Factors controlling pCO 2 variability in the eastern Gulf of Cádiz ( SW Iberian Península )

Spatiotemporal variations of the partial pressure of CO2 (pCO2) were studied during 8 oceanographic cruises conducted between March 2014 and February 2016 in surface waters of the eastern shelf of the Gulf of Cádiz (SW Iberian Península) between the Guadalquivir River and Cape Trafalgar. pCO2 presented a range of variation between 320.6 and 513.6 μatm, with 15 highest values during summer and autumn and lowest during spring and winter, showing a linear dependence between pCO2 and temperature. The distributions of pCO2 were not homogeneous. Spatially, there was a general decrease from coastal to off-shore stations associated with continental inputs and presented an increase in the zones deeper than 400 m due to the influence of the eastward branch of the Azores Current. On the other side, the study area acted as source of CO2 to the atmosphere during summer and autumn and as a sink in spring and winter, with a mean value for the study period of -0.18 ± 20 1.32 mmol m d. In the Guadalquivir and Sancti Petri sections, the CO2 fluxes decreased towards offshore, whereas in the Trafalgar section increased due to the presence of an upwelling. These results highlighted the Gulf of Cádiz as a CO2 sink, with a capture capacity of 14.9 Gg year.


Introduction
Continental shelves play a key role in the global carbon cycle, as this is where the interactions between terrestrial, marine and 25 atmospheric systems take place (Mackenzie et al., 1991;Walsh, 1991;Smith and Hollibaugh, 1993).These zones are considered to be among the most dynamic in biogeochemical terms (Wollast, 1991;Bauer et al., 2013), as they are affected by several factors, particularly high rates of primary production, remineralization and organic carbon burial (Walsh, 1988;Wollast, 1993;de Hass et al., 2002).Continental shelves account for about 10 -15 % of the ocean primary production and they contribute approximately 40 % of the ocean's total carbon sequestration, by particulate organic carbon (Muller-Karger et 30 al., 2005).
Generally, waters over the continental shelf account for ~15 % of the global ocean CO2 uptake (-2.6 ± 0.5 Pg C yr -1 , Le Quéré   et al., 2017).Using direct surface ocean CO2 measurements from the global Surface Ocean CO2 Atlas (SOCAT) database, Laruelle et al. (2014) estimated a sea-air exchange of CO2 in these zones of -0.19 ± 0.05 Pg C yr -1 , lower than other studies published in the last decade (e.g.Borges et al., 2005;Cai et al., 2006;Chen and Borges, 2009;Laruelle et al., 2010;Chen et 35 al., 2013).The discrepancies with respect to this estimation derive from the different definitions of the continental shelf domain and the skewed distribution of local studies (Laruelle et al., 2010).In several studies, it has been observed that the continental Ocean Sci.Discuss., https://doi.org/10.5194/os-2019-6Manuscript under review for journal Ocean Sci. Discussion started: 15 February 2019 c Author(s) 2019.CC BY 4.0 License.
shelves present different behaviours according to their latitude: they tend to act as a sink of carbon (-0.33 Pg C yr -1 ) at high and middle latitudes (30 -90º) and as a weak source (0.11 Pg C yr -1 ) at low latitudes (0 -30º) (Cai et al., 2006;Hofmann et al., 2011;Bauer et al., 2013;Chen et al., 2013;Laruelle et al., 2014Laruelle et al., , 2017)).Laruelle et al. (2010) found differences between 40 the two hemispheres: the continental shelf seas of the Northern Hemisphere are a net sink of CO2 (-0.24 Pg C yr -1 ) and those of the Southern Hemisphere are a weak source of CO2 (0.03 Pg C yr -1 ).
The behaviour of the continental shelf presents a high spatiotemporal variability of the air-sea CO2 fluxes due to various processes, particularly thermodynamic effects, biological processes, gas exchange, upwelling zones and continental inputs (e.g.Chen and Borges, 2009;Ito et al., 2016).Thermodynamic effects are controlled by the inverse relationship between 45 temperature and solubility (0.0423 ºC -1 , Takahashi et al., 1993), which produces changes in CO2 dissociation.Biological processes can induce CO2 uptake or release, deriving respectively from phytoplankton photosynthesis that decreases the concentration of inorganic carbon, and respiration by plankton and all other organisms (Fennel and Wilkin, 2009).Both, thermodynamic effects and biological processes are associated with the sea-air CO2 exchange by physical and biological pumps (Volk and Hoffert, 1985).The effects of upwelling systems generate uncertainty (Michaels et al., 2001).Although this process 50 produces a vertical transport that brings up CO2 and remineralized inorganic nutrients from deep seawater (Liu et al., 2010), upwellings are also responsible for high rates of primary production and a reduction of pCO2 under the equilibrium with the atmosphere (e.g.van Geen et al., 2000;Borges and Frankignoulle, 2002;Friederich et al., 2002).Several studies indicate that these systems act as either a source or sink of CO2 depending on their location (Cai et al., 2006;Chen et al., 2013) and the ocean considered.Upwelling systems at low latitudes act mainly as a source of CO2 but as a sink of CO2 at mid-latitudes 55 (Frankignoulle and Borges, 2001;Feely et al., 2002;Astor et al., 2005;Borges et al., 2005;Friederich et al., 2008;González-Dávila et al., 2009;Santana-Casiano et al., 2009).Upwelling systems in the Pacific and Indian Oceans act as sources of CO2 to the atmosphere, whereas in the Atlantic Ocean they are sinks of atmospheric CO2 (Borges et al., 2006;Laruelle et al., 2010).
The Gulf of Cádiz, located on the south-west of the Iberian Peninsula, is part of the Iberian/Canaries Current System and the Eastern Boundary Upwelling System (EBUS) (Borges et al., 2006;Relvas et al., 2007;Arístegui et al., 2009;Laruelle et al., 60 2010).Although it is really a sub-region of this upwelling system; however, it has a seasonal behaviour due to the coastline configuration and the exchange of water masses with the Mediterranean Sea (Arístegui et al., 2009).Finally, the inner shelf because it is more affected by riverine inputs of nutrients and terrestrial carbon (e.g.Gypens et al., 2011;Vandemark et al., 2011) and by human impact (Cohen et al., 1997).The influence of both factors, riverine inputs and human impact, decrease towards offshore (Walsh, 1991).Several studies have determined that the inner shelf tends to act as a source of CO2 and the 65 outer shelf as a sink (e.g.Rabouille et al., 2001;Cai, 2003;Jiang et al., 2008Jiang et al., , 2013;;Arruda et al., 2015).The inner platform (depth less than 40 m) also presents greater seasonal variability of temperature than the outer platform, and consequently the effect of temperature on pCO2 will be greater in the inner platform (Chen et al., 2013).
The Gulf of Cádiz is a geographical domain of considerable interest due to its location.In addition to receiving the outflow of Mediterranean waters through the Strait of Gibraltar, it receives freshwater inputs from several major rivers, i.e.Guadalquivir, 70 Tinto, Odiel and Guadiana.Various studies have been conducted in this area to evaluate the variability of the sea surface pCO2, although they cover smaller areas and shorter duration of time than this work (González-Dávila et al., 2003;Aït-Ameur and Goyet, 2006;Huertas et al., 2006;Ribas-Ribas et al., 2011) or only a specific area like the Strait of Gibraltar (Dafner et al., 2001;Santana-Casiano et al., 2002;de la Paz et al., 2009).All of these studies, however, have determined that this zone behaves as a global sink of CO2, with seasonal variations mainly induced by the combination of the fluctuations of biomass In the study reported in this paper, the main objective is to evaluate the spatial and seasonal variation of the sea-surface CO2 on the eastern shelf of the Gulf of Cádiz.In addition, we aim to assess the relative contribution of the temperature and nontemperature effects to the total CO2 concentration, and to determine if the area as a whole acts as a sink or a source of CO2 to the atmosphere over time.To do this, we have analysed a surface measurement database of >26000 values of pCO2 during 80 cruises made between 2014 and 2016 and covering a 0.8º x 1.3º area of the Gulf of Cádiz.

