Continental Shelf Research 30 (2010) 1601–1613

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Continental Shelf Research

0278-43

doi:10.1

n Corr

8005-13

E-m1 Pr

8005-13

journal homepage: www.elsevier.com/locate/csr

An upwelling filament off southwest Iberia: Effect on the chlorophyll a andnutrient export

Alexandra Cravo a,n, Paulo Relvas a,1, Sara Cardeira a, Filomena Rita a,Miguel Madureira a, Ricardo Sanchez b

a CIMA, FCT, University of Algarve, Campus de Gambelas, 8005-139 Faro, Portugalb Centro Oceanografico de Cadiz, Muelle de Levante s/n, P.O. Box 2609, E-11006 Cadiz, Spain

a r t i c l e i n f o

Article history:

Received 1 September 2009

Received in revised form

27 April 2010

Accepted 10 June 2010Available online 22 June 2010

Keywords:

Iberian peninsula

Upwelling

Filaments

Nutrients

Chlorophyll a

Dissolved oxygen

43/$ - see front matter & 2010 Elsevier Ltd. A

016/j.csr.2010.06.007

espondence to: CIMA, FCT, University of Alg

9 Faro, Portugal. Tel.: +351 289 800 900; fax

ail address: acravo@ualg.pt (A. Cravo).

esent address: CCMAR, University of Algarve

9 Faro, Portugal.

a b s t r a c t

The present paper describes the chemical and biological characteristics of an upwelling filament off

southern Iberia in October 2004, based on a total of 42 CTD casts, along with ADCP velocity

measurements distributed by an almost regular grid of 15 km mean spacing. Stations were sampled

from the surface to a maximum depth of 400 dbar, for nutrients (nitrate, phosphate and silicate) and

chlorophyll a determination. The effect of cross-shelf exchange of nutrients and chlorophyll a between

the coastal and oceanic waters was also investigated. Data revealed that, during the cruise conducted

under relaxed winds, a relatively small filament was formed down to 75 m. However, an amount of

180 t of chlorophyll a was estimated there accompanied by low nutrient concentrations. The observed

water properties reveal that SW Iberia is similar to NW Iberia, showing the continuity of the Canary

Upwelling System along the Iberian Peninsula, one of poorest upwelling systems of the oceanic eastern

boundaries. Nevertheless, the amounts of chlorophyll and nutrients transported through the filament

are significant, revealing that those play a key role in the seaward export of matter with an important

impact on the regional oceanography of this region. Considering the periods of strong upwelling events

and the extent of their duration along the year, the amounts of exported matter must be hugely

increased and responsible for the high productivity of these waters.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Filaments of cold subsurface water, upwelled in the coastalregions under favourable winds, are recognized as efficientcontributors for the exchange processes between the coastalproductive waters and the open ocean oligotrophic waters (Levı,2008). At the eastern boundary regions, filaments have shown toplay a key role in the transport of nutrients and chlorophyll a tothe open ocean (Jones et al., 1991; Basterretxea and Aristegui,2000; Alvarez-Salgado et al., 2001, 2007). Horizontal offshoremass transport is favoured at sites where large upwellingfilaments develop (e.g. Brink and Cowles, 1991; Barton, 1998;Alvarez-Salgado et al., 2001). In the NE Atlantic it has beensuggested that a large fraction of primary production generated inthe eutrophic coastal upwelling regions could be advected intothe waters of the open oceans (Alvarez-Salgado et al., 2001, 2007;Garcia-Munoz et al., 2004). However limited knowledge of the

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; Campus de Gambelas,

dynamics and variability of upwelling filaments makes it difficultto draw any global conclusions on their biological effects. Suchstructures develop seaward from the contorted upwelling frontreaching up to 250 km of length and about 50 km width and leavea clear signature in sea surface temperature satellite imagery.Filaments are upper layer structures, barely exceeding the top150 m. Offshore velocities inside the filament jet are surfaceintensified reaching almost 1 m s�1 (e.g. Kosro et al., 1991). Aweaker return flow accompanies the filament in the downstreamside. Filaments can persist even during periods when there are noupwelling favourable winds, meaning that these structures havegreater inertia than the costal upwelling itself (Pelegrı et al.,2005). This represents a continuous source of matter to the openocean.

Western Iberia is part of the Canary Current System that formsthe Eastern Boundary Upwelling System of the N. Atlantic. Theculminating point of SW Iberia, the Cape S~ao Vicente (Fig. 1), isthe root of a recurrent well developed filament observed insatellite imagery during the upwelling season (Haynes et al.,1993), defined as April–October for the western Iberia (Woosteret al., 1976). This filament was sampled in October 2004, duringthe final stages of the upwelling season. Nevertheless, thephysical description of the filament carried out by Sanchez et al.

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Fig. 1. Wind velocity at the Sagres meteorological station, from 30 September to 27 October 2004 (top) and from the onboard meteorological station, during the cruise,

from 22 to 25 October 2004 (bottom).

