Marine Chemistry 123 (2011) 56–66

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Marine Chemistry

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Air–sea CO2 fluxes in the north-eastern shelf of the Gulf of Cádiz(southwest Iberian Peninsula)

Mariana Ribas-Ribas ⁎, Abelardo Gómez-Parra, Jesús M. ForjaDepartamento de Química-Física, Facultad de Ciencias del Mar y Ambientales, Universidad de Cádiz, Campus Río San Pedro, s/n, Puerto Real, Cádiz, 11510, Spain

⁎ Corresponding author. Tel.: +34 956016163; fax: +E-mail addresses: mariana.ribas@uca.es (M. Ribas-Ri

(A. Gómez-Parra), jesus.forja@uca.es (J.M. Forja).

0304-4203/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.marchem.2010.09.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 January 2010Received in revised form 27 September 2010Accepted 27 September 2010Available online 4 December 2010

Keywords:Carbon dioxideINTRA-annual variationsAir–water exchangeSpainGulf of Cádiz36.9°N–36.3°N6.9°W–6.2°W

An intra-annual investigation of the fugacity of CO2 (fCO2) has been conducted in surface waters of the north-eastern shelf of the Gulf of Cádiz (SW Iberian Peninsula) in four cruises made in 2006 and 2007. Intra-annualvariability of fCO2 was assessed and is discussed in terms ofmixing, temperature and biology. In the study area ofthe shelf, thermodynamic control over fCO2 predominates from early May to late November, and this is oppositeand similar in magnitude to the net biological effect. However, biological control over fCO2 predominates duringwinter. The results suggest that surfacewaters in the coastal area are under-saturatedwith respect to atmosphericCO2 during most of the year; therefore they represent a sink for atmospheric CO2 between November and May(−1.0 mmol m−2 day−1), but a weak source in June (1.3 mmol m−2 day−1). In contrast, the coastal ecosystemsstudied (the lower estuary of Guadalquivir Estuary and Bay of Cádiz) acted as a weak sink for atmospheric CO2

during February (−1.3 mmol m−2 day−1) and as a source betweenMay andNovember (2.6 mmol m−2 day−1).The resulting mean annual CO2 flux in the north-eastern shelf of the Gulf of Cádiz was −0.07 mol m−2 year−1

(−0.2 mmol m−2 day−1), indicating that the area acts as a net sink on an annual basis.

34956016040.bas), abelardo.gomez@uca.es

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Although thecontinentalmarginal seasoccupyonly a little over8%ofthe ocean surface area and less than 0.5% of the ocean volume, they playa major role in the biogeochemical cycles of carbon and associatedelements (Alongi, 1998; Chen and Borges, 2009). The coastal and shelfseas act as a medium of exchange for oceanic, atmospheric, riverine,sediments, biota and terrestrial carbon pools. Moreover, about 60% ofhuman population is concentrated within 100 km of the coasts(Vitousek et al., 1997). Many studies of carbon dynamics have beencarried out in recent years. Most of them present encouraging results atthe global ocean scale (Sabine et al., 2004; Takahashi et al., 2009) but donot specifically address the ocean margins and highly variable coastalwaters. Quantifying regional fugacity of CO2 (fCO2) in surfacewater andair–sea CO2 fluxes (FCO2) will provide a basis for determining theseasonal physical and biogeochemical controls on the ocean carboncycle. Recently, a global synthesis of air–sea CO2 fluxes for continentalshelves and estuaries has been presented (Borges et al., 2005; Cai et al.,2006; Chen and Borges, 2009; Chen et al., 2003) in an attempt to answerthe ongoing and debated question ofwhether the coastal oceans act as asource or a sink of CO2 to the atmosphere. Moreover, the extremevariability of ocean surface fCO2 in shelf and coastal waters is somewhatproblematic for accurately quantifying gas exchange (Cooper et al.,1998). To resolve these small spatial scale features on a regional scale,

data from studies with extensive spatial and temporal coverage areneeded.

It must be pointed out that direct fCO2measurements in the Gulf ofCádiz are very limited and they have been mostly in the open area ofthe Gulf of Cádiz. Only a few studies have been carried out in the Gulfof Cádiz to evaluate the variability of the sea surface fCO2 (Aït-Ameurand Goyet, 2006; González-Dávila et al., 2003; Huertas et al., 2006)which clearly hampers our ability to estimate the air–sea FCO2 of thenorth-eastern shelf of the Gulf of Cádiz.

A better understanding of the Gulf of Cádiz flux is fundamentalbefore it canbeplaced in a global context. In this study,we looked at fourdirect fCO2 and dissolved oxygen measurement surveys in the north-eastern shelf of the Gulf of Cádiz and its associated inner ecosystems(the lower estuary of the Guadalquivir Estuary and the Bay of Cádiz).This data set provides us fCO2 with enough resolution to examine theintra-annuality of the air–sea FCO2 of the north-eastern shelf of the Gulfof Cádiz. This region, covering a 0.6°×0.6° area, represents ~10% of thenorthern shelf of the Gulf of Cádiz surface area (Fig. 1), and is a highlydynamic zone both in terms of physical and biogeochemical processes.

This region is of special interest, being immediately adjacent to theStrait ofGibraltar (Fig. 1). It has alreadybeen emphasized that theGulf ofCádiz plays an important role in the carbon cycle of the eastern NorthAtlantic (Parrilla, 1998) and theMediterranean Sea (Dafner et al., 2001).In the Gulf of Cádiz several water masses mix together to form the“Atlantic inflow”, which is responsible for supplying water to theMediterranean Sea from the west (Minas et al., 1991). Moreover, thisarea is on the pathway of the “Mediterranean outflow”which thereafterenters the open ocean and influences water circulation in the entire

Fig. 1. Map of the north-eastern shelf of the Gulf of Cádiz. Isolines represent the bathymetry. The four different zones into which the study site was divided for the discussion aredepicted: two coastal ecosystems (Guadalquivir Estuary and Bay of Cádiz) and the coastal zone separated in two parts (shallower and deeper). The cruise track is shown by thebroken gray line. The location of the buoy from which meteorological and oceanographical data were obtained is also shown.