Study area
This study was carried out over the eastern shelf of the Gulf of Cádiz (Fig. 1), which forms a large basin between the southwest of the Iberian Peninsula and the northwest of Africa, where the Atlantic Ocean connects with the Mediterranean Sea through 85 the Strait of Gibraltar.In the Strait of Gibraltar takes place a bilayer flow, with an upper Atlantic layer flowing towards the Mediterranean basin and a deeper outflow of higher-density Mediterranean waters to the Atlantic Ocean (e.g.Armi and Farmer, 1988;Baringer and Price, 1999;Sánchez-Leal et al., 2017).A similar circulation pattern of opposing flows is found in the Gulf of Cádiz where three main water masses are distributed at well-defined depth intervals and areas: the Surface Atlantic Water (SAW), with coastal and atmospheric influence, inflowing at the shallowest depths; the Eastern North Atlantic Water 90 (ENACW), at an intermediate depth, characterised by low salinity; and the Mediterranean Outflow Water (MOW), entering at the deepest level (Criado-Aldeanueva et al., 2006;Bellanco and Sánchez-Leal, 2016).
The Gulf of Cádiz is part of one of the four major EBUS of the world, the North Atlantic upwelling (e.g.Alvarez et al., 2009), that extends from south of Cape Verde (Senegal) to Cape Finisterre (northwest of Spain).For this reason, the Gulf of Cádiz presents characteristics typical of this EBUS: seasonal variability of a winds system favourable to the coastal upwelling (Fiúza 95 et al., 1982), high biological productivity (Navarro and Ruiz, 2006), a system of fronts and zonal currents (García Lafuente and Ruiz, 2007) and a zone of water exchange between the coastal zone and open ocean (Sánchez et al., 2008).However, the fact that the coastline of the study area runs more in a W-E direction than the overall N-S direction common to all the EBUS phenomena, and the bilayer flow through the Strait of Gibraltar, are two factors that complicate the simple EBUS conceptual model (Peliz et al., 2009).

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In addition, the surface circulation in the Gulf of Cádiz is characterised by several different processes.These are: first the presence of an anticyclonic water flow towards the east over the shelf edge as far south as the Strait of Gibraltar, known as the Gulf of Cádiz Current (Sánchez and Relvas, 2003;Peliz et al., 2007); second, in the Trafalgar area an upwelling process occurs, produced by tidal interaction with the topography of the zone; and third, the mixing of surface layers induced by the wind (Vargas-Yáñez et al., 2002;Sánchez and Relvas, 2003;Peliz et al., 2009;Sala et al., 2018).In addition, the centre of the Gulf 105 is under the influence of the eastern-end branch of the Azores Current, producing a front subjected to a mesoscale variability (Johnson and Stevens, 2000;García-Lafuente and Ruiz, 2007;Peliz et al., 2007;Sala et al., 2013).