A. Cravo et al. / Continental Shelf Research 30 (2010) 1601–16131602

(2008) reveals that this feature is a powerful mechanism of massexchange between the coastal waters and the open ocean, even inits decaying period. Offshore transports of about 0.9 Sv arereported by the authors, which corresponds to more than oneorder of magnitude above what could be expected by the Ekmanmechanism acting alone. Regarding such physical settings, thisstructure must impact the ecosystem functioning in the region. Inthis paper we investigate the effect of the Cape S~ao Vicentefilament in the cross-shelf exchange of chemical and biologicalproperties between the coastal and oceanic waters.

Up to now, published research for the southern margin ofIberia has been quite limited to physical oceanography issues(Relvas and Barton, 2002; Sanchez and Relvas, 2003; Sanchezet al., 2008). Consequently, there is a lack of knowledge abouthow the physical dynamics affects the chemical and biologicalprocesses. Satellite remote sensing observation of filaments ofcool upwelled water, rich in chlorophyll, is recurrent in thisregion. The present multidisciplinary study aims to contribute tofulfil this gap of knowledge by estimating the magnitude andconsequences of the export of matter (nutrients and chlorophyll a

as a proxy of phytoplankton) associated with filaments within thishighly productive area (Cunha, 2001; Moita, 2001). Moreover,direct observations of filament cross-shelf biological and chemicalfluxes are restricted to the NW Iberia (e.g. Alvarez-Salgado et al.,2001; Joint et al., 2001a; Barton et al., 2001), Canary regions (e.g.Arıstegui et al., 2004; Barton et al., 2004; Garcia-Munoz et al.,2004) and NW Africa (Alvarez-Salgado et al., 2007; Pastor et al.,2008). With the description of the filament at the South Iberia, thecontinuity of the upwelling characterisation along the CanarySystem will be provided. The estimation of the contribution ofonshore–offshore nutrients and chlorophyll transports in afilament in the SW Iberia region is reported here for the first time.

2. Data, processing and methods

From 22 to 25 October 2004, during about 63 h, the filamentwas surveyed on board the R/V D Carlos I from InstitutoHidrografico, Lisbon (ATOMS04 cruise), attempting a set ofobservations close to synoptic. Guided by satellite SST imagerytransmitted to the ship in near-real time, the upwelling filamentwas identified and a total of 42 CTD casts were carried out,distributed by 6 transects across the filament on an almost regular

grid of 15 km mean spacing (Fig. 2a). Stations were sampled fromthe surface to a maximum depth of 400 dbar using a IdronautOS316 CTD attached to a General Oceanics Rosette sampler. TheCTD sampling rate was 20 Hz and the lowering speed was1 m s�1. Ten levels were selected for water collection: 5(surface), 10, 20, 30, 50, 75, 100, 150, 200 and 400 m depth todetermine chlorophyll a (Chl a), nutrients and dissolved oxygen.Water samples for the determination of Chl a were collected onlyto 200 m, since at deeper depths the limitation of light inhibits thealgal development.

2.1. Velocities and transports

Current velocities were measured along the ship track using ahull-mounted 38 kHz RDI acoustic Doppler current profiler(ADCP). Details about the data processing and the observedvelocity field can be found in Sanchez et al. (2008). Due to theADCP configuration, velocities in the top 30 m of the watercolumn were not directly sampled. Since some parametersrevealed relatively high values in the surface layer, in particularChl a, geostrophic velocities computed from the dynamic heightof the sea surface relatively to the 400 dbar reference level wereused in the top 30 m.

Velocities perpendicular to the filament cross sections wereobjectively interpolated onto a regular grid. The biologicalproperties were interpolated onto the same grid. The horizontaladvective fluxes were computed at each grid node by multiplyingthe velocity by the correspondent variable value. Transports werecalculated by spatially integrating the fluxes over the desiredregion.

2.2. Chlorophyll a, nutrients and dissolved oxygen: sampling and

analytical procedures

Water samples (1 L) were filtered through 0.45 mm deconta-minated and weighed membrane filters and frozen at–20 1C priorto analysis for nutrient concentrations. Nitrate (NO3

�), phosphate(PO4

3�) and silicate (SiO44�) were analysed using a Skalar

autoanalyser according to the methods described by Grasshoffet al. (1983). Detection limits were 0.1 mM for nitrate, 0.08 mM forsilicate and 0.04 mM for phosphate. The marine nutrient standardkit (OSI) was used as reference material to ensure accuracy. The

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Fig. 2. SST satellite image from 22 October 2004 with the location of the sampling sites, zonal and meredional transects (a) and the concurrent of Chl a satellite image from

23 October 2004 (b).

A. Cravo et al. / Continental Shelf Research 30 (2010) 1601–1613 1603

procedure was highly accurate, with a relative error of less than2.5% in determining nutrient concentrations. Precision wascalculated as�1% for silicate and phosphate, and�2% for nitrate.

