworthy to observe that both the C/N and POC frontal gradients extended across different water masses in depth, down to at least 3000 m.

image

Figure 2. POC concentrations and C/N ratios along each section according to neutral density layers: (a) north, (b) south, (c) west, and (d) east transects. Locations and depths of sampling for POC are shown with black dots.

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[11] The south transect (20°N) showed variable but lower POC concentrations in comparison with the north transect, except in the upper 150 m where they reached 6 μmol L−1 west of 22°W (Figure 2b). The variability in the surface POC distribution and C/N ratios reflects the intersection of the south transect across the irregular meandering of the frontal system, caused by the confluence of the NACW and South Atlantic Central Water (SACW) and their interaction with the giant upwelling filament of Cape Blanc [Hernández-Guerra et al., 2005]. The zonal POC gradient at deep layers seen in the north transect is however not observed in this section. Below 500 m, POM concentrations were very low (<1 μmol L−1 POC), presenting high average (13.8 ± 2.2, n = 35) C/N ratios.

[12] Along the west transect (26°W), POC concentrations were in general >4 μmol L−1 in the upper 200 m of the section, decreasing sharply with depth (<1.5 μmol L−1 below 1000 m) (Figure 2c). At 21–23°N POC concentrations were, however, higher in the upper 1000 m than in the rest of the section, showing also lower C/N ratios. This region coincided with the area where the Canary Current turned to the southwest before flowing equatorward to feed the North Equatorial Current [Hernández-Guerra et al., 2005].

[13] The east transect (20°35′W) intersected two areas where high POC concentrations were measured throughout the whole water column (Figure 2d): one at 21°N, coinciding with the presence of the Cape Blanc giant filament, and the second north of 28°N, close to the Canary Islands, where the presence of a Mediterranean eddy (meddy) was reported [Hernández-Guerra et al., 2005]. At 21°N, C/N ratios were the lowest values measured along the section (<10), suggesting a more labile origin of the organic matter, because of the higher productivity of the Cape Blanc waters.

3.2. POC Transport

3.2.1. Mass Transport

[14] Figure 3 illustrates the integrated mass transport for each section using the initial reference level of no motion at γn = 28.072 Kg m−3 for geostrophy, and adding the Ekman transport in the first layer. As observed in Figure 3, the patterns of circulation for these layers are different depending on the section. The north and south transects present a similar pattern, with a northward and southward flow, respectively, at all layers, reaching a maximum transport value (1 × 109 kg s−1) at intermediate waters (27.38–27.922 neutral density layers). Both the east and west transects show, however a considerable westward flow (11 × 109 kg s−1) at the thermocline waters (Surface-27.38), whereas the east transect shows also a significant westward flow (4 × 109 kg s−1) at intermediate waters. Remarkably, the most important water input within the box took place along the east transect, being a coastal to open ocean flow. Considering all layers together, the net integrated transport was almost zero, indicating that the mass transport was in balance inside the box.

image

Figure 3. Integrated mass as a function of density layer for the north (black dash-dotted line), south (gray dashed line), west (gray dash-dotted line), and east (black dashed line) transects with their net transport (black solid line). For each transect, positive and negative values mean outputs and inputs, respectively. The sign of the net transport is positive (negative) for divergence (convergence) flow out of (into) the box.

