Spatio-temporal variations in the partial pressure of CO2 (pCO2)
were studied during eight oceanographic cruises conducted between March 2014 and
February 2016 in surface waters of the eastern shelf of the Gulf of
Cádiz (SW Iberian Peninsula) between the Guadalquivir river and Cape
Trafalgar. pCO2 presents a range of variation between 320.6 and 513.6 µatm with highest values during summer and autumn and lowest during
spring and winter. For the whole study, pCO2 shows a linear dependence
with temperature, and spatially there is a general decrease from coastal to
offshore stations associated with continental inputs and an increase in the
zones deeper than 400 m related to the influence of the eastward branch of
the Azores Current. The study area acts as a 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±1.32 mmol m-2 d-1. In the Guadalquivir and Sancti Petri transects, the CO2 fluxes decrease towards offshore, whereas in the Trafalgar transect fluxes
increase due to the presence of an upwelling. The annual uptake capacity of
CO2 in the Gulf of Cádiz is 4.1 Gg C yr-1.
Introduction
Continental shelves play a key role in the global carbon cycle as this is
where the interactions between terrestrial, marine and 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
Haas et al., 2002). Continental shelves account for about 10 %–15 % of
the ocean primary production, and they contribute approximately 40 % of
the total carbon sequestration through the mechanism of the biological pump
(Muller-Karger et 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., 2018). 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 that estimated in
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 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 works, it has been
observed that the continental shelves present different behaviour according
to their latitude: they tend to act as a sink of carbon (-0.33 Pg C yr-1) at high and middle latitudes (30–90∘) and as a
weak source (0.11 Pg C yr-1) at low latitudes (0–30∘) (Cai et al., 2006; Hofmann et al., 2011; Bauer et al., 2013; Chen et al.,
2013; Laruelle et al., 2014, 2017). Laruelle et al. (2010) found differences
between 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).
At the continental shelf, a high spatio-temporal variability in the air–sea
CO2 fluxes occurs due to various effects, such as the thermodynamic
effects, the biological processes, the gas exchange, the upwelling zones and
the continental inputs (e.g. Chen and Borges, 2009; Ito et al., 2016).
Thermodynamic effects are controlled by the inverse relationship between
temperature and solubility (0.0423 ∘C-1; Takahashi et
al., 1993). 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 that increases the concentration of inorganic carbon (Fennel and
Wilkin, 2009). Both factors (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 are not
clearly defined (Michaels et al., 2001). Although this process 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 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).
Upwelling systems at low latitudes act mainly as a source of CO2 but as
a sink of CO2 at mid-latitudes (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 Ocean and Indian Ocean 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).
Additionally, the inner shelf 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) than the outer shelf.
The influence of both factors (riverine inputs and human impact) decreases
towards offshore (Walsh, 1991). Several studies have determined that the
inner shelf tends to act as a source of CO2 and the outer shelf as a
sink (e.g. Rabouille et al., 2001; Cai, 2003; Jiang et al., 2008, 2013;
Arruda et al., 2015). The inner platform (depth of less than 40 m) also shows
greater seasonal variability in temperature than the outer platform, and
consequently the effect of temperature on pCO2 will be greater in the
inner zone (Chen et al., 2013).
The Gulf of Cádiz is strategically located, connecting the Atlantic Ocean
with the Mediterranean Sea through the Strait of Gibraltar, and in addition
it receives continental inputs from several major rivers, i.e. the
Guadalquivir, Rio Tinto, Odiel and Guadiana. Various studies have been conducted
in this area to evaluate the variability in the sea surface partial pressure
of CO2 (pCO2), although they cover smaller areas and a 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 sink of
CO2 with seasonal variations induced mainly by the combination of the
fluctuations of biomass concentration and temperature.