To evaluate the Chl a and phaeopigments concentration as aproxy of phytoplankton biomass, 250 mL of seawater sampleswere immediately filtered on board using GF/F glass fibre filtersand deep-frozen at �20 1C. Under analysis pigments wereextracted from filters with 90% acetone and determined on aPerkin-Elmer spectrofluorometer, as the method described byYentsch and Menzel (1963).

The dissolved oxygen was determined in duplicate on boardfollowing the Winkler method, modified by Carrit and Carpenter(1966), using a whole-bottle manual titration. The standarddeviation for the duplicates was always lower than 5%. Thedissolved oxygen percentage saturation was calculated using thealgorithm given by UNESCO (1973).

3. Results

3.1. Wind pattern

The direct observation of the filament took place during earlyfall, when the northerly upwelling favourable winds tend toweaken in the region. The winds measured at the Sagresmeteorological station, located on the Cape S~ao Vicente cliffs,confirm this pattern (Fig. 1, top). They show a relatively strong (upto 8 m s�1) northerly component since the beginning of October2004, favourable to the upwelling off the west coast, interruptedby relaxation periods during 8–10 October and from 17 Octoberonwards. This last relaxation was coincident with the in situ

sampling of the filament that started on 22 October. The onboardmeteorological station revealed smaller scale temporal variability(Fig. 1, bottom), with weak southwesterlies during the first half ofthe cruise and predominant northwesterlies from the end of 23October onwards. Thus, our observations of the filament did occur

during the decaying phase of the upwelling season, throughout arelaxation event, but just after a relatively intense upwellingperiod. This is evident in the satellite sea surface temperaturefield presented in Fig. 2a, where the upwelling signal and theassociated filament is noticeable.

3.2. Biological and chemical evidence of the filament

A detailed physical characterisation of the filament (Fig. 2a) isdescribed by Sanchez et al. (2008). Its length was about 90 km,plus an offshore curling segment with about 77 km. The widthwas �45 km at �9.501W. Its SST signature was o18.5 1C,contrasting with the ambient waters 420.5 1C. The upwellingof waters above the 150 m was clearly identified by these authors.Horizontal and vertical distributions of Chl a, dissolved oxygensaturation and nutrients, are in close agreement with thetemperature and salinity fields derived by Sanchez et al. (2008)and reveal the impact of the upwelling filament on the chemicaland biological properties in the region. The Chl a satellite image(Fig. 2b) follows quite well the upwelling filament observed bySST (Fig. 2a). Within it, where temperature was lowest the Chl a

concentration (Z0.2 mg m�3) was significantly higher thanoffshore (o0.1 mg m�3).

3.2.1. Horizontal distributions of Chl a, dissolved oxygen saturation

and nutrients

Chl a was most intense at subsurface levels (20 and 30 m –Fig. 3a and b, respectively). The maximum at 20 m (3.6 mg m�3),close to the Cape, is high for this time of the year (late fall), whenChl a is generally lower than in Spring-Summer. At this depth thehorizontal distribution (Fig. 3a) mimics quite well the filamentobserved by satellite imagery (Fig. 2b). A concentration ofZ0.2 mg m�3 was considered for defining the filament limits.Concentrations below this value were found in oceanic waterswhich are typically oligotrophic (Carr et al., 2006).

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Fig. 3. Horizontal distribution of Chl a concentration (mg m�3) at 20 m (a) and 30 m (b).

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Fig. 4. Horizontal distribution at 75 m of: % saturation of dissolved oxygen (a) and nutrients in mM: NO3 (b), PO4 (c) and SiO4 (d).

A. Cravo et al. / Continental Shelf Research 30 (2010) 1601–16131604

At 20–30 m depths where Chl a was highest, the dissolvedoxygen saturation was highest at all stations (as shown in nextsection; Fig. 5e and f). This indicates that the phytoplankton isproducing more oxygen than that used in the respiration process,leading to supersaturation of oxygen (100–110%).

The level where the filament is identified in terms of dissolvedoxygen is at 75 m (Fig. 4a), where Chl a almost decreased todepletion. At this level, near to the coast, oxygen concentrationsdecreased suggesting the colder water that upwelled from deeperlevels had lower oxygen concentrations. There, dissolved oxygen

A. Cravo et al. / Continental Shelf Research 30 (2010) 1601–1613 1605

saturation (o95% – Fig. 4a) is typical of undersaturation values ofoxygen that corresponds to a concentration o7.6 mg L�1. The75 m depth was also the level where the highest horizontalgradients of nutrients (nitrate, phosphate and silicate) were found(Fig. 4b–d, respectively). Interestingly at this level, station 18located just out of the southern edge of the filament reveal asignificant increase of nutrients. This may result from the lowphytoplankton density (Chl ao0.1 mg m�3; Fig. 5) andadditionally this depth corresponds to the base of the nutricline,where nutrients usually are trapped. Here, dissolved oxygen (%SatO2; Fig. 4a) also increased not associated with phytoplanktongrowth, but due to the marked temperature decrease along thethermocline (almost 2 1C from 50–75 m, not shown).