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3.2.2. Lateral POC Flux

[15] POC fluxes were calculated excluding stations 24 (the easternmost station of the north transect), and the extremely high value observed at 700 m at station 34, since it would overestimate the average flux. Station 24 intersected part of a meddy and hence the fluxes were biased, since the flux at that station was northward albeit the eddy was drifting southwestward [Hernández-Guerra et al., 2005]. The resulting average transports of suspended POC for each section and water density layer are depicted in Figure 4 and Table 1. In the north transect the integrated POC transport through the surface layers was of 2.15 × 108 mol C d−1 southward, at a rate of 2.86 mol C m−2 d−1. In the NACW a total of 5.37 × 108 mol d−1 of POC was also transported southward at a rate of 1.49 mol C m−2 d−1. However, at intermediate (MW and AAIW) and deep (NADW) waters 4.11and 1.77 × 108 mol d−1 of POC was transported northward at rates of 0.76 and 0.23 mol C m−2 d−1, respectively. The south transect followed the same pattern of circulation as the north one. Surface and central waters transported southward 0.77 and 1.97 × 108 mol d−1 of POC at rates of 1.14 and 1.55 mol C m−2 d−1, while intermediate and deep waters transported 0.34 and 0.10 × 108 mol d−1 of POC at rates of 0.062 and 0.01 mol C m−2 d−1, to the north. The surface waters along the west and east transects showed a westward transport of 11.62 and 15.33 × 108 mol C d−1, at rates of 7.1 and 11.39 mol C m−2 d−1, respectively. These estimates are much higher both in absolute terms and rates than in the north and south transects. The central waters also transported POC to the west (7.91 × 108 mol C d−1 and 13.02 × 108 mol C d−1 across 26°W and 20°W), although the average rate transport was considerably higher at 20°W (2.2 mol C m−2 d−1) than at 26°W (1.5 mol C m−2 d−1). The intermediate waters also displayed a westward component, transporting 3.24 and 2.85 × 108 mol d−1 of POC at rates of 0.32 and 0.3 mol C m−2 d−1 in the east and west sections, respectively. These transports were less intense than the southward component observed at intermediate waters in the north and south transects. At the NADW layers, the transports (0.46 × 108 mol C d−1 for the east and 1.54 × 108 mol C d−1 for the west) and rates (0.05 mol C m−2 d−1 for the east and 0.12 mol C m−2 d−1 for the west) were low, like in the other sections, but in this case the flows were always toward the interior of the box. In summary, the meridional POC circulation in this area may be divided into a southward transport between surface and the bottom of the central waters and a northward transport below the central waters. The zonal POC circulation was dominated however by a significant coastal-open ocean transport affecting both the surface and central waters.

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Figure 4. POC fluxes (108 mol C d−1) as a function of neutral density layers at each transect. Negative values for POC transport indicate inputs, and positive values indicate outputs. The y axes are labeled on the left by the neutral density and on the right by the average depth of the interfaces of each layer.

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Table 1. POC Fluxes in the Different Water Masses for the North, South, West, and East Transectsa
Water MassesNorth TransectSouth TransectWest TransectEast TransectBalance
FluxRateFluxRateFluxRateFluxRate
  • a

    POC fluxes are in 108 mol C d−1. The rates per unit area (mol C m2 d−1) are also shown. POC flux is positive (negative) for divergence (convergence) flow out of (into) the box.

Surface waters−2.152.860.771.1411.627.1−15.3311.39−5.09
NACW/SACW−5.371.491.971.557.911.5−13.022.2−8.51
MW and AAIW4.110.76−0.340.0622.850.3−3.240.323.38
NADW1.770.23−0.100.01−1.540.12−0.460.05−0.33

[16] Mass balance calculations in the upper surface water layer (above the neutral surface of 26.44) and the central waters (26.44–27.38) resulted in negative balances of 5.09 × 108 mol C d−1 and 8.51 × 108 mol C d−1, respectively (Table 1), with the maximum transport concentrated through the more coastal section (20°W). Intermediate waters (27.38–27.922) presented lower transports, except in the north transect. The overall balance at this depth layer yielded a positive carbon export of 3.38 × 108 mol C d−1, with most of the transport channeled through the north section. Interestingly, the exported carbon is not transported from the south, where POC values at intermediate layers are low. Rather it seems to be transported from the east, and particularly from the Cape Blanc and Canary Islands regions, as suggested by the high POC values observed at these latitudes along the eastern transect. Finally, in the deep waters (27.922–28.072), the suspended POC transport was almost negligible.

4. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

4.1. Coastal Ocean Gradients in Suspended POM

[17] Particles in the ocean exist in a continuum of sizes, but two classes are frequently recognized: suspended and sinking matter. They are distinguished operationally by the sampling method used to collect them: suspended matter with oceanographic bottles or large-volume in situ pumps [e.g., Turnewitsch et al., 2007], and sinking matter with sediment traps. Nevertheless, bottles may also trap particles with moderate to fast sedimentation rates [Gardner, 1977], whose contribution to the overall POM will depend on the relative importance of the sinking pool versus the suspended pool. The majority of POM in the water column is thought, however, to be formed by fine suspended material or particles with almost negligible sedimentation rates compared with horizontal fluxes [McCave, 1984; Kepkay, 2000; Verdugo et al., 2004]. These particles, referred to here as POCsusp, would be characterized by a conservative behavior in the water column undergoing lateral transport [Brun-Cottan, 1976].

[18] In this study we have assumed that all POM collected with bottles represents suspended material or particles with very low sedimentation rates, susceptible of being transported horizontally. Strong evidence supports this hypothesis. Recent studies south of the Canary Islands, based on particle settling velocities using IRSC (Indented Rotating Sphere Carousel) traps [Peterson et al., 1993] during a 6 month period (summer to autumn 2005), indicate that about 60% of the POM collected in the traps have low sinking rates (<5 m d−1), allowing particles to be laterally transported I. J. Alonso-González, unpublished data, 2007). Comparatively, the average POC sinking velocity recorded using the same sediment traps at the DYFAMED station in the Mediterranean Sea, is of 350 m d−1 [Peterson et al., 2005; Armstrong et al., 2009]. The reason for the low sedimentation rates in the Canary region is still a matter of debate [e.g., Arístegui et al., 2003], but would explain the low export ratio with respect to the f ratio, reported by Neuer et al. [2002b] at the ESTOC station, north of the Canary Islands, compared to the Bermuda Time Series Station (BATS). Paradoxically, the POM concentrations collected with bottles in the water column are much higher in the Canary region. Indeed, POC at the BATS [Steinberg et al., 2001], like in other open ocean regions [e.g., Menzel and Goering, 1966], decrease exponentially with depth, reaching values typically <1 μmol L−1 at depths >200 m. In our study, POC concentrations in the deep ocean (down to 3000 m) were >2 μmol L−1 in the eastern sector of the box, decreasing toward the open ocean. An intermediate situation (range 1–3 μmol L−1 POC at depths >200 m) is found in the Azores region [Vezzulli et al., 2002], a transition zone between the Canary Current and the central waters of the North Atlantic subtropical Gyre (NASG).

[19] Our results agree with the autumn values reported by Neuer et al. [2007] for the 200–1000 m layer at the ESTOC, which were the lowest recorded during the annual cycle. The highest average POM values in the upper 1000 m (as high as 8 μmol L−1) were reported for the spring months. No measurements were, however, carried out during summer [Neuer et al., 2007], when the upwelling activity, and hence the potential offshore export, is higher. Indeed, POC concentrations measured between the Canary Islands (28°N) and Cape Blanc (21°N), along four cruises carried out during spring (2003) and summer (2002), reached typical average concentrations of about 6 μmol L−1 down to at least 2000 m J. C. Vilas, personal communication, 2007), without any indication of an exponential decrease with depth. Thus, lateral transport of organic matter generated over the NW Africa continental shelf may largely contribute to the POMsusp observed in the Canary Current region. The transport would be more pronounced during the periods of higher upwelling intensity, decreasing the concentrations toward the open ocean because of remineralization processes. Vilas et al. [2009] reached the same conclusion after studying the distribution of POM around Seine, a seamount placed east of Madeira Island, in the Canary Current. These authors observed peaks of POC in the 200–1000 m layer at the stations closer to the continental shelf and coinciding with the highest activity of the upwelling system.