57M. Ribas-Ribas et al. / Marine Chemistry 123 (2011) 56–66

North Atlantic, and the climate in general (Rahmstorf, 1998). The basinreceives significant fluvial inputs associated with the discharge of largerivers such as the Guadiana, the Guadalquivir, the Guadalete and theTinto–Odiel (Fig. 1). The Guadalquivir River is the main fluvial sourcedraining into the Gulf of Cádiz margin (Fig. 2).

The main objective of the present paper is to investigate fCO2

dynamics in the surfacewaters of the north-eastern shelf of the Gulf ofCádiz (southwest Iberian Peninsula) using four cruises, covering anannual cycle. From the results obtained, it should be possible to assessthe relative contribution of the physical and biological processesaffecting the fCO2 dynamics in the study area, and to compute the air–sea FCO2, taking into account the considerable spatial and seasonalheterogeneity.

The present work is organized as follows:

1) In the following section we briefly present the surface distributionof fCO2 in relation to the physical characteristics of the north-eastern shelf of the Gulf of Cádiz.

Fig. 2. Rainfall and river discharge from the mouth of the Guadalquivir River, during theperiod of study.

2) Next we evaluate the relative contributions of advection, mixing,air–sea exchange, and biological activity to the intra-annual cycleof CO2.

3) Finally, the intra-annual variability of CO2 and its fluxes is thenconsidered.

1.1. Study site

The study was carried out over the north-eastern shelf of the Gulfof Cádiz, which is located on the southwestern coast of the IberianPeninsula (Fig. 1). The circulation in the north-eastern shelf of the Gulfof Cádiz is controlled mainly by the North Atlantic Surface Water(NASW), which flows towards the east and southeast to the Strait ofGibraltar, as well as by an intermittent counter-current system, whichseems to be strongly linked to the wind regime (Lobo et al., 2004). Inparticular, coastal waters near the mouth of the Guadalquivir Riverand the Bay of Cádiz present the highest primary production withinthe Gulf of Cádiz (Navarro and Ruiz, 2006). The coastal fringe of theGulf of Cádiz is also characterized by the presence of waters warmerand colder than those detected in the rest of the basin during June andFebruary, respectively (Navarro and Ruiz, 2006; Vargas et al., 2003)and by considerable meteorological forcing caused by quasi-perma-nent episodes of winds. For example, the predominance of westernwinds is always linked to the generation of upwelling events andtherefore to an increase in primary production; on the other handeasterlies lead to a decrease in phytoplankton (García-Lafuente andRuiz, 2007). Furthermore, the alternation of mixing and stratificationperiods in the region affects the position of the nutricline and thusalso regulates the primary production (Navarro et al., 2006).

2. Materials and methods

2.1. Field sampling and analysis

Four cruises have been undertaken: 17–28 June 2006, 19–30November 2006, 31 January–9 February 2007 on board the R/VMytilus;

58 M. Ribas-Ribas et al. / Marine Chemistry 123 (2011) 56–66

and 21–24May 2007 on board the R/V Ucádiz. The study area is locatedbetween 36.39 and 36.93°N and between 6.83 and 6.25°W.

Sea surface salinity (SSS), sea surface temperature (SST) and fCO2

were sampled with a frequency of 30 s from the surface seawatersupply of the ship (pump inlet at a depth of 3 m). SSS and SST weremeasured, using a SeaBird thermosalinograph (Micro-SeaBird 45),before water entry into the gas equilibrator. SST and SSS are estimatedto be accurate to ±0.004 °C and ±0.005, respectively, according tothe SeaBird calibration data.

The surface water CO2 molar fraction (xCO2) was measured with anon-dispersive infrared gas analyzer (Licor®, LI-6262). At thebeginning and the end of each day, the equipment was calibratedwith two standards: CO2 free-air and a high CO2 standard gas with aconcentration of 530 ppm (with pre-deployment laboratory calibra-tion against Air-Liquide France standard). The temperature inside theequilibrator was measured continuously by means of a platinumresistance thermometer (PT100 probe). The temperature differencebetween the ship's sea inlet and the equilibration system was lessthan 0.8 °C during all the cruises. A pressure transducer (SetraSystems, accurate to 0.05%) was used to measure the pressure insidethe equilibrator. The accuracy (precision) of seawater fCO2 measure-ments was ±3 (±0.5) μatm.

The water-saturated fCO2 in the equilibrator was calculated fromthe xCO2 in dry air; the atmospheric pressure data was provided bythe Spanish national government (Organismo Público Puertos delEstado); and equilibrium water vapor was calculated according to theprotocol described in Dickson et al. (2007). The formulation proposedby Takahashi et al. (1993) was employed for the partial pressurecorrections to in situ water temperature.