Field sampling and analysis
The database for this study has been obtained following two different sampling strategies.The first one consisted on taking sea surface measurements underway.Meanwhile, the second one acquired the measurement at several discrete surface stations 110 along three transects perpendicular to the coastline: the Guadalquivir transect (GD), the Sancti Petri transect (SP) and the Trafalgar transect (TF) (Fig. 1).Data was recollected during 8 cruises carried out with a seasonal frequency (spring: ST1 and ST5; summer: ST2 and ST6; autumn: ST3 and ST7; winter: ST4 and ST8) during 2014, 2015 and 2016 (precise dates are indicated in Table 1).All the cruises were carried out on board the R/V Ángeles Alvariño, except the one of summer 2015

Underway measurements
Sea surface temperature (T), sea surface salinity (S) and the CO2 partial pressure (pCO2) were recorded continuously and were averaged with a frequency of 1 min intervals, from the surface seawater supply of the ship (pump inlet at a depth of 5m).T and S were measured using a SeaBird thermosalinograph (SeaBird 21) with an accuracy of ±0.01 ºC and ±0.003 respectively.

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The equilibrator design for determining the pCO2 is a combination of a laminar flow system with a bubble type system, similar to that developed by Körtzinger et al. (1996) and described by Padin et al. (2009Padin et al. ( , 2010)).
The surface water CO2 molar fraction (xCO2) and H2O were determined using a non-dispersive infrared gas analyser (Licor®, LI 6262) that has a minimum accuracy of ±0.3 ppm.It was calibrated daily using two standards: a CO2 free-air for the blank and a CO2 sub-standard gas of known concentration (413.2 ppm).CO2 concentration of the sub-standard gas was determined 125 from the comparison with standard gases of NOAA with an uncertainty of 0.22 ppm and measured with a Licor 6262 (±1 ppm).The temperature inside the equilibrator was measured continuously by means of a platinum resistance thermometer (PT100 probe, ±0.1 ºC).A pressure transducer (Setra Systems, accurate to 0.05 %) was used to measure the pressure inside the equilibrator.
The xCO2 was converted into pCO2 according to the protocol described in DOE (2007): corrections by water vapour pressure 130 and water surface temperature have been made since the equipment quantifies in dry air and the temperature registered in the equilibrator is different to the T. The temperature difference between the ship's sea inlet and the equilibrator was less than 1.5 ºC.

Fixed stations
Discrete surface samples were taken at 5 m depth, using Niskin bottles (10 L) mounted on a rosette-sampler coupled to a 135 SeaBird CTD 911+, to measure pH, chlorophyll-a concentration and nutrients.
The pH was measured by potentiometer in duplicate using 100 mL of seawater with a glass-combined electrode (Metrohm, 905) calibrated on the total pH scale using a TRIS buffer solution (Zeebe and Wolf-Gladrow, 2001).For chlorophyll-a determination, 1 L of seawater was filtered (Whatman, GF/F 0.7 μm) and frozen (-20 °C) until analysis in the lab.Total chlorophyll-a was extracted with 90 % pure Acetone, and quantified after 24 hours by fluorometry analysis (Hitachi F-2500) 140 (Yentsch and Menzel, 1963).Nutrient samples for analysis of nitrate and phosphate content were filtered through precombusted glass-fibre filters (Whatman, GF/F 0.7 μm) and frozen at -20 °C.Analyses were performed in a segmented flow autoanalyzer (Skalar, San Plus) based on classic spectrophotometric methods (Grasshoff et al., 1983).Moreover, at these stations, Apparent Oxygen Utilization (AOU) was calculated applying the solubility expression proposed by Weiss (1974) employing dissolved oxygen values registered by the sensor of the rosette (SeaBird 63) that have been checked 145 using Winkler titrations (±0.1 µmol L -1 ).
The corresponding data of T, S and pCO2 were obtained by the underway measurements averaging data corresponding to 0.5 mile around the location of the fixed stations.

Temperature and biological effects on pCO2 calculations
To determine the relative importance of the temperature and biological effects on the changes of pCO2 in sea water, we follow the method proposed by Takahashi et al. (2002).To remove the temperature effect from the observed pCO2, the data were normalized to a constant temperature, the mean in situ T depending on the focus considered, according to Eq. (1).(1) where the subscripts "mean" and "obs" indicate the average and observed T values, respectively.
The effect of temperature changes on pCO2 has been computed by perturbing the mean pCO2 with the difference between the mean and observed temperature.The pCO2 value at a given observed temperature (Tobs) was calculated based on Eq. ( 2).
pCO2 at Tobs = Mean pCO2•exp[0.0423•(Tobs-Tmean)] (2) When the temperature effect is removed, the remaining variations in pCO2 are due to the effect of biology, such as net biological utilization of CO2, the vertical and lateral transport and sea-air exchange of CO2.
The biological effect on the surface water pCO2 in a given area, (ΔpCO2)bio, is represented by the seasonal amplitude of pCO2 values normalized to the mean T, (pCO2 at Tmean), using Eq. ( 1): The temperature effect of changes on the mean annual pCO2 value, (ΔpCO2)temp, is represented by the seasonal amplitude of (pCO2 at Tobs ), using Eq.( 2): (ΔpCO2)temp = (pCO2, Tobs)max -(pCO2, Tobs)min (4) The relative importance of the temperature and biology effects can be expressed by the ratio, T/B:

Estimation of CO2 fluxes
Fluxes of CO2 across the sea-air interface were estimated using the relationship: where k (cm h -1 ) is the gas transfer velocity; α is the solubility coefficient of CO2 (Weiss, 1974); and ΔpCO2 is the difference between the sea and air values of pCO2.The atmospheric pCO2 (pCO₂ atm ) values were obtained from the monthly atmospheric

Statistical analysis
Statistical analyses were performed with IBM SPSS Statistics software (Version 20.0.Armonk, NY).The dataset was analysed 185 using one-way analysis of variance test (ANOVA) for analysing significant differences between cruises for discrete and continuous surface data on hydrological and biogeochemical characteristics.The threshold value for statistical significance was taken as p < 0.05.