4.2. Offshore Suspended Organic Carbon Pumping to the Interior of the North Atlantic Ocean

[20] To estimate the net lateral flux of POCsusp in the mesopelagic and bathypelagic waters, we first subtracted the open ocean “baseline” POC concentration, obtained from averaging the monthly POC profiles in each depth layer at the Bermuda Atlantic Time-series (BATS) station, from the POC concentrations at the east and west transects. The BATS station, placed at the center of the NASG, presented a typical exponential decrease of POC with depth, reaching values <1μM below 200 m. In a comparative study of three subtropical time series stations (BATS, HOT and ESTOC), Neuer et al. [2002b] concluded that the annually integrated net primary production was similar at all three sites, with slightly lower values at ESTOC. However, the POC sinking flux (measured with drifting traps at about 150–200 m) was approximately 2–3 mmol C m−2 d−1 at BATS (Bermuda) and HOT (north of Hawaii), while at ESTOC (north of the Canary Islands) was only about 20% of these values (0.55 mmol C m−2 d−1). Recent results J. Arístegui, unpublished data, 2007; Alonso-González, unpublished data, 2007) of sinking POC, collected with similar traps at 150–200 m, from 30N to 21N, outside the influence of the intense eddy field, indicate that the POC flux varies on average from 0.7 to 2 mmol C m−2 d−1 from spring to autumn, increasing to 3–4 mmol C m−2 d−1 during the late winter bloom. Since sinking fluxes in our region of study are not higher than in BATS, we consider that the BATS POC would characterize the typical POC profile in the western Canary region, assuming only vertical flux of POC.

[21] Lateral fluxes were particularly intense along the east and west transects. Thus, we focused our analysis on the net balance considering the fluxes through these two transects. We calculated a lateral flux of suspended POC at the central and intermediate waters (100–1700 m depth layer) of 8.77 × 108 mol C d−1 and 4.8 × 108 mol C d−1 for the east and west transects respectively, extending along 1.01 × 106 m in length (Figure 5). In any case, a simple sensitivity analysis was added to assess the effect of baseline subtraction to the magnitude of the lateral POC fluxes. First, the BATS POC data were fitted to a power law function, and then a “fake baseline” was generated by applying the same equation observed at BATS to higher surface POC values (Figure 6). Subtracting the fake baseline from the POC concentrations at the east and west transects, the lateral fluxes of suspended POC at Central and intermediate waters were 5.78 and 4.26 × 108 mol C d−1 for the east and west transects respectively. The POC increase over the BATS baseline (≈1.5 times higher) results in a decrease in the lateral POC fluxes of 34% for the east transect and 11% for the west transect.

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Figure 5. Offshore suspended organic carbon fluxes (108 mol C d−1) in the different water masses. The values were obtained by subtracting the open ocean baseline POC concentration calculated from the Bermuda Atlantic Time-series Study (BATS). Note that the highest POC fluxes are in central waters of the more coastal section.

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image

Figure 6. Comparison of the average POC profile measured during this study (CORICA) with the baselines used in the sensitive analysis to compute lateral POC fluxes. The BATS baseline was generated by fitting a power law equation to the average BATS POC profiles (black line). A fake baseline (dashed line) was derived by applying the same equation observed at BATS to higher surface POC values (see text for details).

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[22] Table 2 compares the vertical and lateral POC transport in the Canary region. The lateral fluxes correspond to this study, whereas the vertical fluxes were obtained from surface-tethered and deep-moored traps [Neuer et al., 1997, 2002b, 2007; Arístegui et al., unpublished data, 2003]. As observed, the lateral POC fluxes are 2 or 3 orders of magnitude higher than vertical fluxes depending on water masses, confirming our hypothesis of a more relevant horizontal versus vertical flux of POM per unit area. This conclusion is partly in agreement with the work of Bauer and Druffel [1998] who suggested that suspended POC inputs from ocean margins to the open ocean interior might be more than an order of magnitude greater than inputs of recently produced organic carbon derived from the surface ocean. However, we should keep in mind that the overall differential effect of the POC transport will depend on the aerial extension of the vertical versus lateral fluxes under consideration.