3.2.2. Vertical distributions of Chl a, nutrients and dissolved oxygen

saturation along the filament

The filament associated with this upwelling area affects mainlythe top 75 m (Fig. 5), although at 150 m depth it is possible toobserve the uplift of the various isolines, as confirmed by Sanchezet al. (2008) for temperature, salinity and density. At 150 m, wellbelow the thermocline, from where the water upwelled, thenutrient concentrations were �6 mM NO3, �0.3 mM PO4 and�2.5 mM SiO4, noticeably higher than at surface. In both sections,in agreement with the upwelling pattern revealed by the densityfields (Fig. 5g–h), the isolines of nutrients and dissolved oxygensaturation rise shoreward with a contorted shape, steeper close tothe coast. There, the occurrence of lower dissolved oxygensaturation values in the upper levels, in opposition to whatobserved further offshore, are indicative of oxygen undersatura-tion typical of recently upwelled waters.

The zonal sections C and D (Fig. 2a) along the filamentconfirmed that in terms of Chl a (Fig. 5a and b, respectively) it had�80 km long, with the highest concentrations close to the coast,developing from surface down to 75 m. Moreover, at thesedepths the percentage of Chl a in relation to total pigments(including phaeopigments) was high (460%). This means thatphytoplankton population is in a growing phase there, whilebelow the Chl a concentration markedly decreased ({0.1 mgm�3), showing a major contribution of phaeopigments due tolight limitation.

If we define the tongue formed by the filament as the volumeinside the iso-surface of 0.2 mg m�3 that falls in the box 8.95–10.101W; 36.75–37.351N from surface till 75 m depth, then anestimation of the total amount of Chl a in the filament is about180 t (180,000 kg).

At both sections (Fig. 5a, b), offshore of 9.41W the filamentsubducts in agreement with what is regularly observed in similarfilaments in other upwelling systems (e.g. Jones et al., 1991;Kadko et al., 1991; Washburn et al., 1991; Moisan and Hofmann,1996a, b). The deepening of chlorophyll resulted from physicalprocesses such as sinking of the colder and denser coastallyupwelled water as it advects seaward, in agreement with thevertical negative velocities computed by Sanchez et al. (2008)along these zonal sections.

The Chl a pattern shown by satellite imagery (Fig. 2b) isquite well represented by its distribution at 20 m (Fig. 3a).If we consider only the upper levels to 20 m depth, capturedby the satellite imagery, and infer the amount of Chl a, it willgive only 22% of the total present in the filament. The filamentsubducts below the surface particularly in Sections III and IV,what makes it undetected through remote sensing andjustify surveys such as the present one to understand oceanproductivity.

As the vertical distributions for all the nutrients were similar,only nitrate (most abundant) is shown in Fig. 5c–d. This shows

that along these two zonal sections, in the upper 50 m theconcentrations were low, both over the continental shelf andalong the filament. Nutrient concentrations were almost ex-hausted by rapid assimilation, particularly where Chl a concen-tration is highest. There, nitrate (o1 mM), phosphate (o0.1 mM)and silicate (o1 mM) were minimal and dissolved oxygensaturation maximum (100–105%; Fig. 5c–f). Assuming that thenutrient concentrations upwelled from 150 m depth (0.3 mM PO4,2.5 mM SiO4 and 6 mM NO3) at a rate of 15 m/d, as estimated forthis filament (Sanchez et al., 2008), it will take 10 d to reach thesurface. Then, in the initial period of upwelling, the nutrientconcentrations in the filament must be higher than found afterthe period of wind relaxation. Below depths 475 m depth, therewas a general increase of nutrients and a decrease of oxygensaturation values due to the increase of the remineralizationprocess while the Chl a decreased drastically due to lightlimitation.

3.3. Filament cross sections

Three meridional cross sections surveyed during the cruise(Fig. 2) show that the core of Chl a, with concentrationsZ0.2 mg m�3 (Fig. 6a–c), was generally stronger in the centralarea, more intense and wider at Section II, closer to the coast.Here, the filament occupied all the section (�80 km), between 30and 75 m depth. Along the three contiguous transects theintensity of the Chl a signal in the filament diminished with thedistance from the coast. Notice that the highest values of Chl a ateach section match the lowest absolute velocities at the frontalboundary between seaward and shoreward flows (Fig. 6d–f).Opposite flows are associated with contorted recirculating gires(Jones et al., 1991; Strub et al., 1991; Garcia-Munoz et al., 2005;Pelegrı et al., 2005). It is important to remark that the highestvelocities were recorded at Section IV (0.4–0.6 m s�1, Fig. 6f) asthe filament was elongating, where the Chl a concentrations(Fig. 6c) decreased.