2.2. Temperature and biological control in fCO2 calculations

In order to remove the temperature effect from the observed fCO2,the observed fCO2 values have been normalized to a constanttemperature of 19 °C, the mean annual SST at the study site, usingthe equation proposed by Takahashi et al. (2002):

fCO2 at Tmean = fCO2ð Þobs⋅exp 0:0423 Tmean–Tobsð Þ½ � ð1Þ

where T is temperature in °C, and the subscripts “mean” and “obs”indicate the annual average and observed values, respectively. In therest of this article, the fCO2 at the mean temperature will be referredto as fCO2 at 19 °C. The effect of temperature changes on fCO2 has beencomputed by perturbing the mean annual fCO2 in the study area of370 μatm with the difference between the mean and observedtemperature, as Takahashi et al. (2002) proposed:

fCO2 at Tobs = Mean annual fCO2⋅exp 0:0423 Tobs–Tmeanð Þ½ � ð2Þ

where the mean annual temperature of 19 °C was used. The resultingvalues indicate the fCO2 values that would be expected only fromtemperature changes, if a parcel of water having the mean fCO2 valueof 370 μatm is subjected to seasonal temperature changes underisochemical conditions. In the following, the effect of observedtemperature changes on fCO2 will be referred to as fCO2 at Tobs.

The effect of biology on surface water fCO2 in a given area(ΔfCO2)bio is represented by the seasonal amplitude of fCO2 valuesnormalized to themean annual temperature (fCO2 at 19 °C) computedusing Eq. (1):

ΔfCO2ð Þbio = fCO2Tmeanð Þmax− fCO2Tmeanð Þmin ð3Þ

In addition, the changes in fCO2 related to temperature effects follow(ΔfCO2)temp then as:

ΔfCO2ð Þtemp = fCO2Tobsð Þmax− fCO2Tobsð Þmin ð4Þ

The relative importance of each effect can be expressed in termsof the ratio between the temperature effects (T) and biologicaleffects (B):

T= B = ΔfCO2ð Þtemp = ΔfCO2ð Þbio ð5Þ

In areas with high seasonal variability in biological activity, theratio (T/B) would be lower than 1.0; however, in regions with weakerbiological effects or where such effects tend to be fairly constantthroughout the year, the (T/B) ratio would be higher than 1.0.

Dissolved oxygen (DO) was sampled at the various stations, fixedin oceanographicWinkler and stored in darkness for 24 h as describedby Grasshoff et al. (1983), for later analysis by potentiometric titration(Metrohm 670 Titroprocessor). The Apparent Oxygen Utilization(AOU) is defined as the deviation of oxygen from an O2 concentrationin equilibrium with the atmosphere, calculated from the Benson andKrause (1984) solubility equation.

2.3. Flux calculations

Average net FCO2 was estimated based on the formula FCO2=α kΔfCO2 [6], where α is the solubility coefficient of CO2 (Weiss, 1974), kis the gas transfer velocity of CO2 and ΔfCO2 is the mean differencebetween the water and air fCO2. The atmospheric fCO2 data wereobtained from the monthly data at Tercerira Island station (Azores,Portugal), taken from National Oceanic and Atmospheric Administra-tion (NOAA/CMDL/CCGG air sampling network) data available onlineat http://www.esrl.noaa.gov/gmd/dv/ftpdata.html. A negative valueindicates air-to-sea flux and a positive flux value represents the netCO2 exchange from the water body to the atmosphere.

The gas transfer velocity, k, was calculated using the equation k(cm h−1)=0.31 u2 (Sc/660)−0.5 [7] given by Wanninkhof (1992). Scis the Schmidt number of CO2 in seawater; 660 is the Sc value inseawater at 20 °C. The wind speed data, u (m s−1), was recorded atthe Seawatch buoy station (Fig. 1, (36.47°N; 6.96°W)), provided bythe Spanish national government (Organismo Público Puertos delEstado) (Fig. 3), and normalized to 10 m height using the relationshipof Garratt (1977). Daily averaged wind speeds were used for thecalculation of gas transfer velocities. Wind is a crucial climate factorinfluencing marine hydrodynamic conditions on the Cádiz coast,given its frequent strength (Fig. 3). Monthly river discharge data wereprovided by the Confederación Hidrográfica del Guadalquivir and dailyrainfall data at a station situated in Rota were provided by AgenciaEstatal de Meteorología (Fig. 2).

2.4. Statistical analysis

Intra-annual and spatial differences (between the lower estuary ofGuadalquivir Estuary, Bay of Cádiz and the shelf zone) in hydrologicaland biogeochemical characteristics were analyzed using one-wayanalysis of variance (ANOVA) followed by the Bonferroni post hoc test(Statgraphics Plus 5.1). The threshold value for statistical differencewas taken as pb0.05.

SSS, SST, fCO2 and AOU were represented with Ocean Data View(ODV) (Schlitzer, 2009). AOU was interpolated using the DIVAGridding method.

3. Results

Overall, our June cruise reflected the dry summer but not theannual SST maximum, which occurred in August; the Novembercruise took place during the rainy autumn; our February cruiserepresented the cold winter; and finally the cruise in May showed theconditions in spring. Fig. 4 shows surface water temperature, salinityand fCO2 during the four sampling cruises and Table 1 shows themeanand standard deviations of surface salinity, temperature, fCO2 and DO

Fig. 3. Wind roses showing the predominant wind direction in the sampling area in summer, fall, winter and spring.

59M. Ribas-Ribas et al. / Marine Chemistry 123 (2011) 56–66

for the different areas during four cruises. Fig. 5 shows the surfacewater AOU during the four sampling cruises. We do not have datafrom the lower estuary of Guadalquivir Estuary for the May.

3.1. Hydrological settings

Fig. 2 shows the rainfall and river discharge from the mouth of theGuadalquivir River, during the period of study. Water discharge fromthe Guadalquivir River is subject to significant seasonal variation, witha minimum discharge during January and March 2007 and amaximum discharge in November 2006 and May 2007.