Underway variables 190
Table 1 gives the ranges of variation and the mean and standard deviation of T, S and pCO2 during the 8 sampling cruises.
Figure 2 shows the underway distribution of T and pCO2 in the Gulf of Cádiz.
T values were significantly different among all cruises (p < 0.05), varying between 14.3 and 23.4 ºC.In general, the samplings made during 2014 presented temperatures higher than 2015 and 2016 (Table 1).For the whole period, the averaged values for both seasons were highest during summer (21.2 ± 1.3 ºC) and autumn (21.0 ± 0.8 ºC), with the lowest values during spring 195 (15.5 ± 0.5 ºC).During winter, T showed an intermediate value of 17.6 ± 0.9 ºC.Spatially T tended to increase from coastal to offshore areas, with a difference of ~11.8 ºC (Fig. 2A).No substantial differences were found between the three transects studied (GD, SP and TF) in terms of temperature data.1).No spatial or seasonal variations were observed.
However, during December 2014, in the area of the Guadalquivir River was measured the lowest salinity value (34.68), related with a storm period that led to very heavy freshwater discharges.On the other hand, TF was the area that presented the highest mean salinity value for the whole study (36.19 ± 0.25).

205
During our study period, pCO2 values ranged from 320.6 to 513.6 µatm.Highest values were recorded during summer and autumn of 2014 and 2015 (Table 1), with a similar mean value found for both seasons, 411.7 ± 13.3 µatm and 411.3 ± 10.7 µatm, respectively.The lower mean value (390.3 ± 15.2 µatm) was logged during winter and the lowest mean value (383.9 ± 22.1 µatm) during spring.In general, the pCO2 tended to decrease with the distance to the coast (Fig. 2B).ΔpCO2 values ranged between -37.5 ± 14.9 µatm during March 2015, and 16.5 ± 9.5 µatm during October 2014.Moreover, an oversaturation 210 relative to the atmosphere was evidenced during spring and winter for both years.In Fig. 2B a sharply variation of pCO2 can be observed at some zones.These coincide with those stations where discrete water samples were taken.This may be due to the daily variation (day/night) presented by pCO2, since the sampling procedure did not take the same amount of time at each stationit varied in function of the depth of the system in each zone.It is possible that during the daytime, the pCO2 values were higher than at night.215

Discrete surface variables
Table 2 shows the average values and standard deviation of temperature, salinity, pH, AOU, chlorophyll-a, nitrate and phosphate measured at fixed stations along the three transects during the 8 cruises.
The pH presented significant differences among the cruises (p < 0.05), with a range of variation from 7.  1).The minimum value of pH were found in September 2015 (7.49 ± 0.03) and the maximum in March 2015 (8.09 ± 0.12).
AOU was significantly different between all the cruises (p < 0.05), and a clear seasonal variability was not observed.Values measured ranged from -31.9 to 12.3 µmol L -1 , with the highest values in December 2014 (7.7 ± 2.1 µmol L -1 ) and the lowest in March 2015 (-19.1 ± 9.4 µmol L -1 ) (Table 2).During spring was registered the lowest mean value for both years (-10.9 ± 225 11.7 µmol L -1 ); higher mean values were found in summer (-6.3 ± 6.1 µmol L -1 ) and winter (4.2 ± 6.3 µmol L -1 ).All values were negative except for those of December 2014; that exception may have been due to the exceptional mixing of the water column caused by the storm.

Air-sea CO2 exchange
Table 3 summarizes the mean values and standard deviation for atmospheric pCO2, wind speed, gas transfer velocity and the 245 air-sea CO2 fluxes measured in this study.
The mean wind speeds were relatively similar for the whole study period, ranging between 5.5 ± 2.8 m s -1 (March 2015) and 7.7 ± 4.2 m s -1 (December 2014).The gas transfer velocity varied between 6.9 ± 0.1 cm h -1 in March 2015 and 14.4 ± 0.3 cm h -1 in June 2015, since it is very sensitive to changes in wind speed (4.6 cm h -1 / m s -1 at 20 ºC).
There was a clear seasonal variability in the dataset of CO2 fluxes (p < 0.05).The study area acted as source of CO2 to the