In this paper we evaluate the spatial and seasonal variation in the
sea-surface pCO2 on the eastern shelf of the Gulf of Cádiz. In
addition, we aim to assess the relative contribution of the thermal and
non-thermal effects to pCO2 distribution and to determine if the area
as a whole acts as a sink or a source of CO2 to the atmosphere over
time. It has also been possible to estimate the influence that various sea
surface currents have on pCO2 variability since this study considers
deeper areas than previous works. Therefore, we can analyse the change that
has occurred in relation to the CO2 uptake capacity in the Gulf of
Cádiz in the last 10 years in comparison with other studies that
analyse the seasonal variation underway by pCO2 in this area
(Ribas-Ribas et al., 2011). In this work we have analysed a surface
measurement database of > 26 000 values of pCO2 obtained
during cruises made between 2014 and 2016 and covering an area of
0.8∘× 1.3∘ of the Gulf of Cádiz.
Material and methodsStudy 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 the Strait of Gibraltar. In the Strait of
Gibraltar a bilayer flow takes place with an upper Atlantic layer flowing
towards the Mediterranean basin and a deeper outflow of higher-density
Mediterranean waters flowing 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 Central Water (ENACW) at an intermediate depth, characterized by low
salinity; and the Mediterranean Outflow Water (MOW) entering at the deepest
level (Criado-Aldeanueva et al., 2006; Bellanco and Sánchez-Leal, 2016).
Map of the eastern shelf of the Gulf of Cádiz showing the location of the fixed stations located on three transects at right angles to the coastline: Guadalquivir (GD), Sancti Petri (SP) and Trafalgar (TF). The location of the principal surface currents, rivers and capes of the study area are also noted.
The Gulf of Cádiz is part of one of the four major eastern boundary
upwelling systems of the world: the North Atlantic upwelling (e.g. Alvarez et
al., 2009) that extends from south of Cap-Vert (Senegal) to Cape
Finisterre (northwest of Spain). For this reason, the Gulf of Cádiz
presents characteristics typical of this system: seasonal variability of a
winds system favourable to the coastal upwelling (Fiúza 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 eastern boundary
upwelling system phenomena, and the bilayer flow through the Strait of
Gibraltar are two factors that complicate the simple eastern boundary
upwelling system conceptual model (Arístegui et al., 2009; Peliz et al.,
2009).
In addition, the surface circulation in the Gulf of Cádiz is
characterized by several different processes. These are 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); an upwelling process that occurs in the
Trafalgar area, produced by tidal interaction with the topography of the
zone; and the mixing of surface layers induced by the wind
(Vargas-Yáñez et al., 2002; Peliz et al., 2009; Sala et al., 2018).
The centre of the gulf is also 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) (Fig. 1).
Field sampling and analysis
The database for this study has been obtained following two different
sampling strategies. The first consisted of taking sea surface measurements
while underway. The second strategy was to obtain measurements at several
discrete surface stations along three transects at right angles to the
coastline: the Guadalquivir transect (GD), the Sancti Petri transect (SP)
and the Trafalgar transect (TF) (Fig. 1). Data were collected during eight
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 (Table 1). All the cruises were made on the R/V Ángeles Alvariño, except the summer 2015 cruise (ST6) that was undertaken on the
R/V Ramón Margalef. The study area is located between 35.4 and
36.7∘ N and 6.0 and 7.2∘ W (52.8×102 km2).
Date, number of measurements (n), range, average values, and
standard deviation of underway sea surface temperature (SST), sea surface
salinity (SSS), and pCO2 during the eight cruises undertaken: March 2014
(ST1), June 2014 (ST2), October 2014 (ST3), December 2014 (ST4), March 2015
(ST5), June 2015 (ST6), September 2015 (ST7) and February 2016 (ST8).
Sea surface temperature (SST), sea surface salinity (SSS) and pCO2
were recorded continuously and were averaged with a frequency interval of 1 min from the surface seawater supply of the ship (pump inlet at a
depth of 5 m). SST and SSS were measured using a Sea-Bird thermosalinograph
(SBE 21) with an accuracy of ±0.01∘C and
±0.003 units, respectively. 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. (2009, 2010).