Fig. 3 shows the wind roses in the sampling area during differentseasons. The wind speed ranged from 2.7 to 8 m s−1 during thesummer,withwinddirection predominantly from theWSW.During theautumn,wind speed and directionweremore variable, ranging from2.1to12.0 m s−1,winddirectionwaspredominantly fromNE to SE; and theaverage wind speed was 5.4 m s−1, the highest recorded on the fourseasons. During the winter, the predominant direction was again fromthe NE; wind speed ranged from 1.6 to 12.1 m s−1, with an averagespeed of 5.2 m s−1. Finally, during the spring, the wind was predom-inantly northwesterly,with speed ranging between 1.5 and 10.9 m s−1;

thiswaswhen the lowest averagewind speed (4.6 m s−1)was recorded(Fig. 3).Wind speed values at a height of 10 mduring the sampling daysused for the k calculationswere homogeneous, ranging from4.8±1.0 to5.5±2.9 m s−1 in May and November, respectively.

SST and SSS values presented a wide range of variation. The SSTand SSS distribution is generally related to the Guadalquivir Riverdischarge.

3.1.1. Lower estuary of Guadalquivir EstuaryThe SST increased from its lowest values of 14.0±0.5 °C in

February and 18.1±0.5 °C in November, to high values in June(24.3±0.5 °C) (Fig. 4 and Table 1). In general, temperature in thisarea is higher than in the others, because of heating of the shallowarea and the terrestrial input. The SSS decreased from 35.9±1.2 inJune to its lowest value of 30.2±7.8 in November (Fig. 4), when thefreshwater discharge was the highest for the entire study period(Fig. 2). In February SSS returned to a relatively high value of 34.3±1.2 (Fig. 4 and Table 1). De la Paz et al. (2007) reported duringsummer in the outer station of Guadalquivir Estuary values of salinityfrom 32 to 36 and of temperature from 21 to 25 °C. In winter, theyshowed values of temperature slightly lower of those reported in our

Fig.

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60 M. Ribas-Ribas et al. / Marine Chemistry 123 (2011) 56–66

Table 1Mean and standard deviations of surface salinity, temperature, fugacity of CO2 and dissolved oxygen for the deeper coastal zone (zN20 m), shallower coastal zone (zb20 m), the Bayof Cádiz and the lower estuary of Guadalquivir Estuary (see Fig. 1) during the four cruises.

Temperature(°C)

Salinity fCO2

(μatm)DO(μmol kg−1)

Deeper coastal zone Jun'06 22.5±0.5 36.30±0.08 382.4±10.4 223.6±16.1Nov'06 19.3±0.4 36.03±0.38 352.8±22.4 225.3±12.9Feb'07 15.3±0.4 36.26±0.09 358.3±12.7 235.9±8.5May'07 19.2±0.4 36.06±0.04 350.9±8.8 240.7±10.3

Shallower coastal zone Jun'06 23.1±1.0 36.31±0.07 397.0±14.8 216.0±12.9Nov'06 19.0±0.5 35.57±0.88 366.8±25.1 219.7±15.6Feb'07 14.2±0.7 35.67±0.57 338.2±21.1 245.2±12.0May'07 19.5±0.8 36.05±0.12 358.0±23.7 241.3±12.4

Bay of Cádiz Jun'06 22.9±0.7 36.45±0.08 409.1±11.9 208.5±9.7Nov'06 18.7±0.4 35.90±0.22 410.7±19.7 211.7±12.7Feb'07 13.8±0.7 35.04±1.03 341.0±17.6 244.9±3.9May'07 19.3±0.7 36.17±0.02 386.7±13.0 n

Lower estuary of Guadalquivir Estuary Jun'06 24.3±0.5 35.86±1.24 419.5±69.8 207.8±8.4Nov'06 18.1±0.5 30.21±7.77 501.7±301.2 202.8±16.1Feb'07 14.0±0.5 34.28±1.18 361.6±38.6 252.8±16.5May'07 n n n n

61M. Ribas-Ribas et al. / Marine Chemistry 123 (2011) 56–66

study, around 15 °C and salinity of 33. This seasonal and inter-annualvariability are normal taking into account the variation of riverdischarge and rainfall.

3.1.2. Bay of CádizThe SST changed from its lowest values of 13.8±0.7 °C in

February, through 18.7±0.4 °C in November and 19.3±0.7 °C inMay, to high values in June (22.9±0.7 °C) (Fig. 4 and Table 1). The

Fig. 5. Distributions of apparent oxygen utilization (in μmol kg

change in SSS was not as significant as in the lower estuary ofGuadalquivir Estuary but changes could still be identified betweencruises. The SSS decreased from 36.5±0.1 in June, through 35.9±0.2in November, to the lowest value of 35.0±1.0 in February (Fig. 4 andTable 1). This small decrease in SSS was due to the Guadalete River. InMay, SSS returned to a relatively high value of 36.2±0.0. In the Bay ofCádiz, Forja et al. (2001) reported average values of 34.6 and 19.5 °C inan outer station of the Bay of Cádiz.

−1) in surface water, for the four different survey cruises.

62 M. Ribas-Ribas et al. / Marine Chemistry 123 (2011) 56–66

3.1.3. Shelf zoneThe SST varied significantly among all the surveys (pb0.001) and

showed an increase onshore and towards the northern stations inJune 2006 (21.0–24.2 °C) and May 2007 (18.2–20.3 °C), whereas inNovember 2006 (18.5–19.5 °C) and February 2007 (13.5–15.7 °C) thetrendwas opposite and temperature decreased towards the shallowerstations. The SSS was significantly higher (pb0.001) in June 2006(36.23–36.38) relative to the other three surveys, with valuesincreasing onshore. In November 2006 (34.69–36.41), February2007 (35.1–36.35) and May 2007 (35.93–36.17) SSS decreasedonshore, especially at the northern stations near the mouth of theGuadalquivir Estuary (Fig. 4 and Table 1). Similar seasonal trend andvalues range have been described by Navarro and Ruiz (2006) in thecoastal zone between Guadiana and Guadalquivir mouths. Duringwinter, the most coastal station reported by González-Dávila et al.(2003) was characterized by a temperature of 16.1 °C and salinity of36.1, which are in agreement with our observations. They pointed outalso the influence of river discharge in the Gulf of Cádiz area.