General trends
Numerous research studies have determined that temperature is one of the most important factors that control the variability 255 of pCO2 in the ocean (e.g.Millero, 1995;Bates et al., 2000;Takahashi et al., 2002;Carvalho et al., 2017), as a consequence of the dependence of the solubility of CO2 with the temperature (Weiss, 1974;Woolf et al., 2016).When pCO2 is affected only Figure 3A shows the dependence of the values of pCO2 with T for the entire database, where a linear increase of pCO2 with T 260 (r 2 = 0.37, p < 0.0001) was observed.This relationship becomes more significant when it is obtained from the mean values of T and pCO2 of each cruise (r 2 = 0.71, p < 0.01, Fig. 3B).The slope, 4.80 µatm ºC -1 , is lower than the thermal effect on pCO2 described by Takahashi et al. (1993), and indicates the influence of other processes on the distribution of pCO2 in this zone of the Gulf of Cádiz.
Table 4 gives the values for the dependence of pCO2 with temperature on various continental shelves determined in other 265 studies.The authors of these studies describe the influence of other processes on the relationship between pCO2 and temperature, such as the vertical mixing of the water column (Wang et al., 2000;Xue et al., 2012), continental inputs (Shim et al., 2007;Xue et al., 2012), the influence of surface currents at different temperature (Shim et al., 2007;Jiang et al., 2008), upwelling phenomena (Wang et al., 2005;Jiang et al., 2008;Xue et al., 2012) and biological activity (Wang et al., 2005;Shim et al., 2007;Ribas-Ribas et al., 2011).https://www.esrl.noaa.gov/gmd/dv/data/index.php, last access: 9 January 2019); this suggests there may have been changes in 290 the continental inputs of nutrients and C related to anthropogenic activity (Rabouille et al., 2001).
Since the cruises were carried out at the beginning of each meteorological season, it is appropriate to analyse how representative is the range of temperatures that has been obtained.within the ranges of variation of the typical temperature in the Gulf of Cádiz, and the mean temperature found, 18.8 ºC, is similar to the mean value obtained at the mooring (19.2 ºC, Fig. 4).If the dependence of pCO2 with temperature is taken to be 4.80 µatm ºC -1 , one would expect that the mean values of pCO2 obtained in this study would be approximately 2 µatm higher.
The database of this study includes the transition from coastal zones, with depths of the order of 15 -20 m, to distal shelf 300 waters with depths greater than 800 m. Figure 5  Another source of CO2 in the coastal zone results from the net production of inorganic carbon derived from the processes of remineralization of the organic matter in the surface sediments originated from the continuous deposition of organic matter 325 through the water column (de Haas et al., 2002;Jahnke et al., 2005).The intensity of this process decreases in line with the increasing depth of the system, and the influence of the primary production and the continental supplies on the deposition of the particulate organic matter is less (Friedl et al., 1998;Burdige, 2007;Al Azhar et al., 2017).The progressive increase of T and pCO2 with increasing depth of the system measured below 100 -200 m (Fig. 5), it is associated with the presence of a branch of the Azores Current that introduces warmer waters in the central part of the Gulf of Cádiz (Gould, 1985;Käse et al., 1985;Johnson and Stevens, 2000).The influence of warmer surface currents and their 345 influence on the variability of pCO2 has been observed in other studies, such as that on the influence of the Gulf Stream in the south-eastern continental shelf of the United States (Wang et al., 2005;Jiang et al., 2008), and that on the Kuroshio Current in the northern East China Sea (Shim et al., 2007).
Ribas-Ribas et al. ( 2011) also found a decrease of pCO2 towards the deep zones (down to ≈ 100 m) on the north-eastern shelf of the Gulf of Cádiz.In general, an oversaturation of CO2 with respect to the atmosphere in shallower zones and the subsequent 350 undersaturation in distal waters, has also been described in other systems such as, for example, in the southern part of the Yellow Sea (Qu et al., 2014), in the southwestern part of the Atlantic Ocean (Arruda et al., 2015), in the North Sea (Clargo et al., 2015), and on the continental shelf of Maranhense (Lefèvre et al., 2017).

Control factors affecting pCO2
In addition to the influence of temperature, the spatiotemporal distribution of pCO2 in surface seawater is affected by the 355 biological utilization of CO2, the vertical and lateral transport, the sea-air exchange of CO2 and terrestrial inputs (e.g.Arruda et al., 2015;Ito et al., 2016;Xue et al., 2016).
With the object of investigating the influence of the biological activity on the variations of pCO2, Fig. 6 shows the dependence between the mean values of pCO2 at the fixed stations, and the temperature, pH, AOU and the concentration of chlorophyll-a (n = 126).The thermal effect on pCO2 is more intense when the discrete database (pCO2 = 297 + 5.7 T, r 2 = 0.48; p < 0.0001) 360 is considered in comparison to the effect obtained using the whole database.The variation of pCO2 with pH (pCO2 = 1710 -162.8 pH, r 2 = 0.34; p < 0.0001), AOU (pCO2 = 410 + 1.1 AOU, r 2 = 0.21; p < 0.0001), and chlorophyll-a (pCO2 = 413 -20.8 [Chl-a], r 2 = 0.14; p < 0.0001) show the influence of the processes of photosynthesis and respiration on the variations of pCO2 found.For the Gulf of Cádiz, Huertas et al. (2005) found a linear relationship between pCO2 and chlorophyll-a with a slope similar to that obtained in this study , r 2 = 0.32; p < 0.0001; n = 28).Other authors have also 365 described the interrelationships existing between pCO2 and chlorophyll-a in other coastal areas (Borges and Frankignoulle, 1999;Tseng et al., 2011;Zhang et al., 2012;Qin et al., 2014;Litt et al., 2018).Inverse relationships between pCO2 and dissolved oxygen as a consequence of the balance between the processes of photosynthesis and respiration have been found in other studies (Zhai et al., 2009;de la Paz et al., 2010;Xue et al., 2012;Xue et al., 2016).
In order to identify the overall controls of temperature and biological effects, the T/B ratio has been calculated (Takahashi et 370 al., 2002).A T/B ratio greater than 1 implies the dominance of temperature effects over biological processes, on the pCO2 dynamics.Strictly speaking, the term that picks up the biological effect in the model of Takahashi et al. (2002), encompasses all those processes that are not thermal, that influence the variations of pCO2, including the biological utilization of CO2, continental inputs and the existence of upwellings (Xue et al., 2012;Qu et al., 2014).This method was originally designed for open oceanic systems, but it has been widely used by other authors in coastal areas (e.g.Schiettecatte et al., 2007;Ribas-Ribas 375 et al., 2011;Qu et al., 2014;Burgos et al., 2018).
In this study, the global T/B ratio is 1.15, which indicates that temperature is an important factor controlling intra-annual variation of pCO2.This value is similar to that determined by Ribas-Ribas et al. ( 2011 Figure 8 shows the dependence of ΔpCO2 temp with temperature (r 2 = 0.40, p < 0.01) and of ΔpCO2 bio with chlorophyll-a (r 2 = 0.42, p < 0.01) for the fixed stations; these values confirm the importance of both the thermal and biological processes on 390 the variation of pCO2.The existence of a certain linear correlation between the mean values of chlorophyll-a and the concentrations of nitrate (r 2 = 0.66, p < 0.01) and phosphate (r 2 = 0.63, p < 0.01) has also been observed.The increase of the T/B ratio and the decrease of ΔpCO2 bio from the coastal zone to the central part of the Gulf of Cádiz are associated with the variations of the chlorophyll-a and nutrient concentrations that diminish exponentially with the depth of the system.Thus, the mean concentrations of chlorophyll-a, nitrate and phosphate in the distal zone are 66.3, 81.9 and 44.8 % less than the 395 concentrations found close to the coast.
However, the concentrations of chlorophyll-a and nutrients are relatively constant in waters with bottom-depth to 200 m, and do not explain the decrease of the T/B ratio and the increase of ΔpCO2 bio in waters with bottom-depth higher than 400 m.
These variations have been associated with the change in the origin of the surface water masses.Thus, in the central zone of the Gulf of Cádiz, the origin of the surface waters is a branch of the larger-scale Portuguese-Canary eastern boundary current 400 that circulates around a cyclonic eddy off Cape San Vicente and veers eastward into the Gulf of Cádiz (García-Lafuente et al., 2006).While the deepest zone is under the influence of a branch of the Azores current, which in addition to being a warmer stream that could lead to an increase in primary production, it is the northern border of the subtropical gyre (Klein and Siedler, 1989), thus favour the accumulation of CO2 in this area as convergence zone (Ríos et al., 2005).
The T/B ratios have also been calculated for the different transects at right angles to the coast that have been cruised for 405 sampling in the study zone, as shown in Fig. 9.It can be appreciated that the T/B ratio increases with the distance from the coast on the three transects, and that the temperature generally has a greater influence on the distribution of pCO2 than the nonthermal effects.The T/B ratio varies to the east, with values between 1.0 in the zone of the Guadalquivir and 1. near the mouth of the Guadalquivir River present the highest primary production of all waters within the Gulf of Cádiz (Navarro and Ruiz, 2006).The coastal zone close to Cape Trafalgar has been characterized as a region with high autotrophic productivity and biomass associated mainly with the nutrients input due to upwelling waters (e.g.Echevarría et al., 2002;García et al., 2002).The presence of these emerged water masses could be related to the relatively low values of ΔpCO2 temp found in this 415 zone.In fact, the mean temperature in this section is 18.4 ± 2.3 °C, about 0.5 °C lower than in the other two zones.The Sancti Petri zone is the one that receives a smaller supply of nutrients, and presents the lowest concentrations of chlorophyll-a in this study.The high values of ΔpCO2 temp in this part of the Gulf of Cádiz are associated with a higher mean temperature (19.0 °C) and a wider range of variation (6.8 °C).