The surface water CO2 molar fraction (xCO2) and H2O were
determined using a non-dispersive infrared gas analyser
(LI-COR®, 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 substandard gas of known concentration (413.2 ppm). CO2 concentration of the substandard gas was determined from the
comparison with standard gases of NOAA with an uncertainty of 0.22 ppm and
measured with a LI-COR 6262 (±1 ppm). The temperature inside the
equilibrator was measured continuously by means of a platinum resistance
thermometer (PT100 probe, ±0.1∘C). 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 between the
equilibrator and SST were made following Takahashi et al. (1993). The
temperature difference between the ship's sea inlet and the equilibrator was
less than 1.5 ∘C.
Fixed stations
Discrete surface samples were collected at 5 m depth, using Niskin bottles
(10 L) mounted on a rosette sampler coupled to a Sea-Bird CTD 911+
(conductivity–temperature–depth system), to measure pH, dissolved oxygen,
chlorophyll a and nutrient concentrations.
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 (tris(hydroxymethyl) aminomethane; Zeebe and Wolf-Gladrow, 2001). Dissolved
oxygen values were obtained with the sensor of the rosette (SBE 63)
pre-calibrated using Winkler titration (±0.1µmol L-1) of
samples collected from several water depths at selected stations (Parsons et
al., 1984). Apparent oxygen utilization (AOU) was determined as the
difference between the solubility calculated applying the expression
proposed by Weiss (1974) and the experimental values of dissolved oxygen.
For chlorophyll a determination, 1 L of seawater was filtered (Whatman, GF/F
0.7 µm) and frozen (-20∘C) until analysis in the
laboratory. Total chlorophyll a was extracted with 90 % pure acetone and
quantified after 24 h by fluorometry analysis (Hitachi F-2500) (Yentsch
and Menzel, 1963). Nutrient samples for analysis of nitrate and phosphate
contents were filtered through pre-combusted glass-fibre filters (Whatman,
GF/F 0.7 µm) and frozen at -20∘C. Analyses were performed
in a segmented flow auto-analyser (Skalar, San Plus) based on classic
spectrophotometric methods (Grasshoff et al., 1983). The accuracies of the
determinations obtained are the following: ±0.003 for pH, ±0.1µmol L-1 for dissolved oxygen, ±0.1µg L-1
for chlorophyll a, ±0.10µmol L-1 for nitrate, and
±0.02µmol L-1 for phosphate.
The corresponding data of SST, SSS and pCO2 for the fixed stations were
obtained by the underway measurements, averaging data corresponding to approximately 0.9 km
around the location of the fixed stations. SST and SSS data
were compared with the values collected with the CTD coupled to the
rosette sampler, and they do not show differences greater than 0.04 ∘C and 0.01 units, respectively.
Thermal and non-thermal effects on pCO2 calculations
To determine the relative importance of the thermal and non-thermal effects
on the changes in pCO2 in seawater (e.g. Landschützer et al.,
2015; Reimer et al., 2017), we follow the method described by Takahashi et al. (2002). To remove the thermal effect from the observed pCO2, the
data were normalized to a constant temperature (the mean in situ SST
depending on the focus considered) according to Eq. (1).
pCO2atSSTmean=pCO2obs⋅exp0.0423⋅SSTmean-SSTobs,
where the subscripts “mean” and “obs” indicate the average and observed
SST values, respectively.
To analyse the effect of the thermal changes in pCO2 at the given
observed temperatures (SSTobs) the following expression has been used:
pCO2atSSTobs=pCO2mean⋅exp0.0423⋅SSTobs-SSTmean.