Table 2. Estimates of Vertical and Lateral POC Fluxes in the Different Water Massesa
Water MassesRange Depth (m)Vertical Fluxes (mmol C m−2 d−1)Lateral Fluxes (mmol C m−2 d−1)
  • a

    Lateral fluxes correspond to this study, whereas the vertical fluxes were directly measured with surface-tethered (central waters) and deep-moored (intermediate and deep waters) sediment traps. The vertical flux at central waters is the amount of POC that is entering into the mesopelagic zone (export flux at 150–200 m).

  • b

    Here 0.55 is from Neuer et al. [2007] (ESTOC station) and 2 is from Arístegui et al. (unpublished data, 2003) (downstream of the Canary Archipelago).

  • c
  • d
Surface waters0–100 9245
Central waters100–7000.55–2b1850
Intermediate waters700–17000.46c310
Deep waters1700–30000.33d80

4.3. Carbon Budget and Mesopelagic Respiration

[23] The surface waters (0–100 m) inside the box received an overall higher external input (17.48 × 108 mol C d−1; sum of north and east transects) of POCsusp than exported (12.39 × 108 mol C d−1; sum of south and west transects) outside the box (Table 1). The resultant balance is 5.09 × 108 mol C d−1 (0.85 mmol C m−2 d−1). This value is in the range of the sinking POC flux reported above for the region of study, but somewhat higher than that estimated by Neuer et al. [2007] for the ESTOC (European Station for Time series in the Ocean, Canary Islands) north of the Canaries (Table 2). Sedimentation rates measured downstream the Canary Islands are however 2–4 times higher [Arístegui et al., 2004], because of the enhanced production and eddy-filament exchange processes along the intense mesoscale field [Arístegui et al., 1994, 1997; Barton et al., 1998, 2004]. Island eddies are known to increase productivity leading to positive net community production [e.g., Arístegui and Montero, 2005]. A fraction of this excess production not sunk down may be laterally advected to the open ocean, contributing to the surface carbon budget inside our box. Assuming nonsignificant atmospheric inputs, the amount of carbon sedimented into the dark ocean (see above) would match the external inputs into the box, leading to a carbon balance in the surface waters. Nevertheless, Dachs et al. [2005] have reported high average net gaseous diffusive air-water fluxes of organic carbon (25–30 mmol C m−2 d−1) in the subtropical northeast Atlantic, which in case they occur during our study, would shift the balance toward a strong heterotrophy. Independently of whether the surface waters were or not in metabolic balance (see discussions by Duarte et al. [2001], Dachs et al. [2005], and Neuer et al. [2007]), the vertical export flux of POC to the dark ocean, which is the flux in which we are interested in our study, seems to be quite constant from spring to autumn, as derived from the sediment traps records.

[24] In order to calculate the respiration rate that could support the total carbon supply below the surface waters, we considered all the carbon fluxes inside the box. Sinking POC collected with surface-tethered traps range from 0.55 to 2 mmol C m−2 d−1 (see the above section). Drifting sediment traps have been frequently reported to underestimate the export flux [Michaels et al., 1994; Buesseler, 1998; Buesseler et al., 2000; Neuer et al., 2007]. However, results comparing POC flux in BATS, both derived from surface-tethered traps and 234Th [Buesseler, 1998], show that during low-productivity (PP) periods (average PP from March to October: 36 mmol C m−2 d−1) the traps and 234Th estimates reasonably agree, yielding average POC sedimentation rates of 2.6 mmol C m−2 d−1, which corresponded to an export/production ratio (e ratio) of 5–10%. At the ESTOC station (which is closer to the upwelling region than our sampling area), the average annual PP is about 30 mmol C m−2 d−1, with the lowest values (<10 mmol C m−2 d−1) recorded during autumn [Neuer et al., 2007]. Similarly, low PP was measured by Basterretxea and Arístegui [2000] in an offshore station west of the Canary Islands, during summer. An e ratio of 5–10% would therefore lead to a POC flux of 0.5–1 mmol C m−2 d−1 at PP = 10 mmol C m−2 d−1, and 1.5–3 mmol C m−2 d−1 at PP = 30 mmol C m−2 d−1. These calculations provide confidence to the measured range in sedimentation rates with drifting traps (0.55–2 mmol C m−2 d−1), which would represent a reasonable lower/upper scenario for passive sinking of POC in our region of study.