Ocean-atmosphere CO2 exchange 420
In the Gulf of Cádiz, the flux of CO2 presents a range of variation from -5.6 to 14.2 mmol m -2 d -1 .These values are within the ranges observed by other authors in continental shelf zones of the North Atlantic (Table 5).As can be appreciated in Fig. 10, the fluxes of CO2 presented seasonal and spatial variations during the period studied.The Gulf of Cádiz acts as a source of CO2 to the atmosphere during the months of summer (ST2, ST6) and autumn (ST3, ST7), and as a sink in spring (ST1, ST5) and winter (ST4, ST8).Previous studies conducted in the Gulf of Cádiz are consistent with the behaviour found in this study 425 (González-Dávila et al., 2003;Aït-Ameur and Goyet, 2006;Ribas-Ribas et al., 2011).
As has been observed with pCO2, temperature is one of the principal factors that control the fluxes of CO2.In fact, for each cruise, a linear and positive relationship has been found between the mean values of the CO2 fluxes and T (r 2 = 0.72, p < 0.01) (Fig. 11).In parallel, there is a linear and negative relationship between the mean values of the CO2 fluxes and the concentration of chlorophyll-a at the discrete stations sampled (r 2 = 0.74, p < 0.01) (Fig. 11), as a consequence of the biological utilisation 430 of the CO2 (Qin et al., 2014).These relationships have also been found in various studies carried out in zones similar to the area studied (Zhang et al., 2010;Arnone et al., 2017;Carvalho et al., 2017).
The fluxes of CO2 in the Gulf of Cádiz tend to decrease with the distance from the coast (Fig. 10).The coastal zone (< 50 m) presents a mean CO2 flux of 0.8 ± 1.8 mmol m -2 d -1 , that reduces progressively to reach a value of -0.3 ± 1.6 mmol m -2 d -1 in open waters with bottom-depth higher than 600 m.This dependence of CO2 fluxes with distance from the coast has also been

435
described in other systems, such as in the South Atlantic Bight of the United States (Jiang et al., 2008), in the south-western part of the Atlantic Ocean (Arruda et al., 2015), in the Patagonian Sea (Kahl et al., 2017) and on the continental shelf of Maranhense (Lefèvre et al., 2017).This dependence is the consequence of the decrease of influence of the continental supplies ) and the zone comprising the rest of the shelf acts as a sink (-0.44 mmol m -2 d -1 ).
In addition, on both GD and SP transects a decrease of the CO2 flux is found towards the open ocean, due to the continental inputs associated with the estuary of the Guadalquivir and with the Bay of Cádiz, respectively.On the TF transect, in contrast, it was observed that the zone close to the coast acts as a sink of CO2 (-0.4 ± 1.2 mmol m -2 d -1 ), and the deeper zone is a weak source of CO2 to the atmosphere (0.3 ± 1.3 mmol m -2 d -1 ).This finding can be explained by the presence of an upwelling close