When the thermal effect is removed, the remaining variations in pCO2
are due to the non-thermal influences, such as the biological utilization of
CO2, the vertical and lateral transport, the sea–air exchange of
CO2, and terrestrial inputs (e.g. Qu et al., 2014; Arruda et al., 2015;
Ito et al., 2016; Xue et al., 2016). The non-thermal effects on the surface
water pCO2, (ΔpCO2)n-T, can be calculated from the
seasonal amplitude of pCO2 values normalized to the mean SST,
(pCO2 at SSTmean), using Eq. (1):
ΔpCO2n-T=pCO2atSSTmeanmax-pCO2atSSTmeanmin.
The seasonal amplitude of pCO2 values normalized to the observed
SST (pCO2 at SSTobs) represents the thermal effect of changes
in the mean annual pCO2 value, (ΔpCO2)T, and it is
calculated with the following expression:
ΔpCO2T=pCO2atSSTobsmax-pCO2atSSTobsmin.
The ratio between the thermal effects (T) and non-thermal effects (B)
quantifies the relative importance of each effect (Takahashi et al., 2002):
T/B=ΔpCO2T/ΔpCO2n-T.
A T/B ratio greater than 1 implies the dominance of thermal effects over
non-thermal effects on the pCO2 dynamics. However, a T/B lower than 1 reveals a
greater influence of non-thermal processes. This method was originally
designed for open ocean systems, but it has been widely used by other
authors in coastal areas (e.g. Schiettecatte et al., 2007; Ribas-Ribas et
al., 2011; Qu et al., 2014; Burgos et al., 2018).
In addition, Olsen et al. (2008) propose a method in which the seasonal
signal of pCO2 data is decomposed into individual components due to
variations in SST, in air–sea CO2 exchange, in SSS, and in combined
mixing and biological processes, according to Eq. (6).
dpCO2sw,i=dSSTpCO2sw,i+dASpCO2sw,i+dSSSpCO2sw,i+dMBpCO2sw,i,
where the superscript “sw” makes reference to the surface pCO2 in
the seawater and “i” to the mean value between consecutive cruises for
all variables; dpCO2sw,i is the observed
change in pCO2; dSSTpCO2sw,i is
the change due to SST changes; dASpCO2sw,i is the change due to air–sea exchange;
dSSSpCO2sw,i is the change due to
salinity variations; and dMBpCO2sw,i
is the change due to mixing plus biology. At the same time, each process is
calculated with the following equations (Olsen et al., 2008):
dSSTpCO2sw,i=pCO2sw,i⋅e0.0423(ΔSST)-pCO2sw,i,
where ΔSST is the SST difference between two cruises.
dASpCO2sw,i=-d⋅Fi/MLDi,
where d is the number of days passed between two cruises (90 d
approximately); Fi is the mean flux of CO2; and MLDi is the mean mixed layer depth.
dSSSpCO2sw,i=pCO2sw,n+1DICn+1,TAn+1,SSSn+1,SSTi-pCO2sw,nDICn,TAn,SSSn,SSTi,
where the superscript “n” refers to the mean value of each cruise and the
variables DIC (dissolved inorganic carbon) and TA (total alkalinity) have
been estimated from pH and pCO2 using the K1 and K2 acidity constants
proposed by Lueker et al. (2000) in the total pH scale through the program
CO2SYS (Lewis et al., 1998). dMBpCO2sw,i is calculated as a residual, i.e. as the
change in pCO2 that is not explained by other processes. Additionally,
this study includes both coastal areas and deeper areas (the analysis is
divided into a function of the system depth) between coastal (water depth < 50 m) and distal (water depth > 50 m) areas. Thus,
MLDi in distal areas (Table 3) was calculated and derived from the
thermocline position that separates the SAW and the ENACW (71.3–96.8 m),
while the coastal areas correspond to the depth of these areas (15–50 m).