Regarding the effect of the Chl a filament upon the nutrientsdistribution (Fig. 7a–i), it is clear that at the three sectionsnutrient concentration was minimal in the upper levels (r50 m)matching the core of maximum Chl a. However, at these levels,nutrient concentration was higher at Section II, where upwellingwas more intense and decreased along the sections, due tophytoplankton consumption. The central zone of the transectsshows a dome in the nutrients and dissolved oxygen saturationpatterns from 150/200 m (Fig. 7a–l), tilted to the south as movingoffshore. This may result from the southward diversion of thefilament before it curls anticyclonically, as shown in Figs. 2 and3a. These patterns agree with the physical description of thefilament. Below 150 m, nutrient concentrations increased whiledissolved oxygen decreased regularly with depth. The maximumconcentrations at 400 m around 371N may be associated with thepresence of the Mediterranean Water (MW) (Sanchez et al., 2008),frequently identified in the area as a core, flowing in equilibriumas shallow as 350 m (Ambar, 1983). At this depth this feature isenriched in nutrients as reported by Gomez et al. (2000),conversely to that usually observed at levels deeper than500 dbar (Cabec-adas et al., 2002; Ambar et al., 2008).

3.4. Cross shelf transports

To determine the contribution of the filament to Chl a andnutrients exchange with offshore waters, seaward fluxes of Chl a

and nutrients across the filament were calculated at the SectionsII, III and IV (Fig. 8). Additionally an estimate of the seaward

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Fig. 5. Spatial distribution at the zonal transects C and D of the concentration of Chl a in mg m�3 (a and b), NO3 in mM (c and d) and of dissolved oxygen saturation in %

(e and f) and density in sigma-t (g and h), respectively.

A. Cravo et al. / Continental Shelf Research 30 (2010) 1601–16131606

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Fig. 6. Spatial distribution of Chl a concentration in mg m�3 (a, b and c) and of the flow in m s�1 (d, e and f), at the Sections II, III and IV, respectively.

A. Cravo et al. / Continental Shelf Research 30 (2010) 1601–1613 1607

transports of Chl a within the filament, from surface until 75 m,was calculated as well as for nutrients from surface to 150 m atSections II and III, and down to 200 m at Section II due to thesinking of the filament (Table 1).

Section II shows that the seaward export of Chl a (Fig. 8a) bythe filament is intensified in two centers, a major one near thesurface centered at 37.251N and a secondary one to the south,centered at 50 m depth at 37.051N. If we consider only the twomain centers limited by the �2.5�10�2 mg m�2 s�1 flux isoline(maximum depth of 80 m), the offshore transport is 0.21 kg s�1

(0.14 kg s�1 in the upper center and 0.07 kg s�1 in the lower one),which represents a transport of about 18 t d�1.

Further offshore, in section III (Fig. 8b), the seaward transportis almost concentrated in a single core centered at 35 m depth,exporting 0.10 kg s�1 (�8.5 t d�1). Similar values are found inSection IV (Fig. 8c), but the structure of high Chl a concentrationstretched and deepened, according to the sinking of the filamentstructure offshore (Fig. 5a–b).

Closer to the coast, in transect II (Fig. 8a), the Chl a transport ishigher than at the offshore sections. Assuming that the total amountof chlorophyll present in the filament for all the sampling period is�180 t as calculated in Section 3.2.2, we can suppose that it takesabout 10 d to be formed. This period may be considered reasonablefor a filament formation, in agreement with other authors (Pelizet al., 2004). Since these values are obtained during a period ofrelaxed wind, this means that along the year, during periods of

favourable and strong conditions to upwelling events, much higheramounts may be exported seaward.

The fluxes of the three nutrients, nitrate, (Fig. 8d–f), phosphate(Fig. 8g–i) and silicate (Fig. 8j–l), were weak in the top 75 m.Despite the velocity decreased at 150/200 m (Fig. 6d–f), asnutrient concentrations increased in depth (Fig. 7), the nutrientfluxes were generally higher at 150/200 m than in the upperlayers, and directed offshore. These at 200 m are stronger atSection IV as a result of the filament subduction accompanied byan intensification of the current flow. Furthermore, there was alower consumption of nutrients by phytoplankton in the filament(reflected in a less intense core of Chl a). In the three sections,centered around 371N, there was also a seaward flow for all thenutrients at the levels deeper than 300 m associated with anincrease of offshore velocity due to the influence of a shallow coreof the MW (Sanchez et al., 2008, see their Fig. 2).

Nutrient transport in the filament accounted for only 8–30% ofthe total transport in the top 150/200 m across Sections II and III(Table 1), but accounted for as much as 74% of the nitrate fluxacross Section IV. The estimated seaward nutrient transports acrossthe three sections are �1750–3550 t d�1 NO3, �330–540 t d�1

PO4 and �2800–4000 t d�1 SiO4, with Section III being consistentlythe lowest. However it must be pointed out that the transportof nutrients, either in the filament or in the top 150/200 m,corresponds to a residual after consumption by phytoplanktonparticularly during the period of wind relaxation.