445
to the coast that is likely to be causing an increase of the production (e.g.Hales et al., 2005;Borges et al., 2005).With reference to this, on the TF transect there are significant differences between the mean surface concentrations of chlorophyll-a and nitrate in the coastal zone (0.63 ± 0.43 µg L -1 and 1.09 ± 0.77 µmol L -1 , respectively) and in deeper zones (0.17 ± 0.12 µg L -1 and 0.32 ± 0.33 µmol L -1 , respectively).The Gulf of Cádiz, during the period of this sampling, acted as a sink of CO2, with a mean rate of -0.18 ± 1.32 mmol m -2 d -1 , 450 that would give rise to an annual flux of -0.07 mol C m -2 yr -1 .The findings of previous studies carried out in the Gulf of Cádiz coincide with the behaviour observed in this study (Santana-Casiano et al., 2002;González-Dávila et al., 2003;Huertas et al., 2006;de la Paz et al., 2009;Ribas-Ribas et al., 2011), with the exception of the study by Aït-Ameur and Goyet (2006) in which it was estimated that the Gulf of Cádiz acts as a source of CO2 to the atmosphere, although that study only corresponds to the summer season. 455

Conclusions
The mean value of pCO2 in the eastern part of the Gulf of Cádiz found in this study (398.9 ± 15.5 µatm) indicates that it is undersaturated in CO2 with respect to the atmosphere (402.1 ± 3.9 µatm).The spatiotemporal variation of pCO2 found responds to the influence of different factors that usually affect its distribution in the littoral oceans.In global terms, when the mean values of the 8 cruises are considered, temperature (pCO2 = 302.0+ 5.16 T, r 2 = 0.71, p < 0.01) and biological activity (pCO2 460 = 425., r 2 = 0.76, p < 0.01) are the two principal factors that explain the temporal variability of pCO2.Over and above these general tendencies, there are spatial variations associated fundamentally with two other processes.Firstly, the effect of the continental supplies is that, in the coastal zone, principally the area close to the mouth of the Guadalquivir, there is a wider dispersion of the values of pCO2; it is in this area where the lowest and highest values have been observed in the discrete measurements.In this same coastal zone the highest mean values of pCO2 were found -values that diminish 465 progressively in line with increasing distance from the coast, out as far as an approximate depth of some 400 m.Secondly, there is a relative increase of the temperature and pCO2 in the zone furthest from the coast (depth > 400 m) that is the consequence of a change in the origin of the surface water, with the arrival of a warm branch of the Azores current.

175
data of xCO2 (xCO₂ atm ) at the Izaña Atmospheric Station (Spain), (Earth System Research Laboratory; https://www.esrl.noaa.gov/gmd/dv/data/index.php, last access: 9 January 2019).The xCO₂ atm was converted to pCO₂ atm as described in DOE (2007).The gas transfer velocity, k, was calculated using the parameterization formulated by Wanninkhof (u (m s -1 ) is the mean wind speed at 10 m height on each cruise, obtained from the Shipboard Weather Station; Sc is the Schmidt number of CO2 in seawater, and 660 is the Sc in seawater at 20 ºC.Ocean Sci.Discuss., https://doi.org/10.5194/os-2019-6Manuscript under review for journal Ocean Sci. Discussion started: 15 February 2019 c Author(s) 2019.CC BY 4.0 License.