Estimation of CO2 fluxes
Fluxes of CO2 across the sea–air interface were estimated using the
following relationship:
FCO2=α⋅k⋅(ΔpCO2)sea-air,
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 (pCO2atm) values were obtained from the monthly atmospheric
data of xCO2 (xCO2atm) at the Izaña Atmospheric Research Center in
Spain (Earth System Research Laboratory;
https://www.esrl.noaa.gov/gmd/dv/data/index.php, last access: 9 January 2019). The xCO2atm was converted to pCO2atm as described in DOE (2007).
The gas transfer velocity, k, was calculated using the parameterization
formulated by Wanninkhof (2014):
k=0.251⋅u2(Sc/660)-0.5,
where 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 ∘C.
Statistical analysis
Statistical analyses were performed with IBM SPSS Statistics software
(version 20.0; Armonk, New York, USA). The dataset was analysed using
a 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. Moreover, all
reported linear correlations are type I and they are statistically
significant with p values smaller than 0.05 in the entire article unless
indicated otherwise.
ResultsUnderway variables
Table 1 gives the ranges of variation and the mean and standard deviation of
SST, SSS and pCO2 during the eight cruises and Fig. 2 shows the underway
distribution of SST and pCO2 in the Gulf of Cádiz. Among all the
cruises, the SST values vary between 14.3 and 23.4 ∘C.
During 2014, SST values were found to be higher than those in 2015 and 2016
(Table 1). For the whole period, the averaged values were highest during
summer (21.0±0.8∘C) and autumn (21.1±1.2∘C), lowest during spring (15.5±0.5∘C), and intermediate during winter (17.5±0.6∘C). In general, SST tended to increase from coastal to
offshore areas during spring and winter, while in summer and autumn this SST
gradient was inverse (Fig. 2a). No substantial differences were found
between the three transects studied (GD, SP and TF), although near the
Guadalquivir river mouth and Cape Trafalgar (36.19∘ N,
6.03∘ W) the lowest values of SST due to freshwater inputs
and the frequent upwelled waters, respectively, were detected.
Underway distribution of sea surface temperature (SST
(∘C), a) and pCO2 (µ atm, b) during the eight
cruises in the Gulf of Cádiz: March 2014 (ST1), June 2014 (ST2), October 2014 (ST3), December 2014 (ST4), March 2015 (ST5), June 2015 (ST6),
September 2015 (ST7) and February 2016 (ST8).
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. Figure 3 shows the mean value over the
last 10 years of the maximum and minimum temperatures in the Gulf of
Cádiz acquired by an oceanographic buoy (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); the mean values and standard deviations of the eight cruises are
superimposed. It can be observed that the mean values for each cruise are
within the range of variation of the typical temperature in the Gulf of
Cádiz, and the mean temperature found (18.8 ∘C) is very
close to the mean value obtained at the oceanographic buoy (19.2 ∘C, Fig. 3). Sampling during our cruises did not detect the
highest temperatures occurring in the Gulf of Cádiz during August, which
may indicate that the real range of pCO2 variation is greater than that
determined in this study.
Maximum and minimum sea surface temperature (SST) variation during
a 10-year period recorded by an oceanographic buoy located in the Gulf of
Cádiz (36.48∘ N, 6.96∘ W). The red line shows
maximum SST variation. The green line shows minimum SST variation. The grey
line shows the average temperature for the 10-year period. Blue circles show
mean values and standard deviations of underway SST measured during the
eight cruises carried out during this study.
Average values of SSS varied significantly among the cruises, ranging
between 35.03 and 37.06. The highest mean values were recorded during
February 2016 (36.44±0.09) and lowest during September 2015 (35.64±0.08) (Table 1). The lowest salinity value (35.03) and the most
notable spatial variation (35.03–36.36) was observed during December 2014
in the area of the Guadalquivir river, associated with a period of storms
with consequent major freshwater discharges. The area that presented the
highest mean salinity value for the whole study was TF (36.19±0.25).