[25] Diel migrating zooplankton contributes also to the vertical flux of POC (the so-called “active flux”) by feeding in surface during the night and defecating unassimilated POC at depth during the day. Hernández-León et al. [2001] estimated an active flux of 0.22 mmol C m−2 d−1 in the eddy field downstream the Canaries, similar to the average value (0.17 mmol C m−2 d−1) reported by Steinberg et al. [2000] for BATS. These fluxes represented 25% and 8% of the passive POC fluxes in their respective regions. If we assume that the active flux represents at best a 20% of the passive flux, the vertical POC flux (passive + active) in our box would range from 0.7 to 2.4 mmol C m−2 d−1. Multiplying these values by the box area (6 × 1011 m2) yield a total POC supply of 4.2–14.4 × 108 mol C d−1.

[26] The central waters (approximately 100–700 m; hereafter named mesopelagic zone) inside our box received a net lateral POC supply of 8.5 × 108 mol C d−1 (Table 1). If we assume that about 90% of the vertical POC flux is respired in the mesopelagic zone [Arístegui et al., 2005b], and adding the net lateral carbon supply we obtained a total mesopelagic POC reservoir (POCmeso) of 12.3–21.5 × 108 mol C d−1.

[27] How this POC flux compares with the DOC flux? The relative contribution of dissolved organic carbon (DOC) to total mesopelagic respiration (R) was estimated by Arístegui et al. [2003], in a section spanning the coastal transition zone (CTZ) in the Canary Current. In their study, DOC contributed 30% to the total mesopelagic oxygen consumption, a value 2 times higher than the average (15%) calculated by Arístegui et al. [2002] for the global ocean. Unlike the eastern Canaries-CTZ our region of study was characterized by a stable surface thermocline, and was outside the influence of the eddy field region, which might enhance vertical mixing. Thus, we would expect to have a lower contribution of DOC to total mesopelagic R. In a best case scenario of a 30% contribution of DOC, the total carbon fluxes (lateral and vertical fluxes of DOC and POC) would support an integrated (100–700 m) mesopelagic respiration rate of 2.9–5.1 mmol C m−2 d−1 (computed by dividing POCmeso by both the box area and 0.7). If DOC contributed 15% (a more reasonable contribution) the total R would be 2.4–4.2 mmol C m−2 d−1 (POCmeso/box area × 0.85). From these calculations we can infer that the lateral POC would account for 28–49% of total mesopelagic R in the 30% DOC scenario, and 34–59% in the 15% DOC scenario.

[28] Total R is about an order of magnitude lower than that estimated by Arístegui et al. [2005a] for the Canary Current region, during summer time, combining actual oxygen consumption measurements and enzymatic activities (ETS activity). The discrepancy in the magnitude of the rates may be explained by the fact that, during the summer sampling, the POCsusp concentrations were about 4–6 times higher in the water column than during this study (not shown) and that the summer rates were averaged including near-coastal stations downward the Canary Islands region, where vertical POC flux is several times higher.

[29] Our calculated R estimates are also about half of the mesopelagic oxygen consumption rates (9 mmol C m−2 d−1) reported by Jenkins and Goldman [1985] for the NASG, inferred from changes in the apparent oxygen utilization (AOU) and the use of tracers to calculate the apparent age of the water mass. A similar twofold imbalance was observed in BATS between the AOU/tracers approach when compared with estimations of POC and DOC 1-D fluxes in the mesopelagic zone [Carlson et al., 1994; Michaels et al., 1994]. Although there are known pitfalls in the accurate determination of carbon fluxes [e.g., Buesseler, 1998; Hansell, 2002; Arístegui et al., 2005b], we must not exclude the uncertainty associated with the inference of mesopelagic respiration from changes in the oxygen field, because of eddy diffusivity [Jenkins and Wallace, 1992].