3.2. Distributions of sea surface fCO2 and other biogeochemical properties

3.2.1. Lower estuary of Guadalquivir EstuaryAround the lower estuary of Guadalquivir Estuary, rich CO2 water

was observed all year around. In our June survey, the fCO2 values for thewater ranged between 350 and 850 μatm. In November fCO2 values fellin the range between 400 and 1700 μatm, the highest value sampledduring the studied period. It should be noted that, during November2006, sampleswere taken further upstream in the river than in theothercruises. In contrast,we observed fCO2 values between 260 and 460 μatmin February; some of these values are slightly lower than those for theatmosphere (Fig. 4 and Table 1).

3.2.2. Bay of CádizIn our June and November cruises, fCO2 values in the Bay of Cádiz

ranged from390 to 430 μatm; values in this range arehigher than those ofthe atmospheric fCO2 level. In contrast, in Februarywe observed a level offCO2generally lower than that of the atmosphere. Then inMay fCO2valueswere again higher, reaching 410 μatm inside the Bay of Cádiz (Fig. 4).

The AOU values ranged from 0 to 10 μmol kg−1 during June,November and February. Values lower than −20 μmol kg−1 werefound inMay in the zonedirectly opposite themouth of the Bay of Cádiz(Fig. 5).

3.2.3. Shelf zoneThe part of this zone found to present most variation is the

northern area directly facing the lower estuary of GuadalquivirEstuary. In the southern part of the shelf zone, fCO2 values were lessvariable (at around 360 μatm) (Fig. 4).

fCO2 was significantly lower during winter and spring (pb0.001)relative to summer and fall. In June 2006, fCO2 surface water increasednorthwards and onshore and varied between 370 and 412 μatm.During fall, the fCO2 ranged between 270 μatm (coinciding with thelowest water salinity and a lower AOU values) and 390 μatm. InFebruary 2007, the lowest fCO2 values were recorded, ranging from260 μatm in the northern part of the study area to 370 μatm in thesouthern part. These data showed a slight increase towards the south.In May 2007, the values range from 300 μatm in the northern part ofthe study site to 380 μatm in the outer area and in the south.Generally, the equilibrium value is found closer to the coast in thesouth than in the north, where the platform is wider. During spring,fCO2 reaches the equilibrium value with the atmosphere closer to thecoast than in summer and fall.

AOU ranged from a value lower than−40 μmol kg−1 to one higherthan 10 μmol kg−1 (Fig. 5). AOU values showed a strong gradient inthe northern part of the shelf area during June, reaching values lowerthan −40 μmol kg−1 (Fig. 5), decreasing towards the south. Between

November and February the lowest values coincided with the lowestfCO2 values (Figs. 4 and 5). Moreover, surface distribution during Mayshowed the lowest values recorded during the study periodwithwell-oxygenated waters in the southwest area and opposite the Guadal-quivir Estuary.

We compare our results with prior researches in the region andwith other regions which have seasonal coverage: during winter, themost coastal station reported by González-Dávila et al. (2003) wascharacterized by fCO2 of 338 μatm and AOU of −5.26 μmol kg−1. ThefCO2 values fit with our observation but not AOU value. This differencecould be due to the storm conditions these authors reported in theirstudy, which suit the oxygenation of the water column. Huertas et al.(2006) reported temporal patterns of carbon dioxide in the north-eastern shelf of the Gulf of Cádiz. Their study area was locatednorthern than ours but we could still make some comparisons. Duringspring, pCO2 average for the whole sampling area was 196 μatm, lowvalue due to the development of the spring phytoplankton bloom. Insummer, their average pCO2 value was 448 μatm. In winter thesituation reversed, with an average of 230 μatm.

González-Dávila et al. (2009) reported in the upwelling coastalregion of Angola-Benguela a complex seasonal distribution with areasof super-saturation and low CO2 concentrations, ranged between 138and 490 μatm. They also defined fourmain regions in order to describethe observed behavior. Zhai and Dai (2009) examined the seasonalvariation of air–sea FCO2 in the outer Changjiang (Yangtze River)Estuary, East China Sea. Their values ranged from less than 220 tohigher than 600 μatm. Their winter cruise showed overall lower thanthe atmospheric pCO2 level, in agreement with our study. Their datashowed significant spatial variability of aqueous pCO2 as well aspossible intra-seasonal variations. Thomas et al. (2005) investigatedthe seasonal variability of the pCO2 in the North Sea. Their pCO2 valuesranged from 450 μatm in summer and 200 μatm in spring, whatmatchup with our result.

4. Discussion

It is important to note that the June survey did not take placeduring the annual SST maximum, which occurs in August orSeptember (data from the buoy, not shown). The February survey,however, took place during the SST minimum (which occurs betweenFebruary and April). This is an important consideration because theJune values found in this study would be greater in magnitude duringAugust–September.

Given the distribution of fCO2 in surface waters for the differentcruises (Fig. 4) and based on the observed statistical differences, 4distinct zones can be identified for the discussion of results. The first isthe Bay of Cádiz, which has a calculated area of 100 km2; this areamatches that described by Sánchez de Lamadrid Rey et al. (2002), forthe mean area of spring tide and low tide. The second is the lowerestuary of Guadalquivir Estuary, with a calculated area of 180 km2. Inrespect of the shelf zone, there is a shallower part (zb20 m) and adeeper part (zN20 m) with calculated areas of 470 km2 and 930 km2,respectively (Fig. 1 and Table 2).