During our study period, pCO2 values ranged from 320.6 to 513.6 µatm. The highest values were recorded during summer and autumn of 2014 and
2015 (Table 1) with similar mean values, 411.6±13.2µatm
and 410.6±10.5µatm, respectively, found for both seasons;
the lowest mean value was logged during spring (382.5±16.9µatm), while winter presented an intermediate value (390.8±15.4µatm). These mean values are not significantly different and the
standard deviations are high, indicating high spatial and inter-annual
variability. In general, the pCO2 tended to decrease with the distance
to the coast (Fig. 2b). When comparing these values with pCO2 values in
the atmosphere, an undersaturation of CO2 was observed during spring
and winter (15.3±15.7 and 18.0±11.4µatm,
respectively) and an oversaturation in summer and autumn (-20.4±24.6
and -8.0±15.3µatm, respectively). In Fig. 2 a sharp
variation of SST and pCO2 can be observed in some zones that
coincides with the stations where discrete water samples were taken. This
may be due to the different sampling times at these stations, which varied
between 2 and 8 h as a function of the depth of the system.
The database of this study includes the transition from coastal zones with
depths of the order of 20 m to distal shelf waters with depths greater than
800 m. Figure 4 shows the general trend of the mean values of pCO2 and
SST for different intervals of depth of the water column based on the
information obtained in the eight cruises. Although there is no statistical
difference in pCO2 or SST with bottom depth, it can be observed that
the highest values of pCO2 (408.3±26.7µatm) correspond
to the coastal zone (< 50 m) and that values decrease down to a
depth of 100–200 m (396.1±23µatm). In addition, towards
open waters (> 600 m) there is a progressive increase in
pCO2 and SST (404.3±16.5µatm and 20.1±2.4∘C, respectively).
Underway variation in pCO2 and sea surface temperature (SST)
at different bottom-depth ranges of the water column (metres) during the eight
cruises. The mean values and standard deviations of pCO2 (blue) and SST
(red) for each range of depth are represented. High standard deviations are
associated with the seasonal and inter-annual variability for the whole
sampling period.
Discrete surface variables
Table 2 shows the average values and standard deviation for the underway
averaged measurements of SST and SSS and for the discrete samples of pH,
AOU, chlorophyll a, nitrate and phosphate at fixed stations along the three
transects during the eight cruises. The pH presented significant differences
among the cruises with a range of variation from 7.84 to 8.34. Lowest mean
values were found during summer (8.00±0.04) and autumn (7.96±0.05) of 2014 and 2015, respectively (Table 2), coinciding with the highest average values
of pCO2 recorded (Table 1). The pH values for spring and winter were practically equal for 2014 and 2015 (8.08±0.08 and 8.07±0.05,
respectively). AOU was significantly different between all the cruises but
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±2.1µmol L-1) and the lowest in March 2015 (-19.1±9.4µmol L-1) (Table 2). For both years, the lowest mean
value was recorded in spring (-11.3±8.9µmol L-1) and
the highest in winter (1.3±2.6µmol L-1). All mean
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
storms. No general trend in the spatial variations in pH and AOU was found.
Number of samples (n), mean values, and standard deviation for
the averaged underway measurements of sea surface temperature (SST), sea
surface salinity (SSS), pH, apparent oxygen utilization (AOU),
chlorophyll a (data from González-García et al., 2018), and nitrate
and phosphate in surface water samples (at depth of 5 m) at fixed stations
during the eight cruises: March 2014 (ST1), June 2014 (ST2), October 2014 (ST3),
December 2014 (ST4), March 2015 (ST5), June 2015 (ST6), September 2015 (ST7)
and February 2016 (ST8).
Chlorophyll a values presented significant differences among the cruises and
between the same seasons of each year. This variable varied from 0.02 to
2.37 µg L-1 with the highest mean value measured in March 2015
(0.76±0.55µg L-1), which coincides with the lowest
(negative) mean value of AOU (Table 2). The lowest mean value was in June 2014 (0.18±0.14µg L-1). With reference to the seasons
of both years, the highest value was in spring (0.71±0.46µg L-1), followed by winter (0.58±0.33µg L-1) and autumn
(0.26±0.30µg L-1), and the lowest value in summer (0.23±0.25µg L-1). The SP transect presented the lowest mean
value of the whole study (0.33±0.31µg L-1) and the TF
zone the highest (0.49±0.37µg L-1).