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Fig. 7. Concentrations in mM of NO3 (a, b and c), PO4 (d, e and f), SiO4 (g, h and i) and % of dissolved oxygen saturation (j, k and l), at Sections II, III and IV, respectively.

A. Cravo et al. / Continental Shelf Research 30 (2010) 1601–16131608

4. Discussion

The filament sampled in October 2004, indeed correspondsto a relatively small upwelling event. We did construct the

longitudinal distribution of monthly mean chlorophyll a derivedfrom satellite data, for a period of 10 years, from January 1998 toJanuary 2008, for the area bounded by 36.5–37.51N and 9–131W(Fig. 9). This longitude span covers the longest filaments observed

-400

-350

-300

-250

-200

-150

-100

-50

0

Depth

(m

)

-400

-350

-300

-250

-200

-150

-100

-50

0

Depth

(m

)

-400

-350

-300

-250

-200

-150

-100

-50

0

De

pth

(m

)

37.5

Latitude (ºN) Latitude (ºN)

-200

-150

-100

-50

0

Depth

(m

)

Latitude (ºN)

Transect II Transect III Transect IV

a

d

g

j

b

e

h

k

c

f

l

i

36.736.836.93737.137.237.337.4 37.5 36.836.93737.137.237.337.4 37.5 36.8 36.736.93737.137.237.337.4

Fig. 8. Fluxes of Chl a in 10�2 mg m�2 s�1 (a, b and c) and nutrients in mmol m�2 s�1: NO3 (d, e and f), PO4 g, h and i) and SiO4 (j, k and l), at Sections II, III and IV,

respectively.

A. Cravo et al. / Continental Shelf Research 30 (2010) 1601–1613 1609

in the region (�400 km) and the latitude span match our studyarea. We notice that spring blooms assume higher impact on thechlorophyll a export than the summer filaments. Even the

summer upwelling pulses are often stronger than that observedduring our survey. Thus, we can infer that the Cape S~ao Vicenteregion corresponds to an area where upwellling events are

Table 1Transport of nutrients and Chl a in Sections II, III and IV considering the top 150 m of the section (S) and only the filament area (F).

Transports (t d�1)

Section II Section III Section IV

Top 150 Filament % F/S Top 150 Filament % F/S Top 150 Filament % F/S

NO3�

�3554 �636 18 �1751 �148 8 �3509 �2583 74

PO43�

�544 �139 26 �329 �63 19 �334 �190 57

SiO44�

�3943 �1184 30 �2837 �675 24 �4123 �2827 69

Chl a (75 m) �18 �8 �8

2008

2007

2006

2005

2004

2003

2002

2001

2000

1999

1998

0.08

13ºW 9ºW9.5ºW10ºW10.5ºW11ºW11.5ºW12ºW12.5ºW

mg/m3

10.02.51.00.70.60.50.40.30.20.1

Fig. 9. Hovmuller diagram of monthly averaged SeaWIFS chlorophyll a along a

section (9–131W) off the southern tip of the Iberian Peninsula, from January 1998

to January 2008, generated by the NASA Giovanni website. Chlorophyll a

concentrations (mg m�3) were averaged between 36.751N and 37.51N. Red arrow

roughly indicates the signal of the sampled filament.

A. Cravo et al. / Continental Shelf Research 30 (2010) 1601–16131610

recurrent and responsible for these highly productive waters interms of phyto, zooplankton, valuable bivalves, crustaceans andfishes as documented by several authors (Chıcharo, 1998; Cunha,2001; Moita, 2001; Galhardo et al., 2006; Loureiro et al., 2008).Catch rates and average sizes of the most important species forrecreational fishing, are greater close to Cape S~ao Vicente (Erziniet al., 2008). However, to date there is no quantification of itscontribution and impacts on nutrients and Chl a export to theoffshore zone. Few studies tackled a chemical approach and theexisting ones are mainly focused on biological productivity(Moita, 2001; Peliz et al., 2004; Navarro et al., 2006). A major

result of the present study is the quantification of offshoretransport in filaments and the impact this has on regionaloceanography. This allows us to compare different upwellingregions along the eastern margin of the Atlantic Ocean.

4.1. Effect of wind stress upon the filament

The duration of upwelling periods is variable, but worldwidegenerally lasts for about 2 weeks (Moncoiffe et al., 2000). Periodsof relaxation alternate between upwelling events. The filamentdescribed in the present paper was observed during a windrelaxation, but just after an upwelling period. This fact hadconsequences upon the chemical and biological properties of theupwelled waters. Our results show that the filament may beconsidered small in comparison with others in areas whereupwelling is recurrent, including those formed in the IberianPeninsula. The total amount of Chl a in the present filament wasabout 180 t, less than the �900 t estimated using satellite data fora much longer filament (�400 km) in the same region in February2001 (Peliz et al., 2004).