4.1. Control over fCO2 in the lower estuary of Guadalquivir Estuary andthe Bay of Cádiz: mixing

Fig. 6 shows the mixing diagram of fCO2 in the lower estuary ofGuadalquivir Estuary and the Bay of Cádiz. It should be noted that, inthe November cruise, we sailed further upstream in the river than inJune; in February only the transport transect was carried out; and inMay we did not sample lower estuary of Guadalquivir Estuary due tologistic problems (see the various cruise tracks in Fig. 4). This is onereason why the lowest salinity values (less than 5) were found inNovember. The other reason is that the highest river discharge andrainfall values during the study period were in November 2006

Table 2Area, average air–water fCO2 gradient, air–water CO2 flux and CO2 emission at the deeper coastal zone (zN20 m), shallower coastal zone (zb20 m), the Bay of Cádiz and the lowerestuary of Guadalquivir Estuary (see Fig. 1) in different cruises.

Surface area(km2)

ΔfCO2 (μatm) Air–water CO2 Flux(mmol m−2 day−1)

CO2 emission(106 mol C day−1)

Deeper coastal zone 930 Jun'06 12.0±10.4 0.9 0.9Nov'06 −22.5±22.4 −0.9 −0.8Feb'07 −22.3±12.7 −1.3 −1.2May'07 −21.8±8.8 −1.0 −1.0

Shallower coastal zone 470 Jun'06 26.6±14.8 2.0 1.0Nov'06 −8.5±25.1 0.1 0.0Feb'07 −42.4±21.1 −2.2 −1.0May'07 −14.8±23.7 −0.7 −0.3

Bay of Cádiz 100 Jun'06 38.7±11.9 1.2 0.1Nov'06 35.4±19.7 3.6 0.4Feb'07 −39.5±17.6 −1.4 −0.1May'07 13.9±13.0 0.7 0.1

lower estuary of Guadalquivir Estuary 180 Jun'06 49.1±69.8 3.3 0.6Nov'06 126.4±301.2 2.8 0.5Feb'07 −19.0±38.6 −1.3 −0.2May'07 n n n

63M. Ribas-Ribas et al. / Marine Chemistry 123 (2011) 56–66

(Fig. 2). In general, the lower estuary of Guadalquivir Estuary showeda profile against salinity typical for estuarine environments with fCO2

increasing with decreasing salinities. It should be pointed out that thelack of data for the estuarine zone during February and May isaffecting Figs. 4 and 6. Thus, more data in the low salinity-high fCO2

range would be expected if these data were available. This pattern hasbeen already observed for the Guadalquivir Estuary (de la Paz et al.,2007) and in other estuarine systems as in the Scheldt plume (Borgeset al., 2008; Schiettecatte et al., 2006); the Cochin Estuary (Gupta etal., 2009) or in the Loire plume (de la Paz et al., 2010).

A different pattern is observed in the Bay of Cádiz. Salinity and fCO2

ranges are lower than in the lower estuary of Guadalquivir Estuary. Ingeneral, there were positive slopes for all seasons. However, in wintertwo data sets could be distinguished. One with higher salinities with apositive slopes; and the other one with a slightly increase of fCO2

while salinity decrease. The first pattern is the typical observed in thecontinental shelf data, with positive or null relation between fCO2 andsalinity. The second pattern is due to the Guadalete River discharge,that had the same behavior of the Guadalquivir River but in a smallerscale, i.e. lower river discharge and hence, higher salinities.

Guadalquivir Estuary

1200

1400

1600

1800

fCO

2 (µ

atm

)

600

800

1000

Jun'06

Salinity

5 10 15 20 25 30 35200

400 Nov'06Feb'07

Fig. 6. Mixing diagrams for fCO2 in the Guadalquivir Estuary and the Bay of Cádiz during dif(dark grey triangles) and May 2007 (pale grey triangles). Note that fCO2 and salinity scalesGuadalquivir Estuary.

4.2. Control over fCO2 in the shelf zone: temperature versus biological factors

The fCO2 in surface seawater is controlled by thermodynamics,biological uptake, air–sea gas exchange, freshwater discharge,sediment/water column exchanges, physical mixing and air–sea FCO2.

It is necessary to differentiate the effect of temperature on seasonalamplitude from the effect of the total CO2 concentration. The relativeinfluence of these two variables for regulating the seasonal amplitude infCO2 is important, not only for understanding the time-space variabilityin the sea–air CO2 exchangebut also for planning futurefield research. Inorder to separate out these factors, the method proposed by Takahashiet al. (2002) was applied to the data from June 2006 to May 2007. Thismethod was originally designed for open oceanic systems. However, ithas beenwidely used by other authors to assess the role of temperatureversus biological factors in the control of fCO2 dynamics in coastal areas(de la Paz et al., 2009; Schiettecatte et al., 2007; Thomas et al., 2005). Inoceanic regions the non-temperature effect can be attributed almostentirely to the “net biology effect”. In contrast, the non-temperatureterm for coastal waters contains all the biochemical processes acting onthe net CO2 utilization (primary production and respiration), including

500

400

450

Bay of Cádiz

fCO

2 (µ

atm

)

250

300

350

Jun'06Nov'06

Salinity

24 26 28 30 32 34 36200

Feb'07May'07

ferent cruises: June 2006 (black circles), November 2006 (gray circles), February 2007are different for the two diagrams and that in the spring no sampling was done in the

Temperature observed fCO2

fCO2@Tobs fCO2@19oC

20

22

24

400

420

440 Annual average fCO2

Tem

pera

ture

[o C]

12

14

16

18

300

320

340

360

380

Date

06/06 09/06 12/06 03/07 06/07

fCO

2 (µ

atm

)

Fig. 7. Seasonal variations in observed fCO2, fCO2 at the mean temperature (fCO2 at19 °C) and the effect of observed temperature changes on fCO2 (fCO2 at Tobs). Otherparameters plotted are the average temperature for each cruise and the annual averagefCO2.