[30] The sources of the lateral mesopelagic carbon fluxes to our region of study may be variable. Upwelling filaments have been identified as playing a key role in coastal ocean export of organic matter [Álvarez-Salgado et al., 2001]. According to Barth et al. [2002], the exported organic matter from filaments may be forced downward along sloping density surface through conservation of potential vorticity along the meandering jet path. These authors found a mesopelagic chlorophyll maximum in the California Current System over 300 Km offshore and between 150 and 250 m, reporting a carbon injection into the adjacent deep ocean of 2.4 × 106 Kg C per event. They suggested that the entire benthic mineralization rate could be supplied by five of these events per year. Eddies downstream the islands [Arístegui et al., 1997; Barton et al., 1998, 2004] are another potential source for deep-water transport of organic matter. Barton et al. [1998] estimated that island eddies may contribute to the nitrogen flux to the Canary region as much as coastal upwelling. McGillicuddy et al. [2007] calculated that carbon export inferred from oxygen anomalies in eddies in the Sargasso Sea accounted for one to three times as much as annual new production in the region. On the other hand, Arístegui et al. [1997] and Arístegui and Montero [2005] showed that anticyclonic eddies may entrain high-chlorophyll water from upwelling filaments with which they interact. Recent unpublished studies (e.g., Arístegui and Alonso-González, 2007) show that eddy entrainment is particularly strong in the frontal regions between eddy pairs and filaments, enhancing sinking of organic matter to depths >800 m. This organic matter could be laterally transported, accounting for some of the deep-water maxima observed in POMsusp. Additionally, meddies, frequently reported for the Canary Basin [Richardson et al., 1991; Shapiro and Meschanov, 1996; Richardson et al., 2000], may largely contribute to the deep-water transport of organic matter. During this study the core of a meddy was sampled (29°N, 20°W; 700–1500 m) yielding an average POC concentration of ≈7 μM. If we consider a southwestward flow of 1.2 Sv (as calculated by Hernández-Guerra et al. [2005]), the POC flux transported by the meddy would be 6.58 × 108 mol C d−1. This value is only slightly lower than the net POC consumption in the Central waters. All these features (filaments, eddies and meddies) are limited in spatial extent and occur episodically, hence the difficulty in observing such events in hydrographic surveys or sediment traps. Therefore, the mesopelagic carbon deficit could be supplied by a few of these mesoscale events per year.

[31] In conclusion, our results suggest that a significant fraction of the mesopelagic carbon budget in the western Canary Current may be fuelled by lateral suspended carbon advection from continental margins or mesoscale activity in the eastern boundary region. A large part of this mesopelagic carbon would be consumed in the boundary current, instead being transported to the open ocean. Our observations indicate that the influence of the lateral advected particulate carbon from the NW African coast on the oligotrophic subtropical gyre region can reach more than 1000 km offshore. If the lateral suspended POC fluxes estimated in this study are confirmed for other boundary regions, the coastal-open ocean POC transport would play a key, but unaccounted, role in the global carbon cycle of the oceans.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

[32] This work has been supported through the Spanish Plan Nacional de I+D (MEC) under the CORICA (REN2001-2649-C02-01), COCA (REN2000-1471-CO2-02/MAR), and RODA (CTM2004-06842-C03-03/MAR) projects. Iván J. Alonso-González was supported by a MEC grant. We thank the captain and crew of the R/V Thalassa for their support at sea. We also thank Cindy Lee for her helpful and insightful suggestions on the manuscript. Special gratitude goes to Verónica Benítez-Barrios for her valuable help with MATLAB. This is part of the work performed by I. J. Alonso-González for the partial fulfillment of the requirements for the degree of Doctor of Philosophy in Oceanography at University of Las Palmas de Gran Canaria.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information
FilenameFormatSizeDescription
gbc1574-sup-0001-t01.txtplain text document1KTab-delimited Table 1.
gbc1574-sup-0002-t02.txtplain text document1KTab-delimited Table 2.

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