4.2. Biological and chemical consequences of the filament

The highest Chl a values occurred in the upper layers (20–30 mdepth, Fig. 3), as usually observed in filaments (Moisan andHofmann, 1996a, b; Joint et al., 2001b; Oliveira et al., 2009). Themaximum value of Chl a (�4 mg m�3) found nearshore close toCape S~ao Vicente is high for the end of the upwelling season. It isat least one order of magnitude higher than those found offshore(r0.1 mg m�3) and higher than the threshold of 1 mg m�3 usedto define the limits of eutrophic areas in the costal upwellingecosystems (Carr et al., 2006). Values 41 mg m�3 were regis-tered only very close to the coast (Fig. 3) and if this limit is appliedto the present study most of the signature of the filament wouldhave been lost. As these high concentrations were recordedduring a period of wind relaxation, this means that filaments canexport material offshore even when upwelling was abated(Garcia-Munoz et al., 2005), since these structures have greatinertia (Pelegrı et al., 2005).

The distribution of Chl a is coherent with a scenario whereupwelled water is associated with a drop in the wind intensity.This reduces the nutrient pumping from below the thermoclineup into the euphotic zone. Afterwards, surface water warms upand becomes more stable due to stratification. This promotesphytoplankton growth that adapted to light and nutrientsavailability leads to a general surface exhaustion of nutrients(Chavez et al., 1991; Levi, 2008). In fact, at 20 m (Chl a maximum)the observed temperature (�16.5 1C) increased almost 2 1C if weconsider that the water was upwelled from a level with �14.5 1C(150/200 m). The surface nutrient concentrations at the top levels(o50 m) and along the filament were low, behaving like a mirrorimage of the Chl a. In consequence, higher values of oxygensaturation were reached due to the dissolved oxygen production

A. Cravo et al. / Continental Shelf Research 30 (2010) 1601–1613 1611

during photosynthesis, in agreement with observations off NWIberia (Castro et al., 2000) and near Cape Guir, NW Africa (Garcia-Munoz et al., 2005). During strong upwelling events nutrientconcentrations must be significantly higher than those measuredin situ, as observed during upwelling events off NW Iberia(Alvarez-Salgado et al., 2001) and in the Gulf of Cadiz (Ruiz andNavarro, 2006). After the exhaustion of nutrients the phytoplank-ton decays along the filament (Figs. 5 and 6), as generally foundby other authors (e.g. Jones et al., 1991; Alvarez-Salgado et al.,2001, Pelegrı et al., 2005) responding either physical losses, suchas sinking and/or mixing, or to biological losses such as grazing(Moisan and Hofmann, 1996a).

The maximum concentrations of Chl a and mean nitrate,phosphate and silicate in the top 50 m, in comparison with otherareas, namely NW Iberia and NW Africa Canary Upwelling system,is presented in Table 2. It can be seen that Chl a is high andconsequently the nutrient concentrations are lower or within thevalues found in other upwelling regions of the NE Atlantic.

4.3. Chemical and biological characteristics in the deeper layers

In the deeper layers, below the filament (475 m), the range ofnutrients and dissolved oxygen saturation was similar to thatfound at analogous levels in other upwelling areas in the NWIberia (Castro et al., 2000; Alvarez-Salgado et al., 2001; Joint et al.,2001a). However, the values are lower than usually observedfurther south in the Canary Upwelling System, in the NW Africa(Pelegrı et al., 2005, 2006; Pastor et al., 2008), or in other EasternBoundary Upwelling System such as California (Jones et al., 1991;Chavez et al., 1991) and Chile – Bay of Mejillones (Valdes et al.,2008). Actually, the consistency of the nutrient data along theAtlantic coast of Iberia reveals that the Iberian upwelling areaforms a coherent region and one of the poorest upwelling systemsof the oceanic eastern boundaries (Castro et al., 2000) where, indepth, nutrient concentrations were relatively lower than else-where. The decreasing of primary production with latitude withineach eastern boundary systems (Carr, 2002) will contribute to thisscenery.

Table 2Maximum Chl a (mg m�3) and mean nitrate, phosphate, silicate concentrations (mM), o

under upwelling events.

Location Comments

Cape S ~ao Vicente (October 2004) Coastal zone upwelling

Cape S~ao Vicente (October 1985) Coastal zone (upwelling)

Cape Roca (August 1985) Upwelling Zone

Outer area Lisbon Bay (September 1999)

NW Iberian Peninsula (August 1998) Coastal zone

Oceanic water

NW Iberia Peninsula (August 1998) Coastal zone

NW Iberian Peninsula (August 1998) Coastal zone

Oceanic zone

NW Iberian Peninsula (August 1998) Coastal zone upwelling

NW Iberian Peninsula (Summer 1994) Continental shelf (upwelling)

Cape Finisterra (May 1991)

(July 1984)

Gulf of Biscay (May 1995) Upwelling

Gulf of Cadiz (upwelling – May 2001) Coastal zone

Canary Islands (October 1991) North zone

South zone

NW Africa – Canary Islands (August 1999) Coastal zone (upwelling)