Shelf zone460

R2= 0.42

360

380

400

420

440

12 14 16 18 20 22 24 26280

300

320

340

Temperature (oC)

fCO

2 (µ

atm

)

fCO2 =269.1+5.03*T

Fig. 8. Diagram showing the relationship between fCO2 and temperature in the shelf.

Deeper coastal zone Shallower coastal zone

Bay of Cádiz Estuary of Guadalquivir

Monthly wind speed

0

1

2

3

5.5

6.0

6.5

7.0

Date

jun'06 sep'06 dic'06 mar'07 jun'07

CO

2 flu

x (m

mol

m-2

d-1

)

-2

-1

u 10

(m s

-1)

4.5

5.0

Fig. 9. Seasonal variability in air–sea CO2 fluxes during the four cruises in the fourdifferent areas shown in Fig. 1. Monthly average wind speed data normalized to 10 mheight is also shown.

64 M. Ribas-Ribas et al. / Marine Chemistry 123 (2011) 56–66

other processes alsogoverning theCO2 suchas riverine input, changes ininorganic carbon by advection or air–sea FCO2. These processes can bevery significant in the coastal ocean. In the originalmethod of Takahashiet al. (2002) these processes would be attributed inherently to thebiological signal (de la Paz et al., 2010). We have applied this methodonly for data from the shelf zone, excluding data from the lower estuaryof Guadalquivir Estuary and Bay of Cádiz.

Changes of fCO2 at 19 °C primarily represent changes in the totalCO2 concentration. Fig. 7 shows the intra-annual variations inexperimental fCO2 and the biological and temperature effectsrepresented by fCO2 at 19 °C and fCO2 at Tobs. The intra-annualevolution of the observed fCO2 exhibited a maximum in June, with aprogressive decrease towards November, and then remained almostconstant in February and May. fCO2 at Tobs is highest in June andlowest in February while fCO2 at 19 °C follows the opposite trend. Thepeak-to-peak amplitudes of seasonal variation in temperature andfCO2 are 7.5 (±0.8) °C and 31 (±15) μatm, respectively. Meanseasonal amplitudes of fCO2 at Tobs of about 115 μatm, and of fCO2 at19 °C of about 88 μatm, are observed during the study period. Unlikethe temperature and “net biological” effect, the observed seasonalamplitude is significantly lower.

From June 2006 to December 2006, the fCO2 is lower than onewould expect due to rising SST, which appears to be caused bybiological uptake of CO2. From January to April, the fCO2 stays higherthan expected from the drop in temperature. This seems to be causedmainly by the deepening of the mixed layer, which introduces waterswith a higher total inorganic carbon content to the surface waters.

The temperature and biological effects are approximately6 months out of phase, because photosynthesis is increased at warmertemperatures. This suggests that the effect of February-to-Juneseawater warming in increasing the fCO2 (115 μatm) is partiallycompensated by the effect of biological utilization of CO2 in decreasingthe fCO2 (88 μatm); this yields a seasonal fCO2 amplitude of 27 μatm(very close to the real value of 31 μatm).

In this study, the T/B ratio is 1.3, indicating that temperature is thekey factor controlling fCO2 intra-annual variation, although thebiological effect is also very important during February. González-Dávila et al. (2009) found valueswhich ranged from 0.8 to 1.4 inwaterwithout offshore filaments influence in the Angola-Benguela region.These authors also found seasonal and spatial variations of this ratio.In the North Sea, Thomas et al. (2005) report that temperature controldominates the biological control at an annual scale in the southernpart of their study, the one closer to the coast and river discharge.

We have plotted the fCO2 of surface waters relative to SST in Fig. 8.Normally, the highest values are observed in the summer months. Itcould be seen that higher temperatures correspond with higher fCO2,which means that in the higher temperature range, CO2 tends to besuper-saturated with respect to the atmosphere; conversely, in thelower temperature range, CO2 tends to be under-saturated. Strongthermal correlations were observed, which were higher in warmersurveys (e.g. June 2006; 388.6±14.5, n=5592) compared to coldersurveys (e.g. February 2007; 357.6±15.1, n=3393).

4.3. Air–sea CO2 exchange

Fig. 9 and Table 2 show the intra-annual andspatial variability in air–sea FCO2 for eachcruise. Theaverage FCO2was1.51 mmol m−2 day−1 inJune and−1.53 mmol m−2 day−1 in February. On the annual scale, theregion acted as a CO2 sink at a rate of −0.20±1.3 mmol m−2 day−1

(−0.07 mol m−2 year−1). A similar value was obtained by Ríos et al.(2005) in the area of the Azores. Aït-Ameur and Goyet (2006)reported that the surface water of the Gulf of Cádiz acts as a very smallnet source of CO2 for the atmosphere during July, coinciding with thetrend observed in this work. A previous study carried out along anoceanic transect in the Gulf of Cádiz in February 1998 computed alarger flux of −19.5±3.5 mmol m−2 day−1; however, the analysis

65M. Ribas-Ribas et al. / Marine Chemistry 123 (2011) 56–66

was performed under storm conditions with a high wind regime(15±3m s−1) that could have had a substantial impact on the gastransfer (González-Dávila et al., 2003).