270
There are previous studies in which the seasonal variations of pCO2 in more coastal zones of the Gulf of Cádiz (depth < 100 m) are described.In 2003,Huertas et al. (2006)  found variations of pCO2 ranging between 196 µatm in March and 400 -650 µatm in August in a zone situated more to the west, between the rivers Guadalquivir and Guadiana.Ribas-Ribas et al. (2011) established during 2006 -2007, a dependence of pCO2 with temperature similar to that found in this study (5.03 µatm ºC -1 ), and mean annual values of 369.3 ± 31.3 µatm.Also in 2006, de la Paz et al. (2009) established a variation of pCO2 between 275 329 µatm in March and 387 µatm in September in the Strait of Gibraltar, a deeper zone situated at the southeastern limit of the Gulf of Cádiz.There are other studies not concerned with seasonal differences that have quantified pCO2 in different zones of the Gulf of Cádiz.Santana-Casiano et al. (2002) found pCO2 values of 332.2 ± 3.9 µatm in the Strait of Gibraltar during September 1997, and González-Dávila et al. (2003) determined pCO2 values of 330.6 ± 5.6 µatm on a transect at right-angles to the coastline in 280 the zone of the Guadalquivir, carried out in February 1998.Aït-Ameur and Goyet (2006) reported variations of pCO2 ranging between 300 and 450 µatm for an extensive region of the Gulf of Cádiz in 2002.Comparing the data given in those previous studies with the mean value found in this study (398.9 ± 15.5 µatm), it is evident that there has been an increase of pCO2 in the Gulf of Cádiz during the last decade, even taking into account the uncertainty associated with the different measurement techniques employed.When we compare this mean value with the value found in 285 the shallower and deeper studied zones of the Gulf of Cádiz byRibas-Ribas et al. (2011), who used the same methodology, there has been an increase of pCO2 of 37.6 µatm in the last decade.For the period of time between 2006 and 2016, the rate of growth of pCO2 in the surface waters of the Gulf of Cádiz (3.8 µatm year -1 ) exceeds the rate of increase of pCO2 in the atmosphere (2.3 µatm year -1 for the last 10 years in Izaña (Earth System Research Laboratory; Figure 4 shows the mean value of the last 10 years for the maximum and minimum temperatures in the Gulf of Cádiz acquired by a mooring (bottom-mounted at 36.48° N -6.96°W; Puertos del Estado; http://www.puertos.es/es-es/oceanografia/Paginas/portus.aspx, last access: 12 July 2018), and the mean 295 values and standard deviations of the 8 cruises are superimposed.It can be observed that the mean values for each cruise are Ocean Sci.Discuss., https://doi.org/10.5194/os-2019-6Manuscript under review for journal Ocean Sci. Discussion started: 15 February 2019 c Author(s) 2019.CC BY 4.0 License.
shows the mean values of pCO2 and temperature for different intervals of depth of the water column based on the information obtained in the 8 cruises.It can be observed that the highest values of pCO2 (408.3 ± 26.7 µatm) correspond to the coastal zone (< 50 m), and that values decrease down to 100 -200 m of depth (396.1 ± 23 µatm).In addition, towards open waters (> 600 m) there is a progressive increase of pCO2 and temperature (404.3 ± 16.5 µatm and 20.1 ± 2.4 ºC respectively).305Severalauthors have described the influence of the continental inputs on the distribution of pCO2 in surface waters; the coastal zone is usually oversaturated with CO2, whereas the continental shelf as a whole acts as a sink of atmospheric CO2 (e.g.Rabouille et al., 2001;Chen and Borges, 2009).The principal continental inputs in the northeast zone of the Gulf of Cádiz take place from the estuary of the Guadalquivir and from the systems associated with the Bay of Cádiz.De la Paz et al. (2007) found values of pCO2 higher than 3000 µatm in the 310 internal part of the estuary of the Guadalquivir, and Ribas-Ribas et al. (2013) established that this estuary acts as an exporter system of C, nutrients and water oversaturated with CO2 to the adjoining coastal zone.The importance of the contributions from the Guadalquivir on the distribution of pCO2 depends on the river's flow rate, as can be appreciated in Fig. 3.In March 2014 (ST1, green) high values of pCO2 (up to 500 µatm) were observed in the zone close to the mouth, as a consequence of the river's high flow rate (between 192.7 and 299.2 m 3 s -1 , Confederación Hidrográfica del Guadalquivir; 315 http://www.chguadalquivir.es/saih/DatosHistoricos.aspx, last access: 19 Juny 2018) while in the spring of 2015 (ST5, dark green) the lowest values of the study were recorded in this zone (as low as 320 µatm) during a period of drought (flow rate 20 m 3 s -1 ) and subject to intense biological activity associated with the highest value found of the concentration of chlorophyll-a (2.4 µg L -1 ).The Bay of Cádiz occupies an area of 38 km 2 , and receives urban effluents from a population of 640,000 inhabitants.This 320 shallow zone is oversaturated with CO2 (Ribas-Ribas et al., 2011) due largely to the inputs of CO2, organic matter and nutrients that are received from the river Guadalete and the Sancti Petri and River San Pedro tidal creeks (de la Paz et al., 2008a, b; Burgos et al, 2018).
Ferrón et al. (2009) quantified the release from the sediment of DIC related to the processes of oxidation of organic matter in the coastal zone (depth < 50 m) of the Gulf of Cádiz, between the Guadalquivir and the Bay of Cádiz.These authors found a mean benthic flux of 27 ± 8 mmol 330 C m -2 d -1 for stations with a mean depth of 23 m.Considering a well-mixed water column, a pH = 8, in the conditions of mean temperature and salinity in the Gulf of Cádiz (18.8 ºC and 36.19,respectively) and using the K1 and K2 acidity constants proposed byLueker et al. (2000) in the total pH scale.This flux of DIC is equivalent to a CO2 flux of 198 ± 80 µmol C m -2 d - Ocean Sci.Discuss., https://doi.org/10.5194/os-2019-6Manuscript under review for journal Ocean Sci. Discussion started: 15 February 2019 c Author(s) 2019.CC BY 4.0 License.ofnutrients and higher concentrations of CO2 (e.g.Liu et al., 2010;Xue et al., 2015;González-Dávila et al., 2017) can affect the distributions of pCO2 found in the Gulf of Cádiz.

Figure 7
Figure7presents the values of the T/B ratio grouped in different bottom-depth intervals of the water column in the system.The variations of ΔpCO2 bio and ΔpCO2 temp found have been superimposed.In the coastal zone (depth < 50 m), the T/B ratio is below 1 (0.9), and increases to values of 1.3 in the central zone of the Gulf of Cádiz, at depths ranging from 100 to 400 m.However, in the deepest zone (depth > 600 m), a progressive decrease to values of 1.1 is found.This variation of the T/B ratio is largely caused by the variations of ΔpCO2 bio, with high values close to the coast (120.2 µatm), low values in the central 385 4 in Sancti Petri, and an intermediate value of 1.2 in the Trafalgar zone.These variations are related to changes in the biological activity and the presence of coastal upwellings.The Guadalquivir zone receives substantial continental supplies that lead to high 410 relative concentrations of chlorophyll-a and nutrients; these give rise to high values of ΔpCO2 bio.In particular, coastal waters Ocean Sci.Discuss., https://doi.org/10.5194/os-2019-6Manuscript under review for journal Ocean Sci. Discussion started: 15 February 2019 c Author(s) 2019.CC BY 4.0 License.
on the CO2 fluxes as one moves towards the open sea.Ribas-Ribas et al. (2011) also found that in the Gulf of Cádiz the CO2 fluxes vary with the distance from the coast; the zone close to the estuary of the Guadalquivir and the Bay of Cádiz acts as a 440 source (1.39 mmol m -2 d -1 Ocean Sci.Discuss., https://doi.org/10.5194/os-2019-6Manuscript under review for journal Ocean Sci. Discussion started: 15 February 2019 c Author(s) 2019.CC BY 4.0 License.