Oceanic water

NW Africa (Sept–Oct 1997) Coastal zone

Oceanic water

Cape Juby (August 1999) Coastal zone

Oceanic water

4.4. Impact of the filament dynamics in the regional oceanography

The filaments effectively exchange matter with offshorewaters contributing to their enrichment throughout the upwel-ling period, i.e. April–October. The estimated Chl a cross shelftransport is maximum at transect II, closer to the coast,transporting �18 t d�1. Considering that within a year there arebetween one and two hundred days of favourable upwellingconditions (Sanchez et al., 2008), it may be assumed that per yearit could be exported roughly 1800–3600 t of Chl a, playing a keyrole in the high biological productivity of this region, much morethan the 450 t for �150 d of favourable upwelling in NW Iberiaestimated by Alvarez-Salgado et al. (2001).

Even considering that the filament is not particularly devel-oped, at Section III it transported 0.210 kg s�1 of Chl a, muchhigher than 0.035 kg s�1 estimated in NW Iberia, where thereported seaward velocity was just �7 cm s�1 (Alvarez-Salgadoet al., 2001). Although relatively weak, these transports aresimilar to the lower limit of export observed in the giant filamentsoff California and NW Africa (�0.170–0.665 kg s�1 Chl a; Joneset al., 1991; Strub et al., 1991; Moisan and Hofmann, 1996a, b;Mackas and Yelland, 1999; Garcia-Munoz et al., 2004).

Because of the intense consumption of nutrients by thephytoplankton within the filament, the ‘‘real’’ nutrients exportby the filament during periods of strong upwelling should becertainly higher than that estimated in this study. Nevertheless,the order of magnitude of these estimated amounts represent, asfor Chl a, an important offshore export of matter, despite lowerthan those found in giant filaments off California. For example, fornitrate (converting data from Table 1) the seaward export acrossall the area of the sections (�80 km), in the top 150/200 m waso3000 mol s�1, smaller than 5200 mol s�1 (Jones et al., 1991)and 4335 mol s�1 (Mackas and Yelland, 1999). The across-shoreflux from productive coastal regions should have also animportant role in the ocean biogeochemical carbon cycle. Thus,filaments represent an effective way to export material (biologi-cal, nutrients or gases) to oligotrophic offshore areas and have astrong influence on the productivity of coastal and offshoreecosystems.

bserved in surface layer (o50 m depth) in different regions of the Atlantic ocean

Chl a NO3� PO4

3� SiO44� References

3.6 o2 o0.2 o2 This study

2 0.1 o0.2 0.1 Moita (2001)

2.5 Moita et al. (2003)

1.8–2.9

o2.9 3 Barbosa et al. (2001)

o0.3 4–6

0.2–2.6 o4 Joint et al. (2001a)

0.2–0.4 Smyth et al. (2001)

0.3–0.45

1.4 o1 Alvarez-Salgado et al. (2001)

2 3.6 1.4 Varela et al. (2001)

4–6 8 0.5 3.2 Castro et al. (2000)

2–4 7 0.5 3

o0.5 o2 o1 Gil et al. (2002)

o2 o0.5 o0.5 Navarro et al. (2006)

o0.15 o1 o1 Barton (1998)

o0.42 o2 o1

o3 Garcia-Munoz et al. (2004)

o0.5

o0.7 o4 Garcia-Munoz et al. (2005)

o1.4 o4

0.5–2 Barton et al. (2004)

0.1

A. Cravo et al. / Continental Shelf Research 30 (2010) 1601–16131612

5. Conclusions

The filament observed by the end of October under relaxedwinds is relatively small, however within it 180 t of Chl a wereexported.

The nutrient concentrations measured during the cruise justrepresent the residual after the phytoplankton consumption.Nevertheless, the amounts of Chl a and nutrients transported perday by the filament is significant, since the order of magnitudereached tens of tons for Chl a and several hundreds of tons fornutrients. Consequently, the filaments in the SW Iberia play a keyrole in the seaward export of matter (nutrients, gases andphytoplankton) with an important impact on the regionaloceanography of this region.

Considering the periods of strong upwelling events and theextent of their duration along the year, the amounts of exportedmatter must be hugely increased and responsible for the highproductivity of these waters.

The observed water properties reveal that SW Iberia is similarto NW Iberia, showing the continuity of the Canary UpwellingSystem along the Iberian Peninsula. This region is one of poorestupwelling systems of the oceanic eastern boundaries.

Acknowledgements

The authors would like to thank Dr. Teresa Moita, from theIPIMAR – Lisboa for the technical assistance on the chlorophyll a

determination. The authors would like also to thank theanonymous reviewers that contribute to improve this manuscript.Analysis and visualization used in Fig. 9 of this paper wereproduced with the Giovanni online data system, developed andmaintained by the NASA GES DISC.

This work was financially supported by FCT (Portugal), underthe ATOMS project (ATOMS – Acoustic Tomography MonitoringSystem), Contract no. PDCTM/P/MAR/15296/1999.

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