The FCO2 values computed here are subject to two (ormore) sourcesof error: i) wind speed variation; and ii) the formulation of the gastransfer coefficient. The first error is low due to the averaging affect in arelatively large database. The other source of error in the FCO2 estimatedepends on the choice of the air–sea CO2 gas transfer coefficient.

Taking the various areas as defined in the previous section incalculating the FCO2, differences may be observed.

4.3.1. Lower estuary of Guadalquivir Estuary and Bay of CádizThe lower estuary of Guadalquivir Estuary and the Bay of Cádiz

acted as a source of CO2 to the atmosphere during June, November andMay and a slight sink during February. The maximum out-gassingtook place in June and November. On an annual basis, the lowerestuary of Guadalquivir Estuary and the Bay of Cádiz have a fluxof +1.39 mmol m−2 day−1 (+0.51 mol m−2 yr−1).

As in most aquatic ecosystems, in the lower estuary of Guadalqui-vir Estuary respiration exceeds autochthonous gross primary produc-tion. The resulting negative net ecosystem production is fuelled byorganic carbon “leaking” from the terrestrial system and causing theGuadalquivir, like most rivers, to be highly super-saturated in CO2 andto act as a source of CO2 to the atmosphere (de la Paz et al., 2007).

4.3.2. Shelf zoneDuringmost of the year, the coastal zone of the Gulf of Cádiz studied

behaves overall as a moderate sink for atmospheric CO2 except in June,following the temperature and biology cycle described in the previoussection.

On an annual basis, the shelf zone has an average FCO2 of −0.44 mmol m−2 day−1 (−0.16 mol m−2 year−1), while the maximumuptake took place in February in the deeper coastal area.

The behavior of the shelf zone as a sink is in agreement with theresults obtained by Huertas et al. (2006), who estimated an annualaverage FCO2 in the western part of the Gulf of Cádiz of −0.15±0.36 mol m−2 year−1.

During November, February and May under-saturation of surfaceseawater with respect to atmospheric CO2 was found in theGuadalquivir River plume which is advected into the more oceanicwaters. There is a dramatic change of the CO2 saturation state fromhighly super-saturated river waters to markedly under-saturatedsurface waters in the plume; this finding is best observed in theNovember cruise data (Fig. 4). This change can be explained by acombination of three effects: 1. CO2 out-gassing from river water; 2.the effect of mixing between river and ocean water on the CO2 systemproperties; and 3. the effect of strong biological carbon drawdown inthe plume.

When the Guadalquivir waters reach the ocean, they becomeincreasingly less super-saturated, and eventually under-saturated,with respect to atmospheric CO2. This finding could be a result of astrong shift in the equilibrium of the chemical CO2 system due tomixing of river water with high-alkalinity ocean waters (Ternon et al.,2000). It could be due also to the onset of phytoplankton blooms, dueto rapid settling of river-borne suspended particles; this phenomenonallows waters to become sufficiently transparent for photosynthesis(Edmond et al., 1981). A wider geographic distribution of samples(northern and oceanic) is needed to identify the influence of thisunder-saturated water in the biogeochemistry of the Gulf of Cádiz ingeneral. This finding could explain the low values found by Huertaset al. (2006) during May. This phenomenon has also been observed inthe Amazon and the Scheldt plumes by Körtzinger (2003) and Borgesand Frankignoulle (2002), respectively.

The differences in behavior observed between the shelf zone, onthe one hand, and the lower estuary of Guadalquivir Estuary and theBay of Cádiz on the other, agree with the result reported by Chen and

Borges (2009), which concludes that continental shelves act as sinksand near-shore ecosystems act as sources of atmospheric CO2.

5. Conclusions

In the study area of the shelf, thermodynamic control over fCO2

predominates from early May to late November, and this is oppositeand similar in magnitude to the net biological effect. fCO2 is controlledmostly by temperature in the shelf area.

High variability of fCO2 between seasons and between zones hasbeen observed. Higher values were observed in June, and in the lowerestuary of Guadalquivir Estuary.

In summary, the coastal zone acts as a sink on an annual scale, andnear-shore ecosystems are a source of atmospheric CO2. The coastalzone of the Gulf of Cádiz (1.68·109 m2) acts as a sink on an annualscale, absorbing 1.5·10−3 Tg–C yr−1 (1.25·108 mol C yr−1). Thisflux was calculated for the year 2006–2007. Nevertheless, in thelight of the results obtained by Huertas et al. (2006) when higherfluxes were observed inMarch 96 comparedwithMarch 95 due to theincreased wind speed, significant inter-annual variability in FCO2 canbe expected.

Acknowledgements

The authors thank the crews of the R/V Mytilus and R/V Ucádiz fortheir valuable assistance during the cruises, and are grateful to theSpanish Puertos del Estado, Confederación Hidrográfica del Guadalquivirand Agencia Estatal de Meteorología for providing data. We also thankthe NOAA/ESRL Global Monitoring Division for providing atmosphericCO2 flask data recorded in the Azores. Dr. de la Paz gave support in thetechnical aspect of the gas equilibrator and subsequent datatreatment. Dr. Körtzinger provided useful ideas in the first stage ofthis work, and Dr. Hernández-Ayón provided valuable comments thatstrengthened the manuscript. The staff from Departamento de FísicaAplicada at University of Cádiz help with theMatlab rutines.We thankthe two anonymous reviewers and Dr. Chen (associate editor) fortheir helpful discussions and comments which have greatly improvedupon the original manuscript. This work was supported by theSpanish CICYT (Spanish Program for Science and Technology) undercontract CTM2005-01364/MAR and P06-RNM-01637. M. R. R. wasfinanced by the Spanish Ministry of Education with a FPU fellowship(AP2005-4791).

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