Nitrate concentration did not show significant differences among the
cruises, ranging between 0.00 and 1.93 µmol L-1. The highest
mean value was recorded in spring (0.82±1.09µmol L-1)
and the lowest in summer (0.25±0.35µmol L-1) of both
years. The TF transect presented the highest mean concentration for the
whole study (0.77±0.76µmol L-1). Phosphate
concentration showed significant differences among all the cruises. By
season, the highest mean value was obtained during autumn (0.31±0.30µmol L-1), although the average data in October 2014 (0.09±0.03µmol L-1) were lower than that of 2015 (0.50±0.55µmol L-1) (Table 2). The lowest mean value was observed
during summer (0.10±0.05µmol L-1). The GD transect
presented the highest mean value of the whole study (0.28±0.39µmol L-1), and the lowest values were found in the TF and SP
transects with similar values in each, 0.15±0.07µmol L-1 and 0.14±0.09µmol L-1, respectively. The mean
N/P ratio in surface waters for the whole study was 3.5±2.0, similar
to that estimated by Anfuso et al. (2010) in the northeast continental shelf
of the Gulf of Cádiz, which indicates a relative phosphate deficit with
respect to the Redfield ratio (Redfield et al., 1963).
Air–sea CO2 exchange
Table 3 summarizes the mean values and standard deviations for atmospheric
pCO2, wind speed, gas transfer velocity and the 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. There was a slight seasonal variation in the CO2 fluxes similar
to pCO2, because they are associated to the spatio-temporal variability
and they present high standard deviations. The study area acted as a source
of CO2 to the atmosphere during summer and autumn (0.7±1.5 and 1.2±0.9 mmol m-2 d-1, respectively)
and as a sink in spring and winter (-1.3±1.6
and -1.3±1.6 mmol m-2 d-1, respectively).
Mean values and standard deviations of mixed layer depth (MLD) in
distal areas (depth > 50 m), atmospheric pCO2 (pCO2µatm), wind speed, gas transfer velocity (k) and air–sea CO2
fluxes for the underway measurements during the eight cruises: March 2014 (ST1),
June 2014 (ST2), October 2014 (ST3), December 2014 (ST4), March 2015 (ST5),
June 2015 (ST6), September 2015 (ST7) and February 2016 (ST8).
CruiseMLD in distalpCO2 atmWind speedkCO2 fluxesareas (m)(µatm)(m s-1)(cm h-1)(mmol m-2 d-1)ST171.3±26.4398.7±1.87.7±3.413.4±0.2-0.3±2.3ST288.6±34.4404.5±0.57.4±3.414.0±0.30.9±1.4ST390.3±34.0397.7±0.66.7±4.011.8±0.41.4±0.8ST496.8±34.1399.4±2.27.7±4.214.3±0.2-1.3±1.7ST591.5±31.6405.5±0.65.5±2.86.9±0.1-2.3±0.9ST689.0±33.0406.1±0.87.5±4.114.4±0.30.5±1.5ST790.2±32.0398.4±0.77.0±3.212.3±0.30.9±1.1ST887.0±40.3406.4±0.36.8±3.110.6±0.1-1.3±1.6DiscussionThermal influence in pCO2
Numerous research studies have determined that temperature is one of the
most important factors that controls the variability in 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 by the temperature, Takahashi et al. (1993)
determined a relative variation in pCO2 of 0.0423 ∘C-1, equivalent to 16.9 µatm∘C-1 for
experimental pCO2 of 400 µatm. In our study, all data from all
seasons together exhibited a linear relationship between pCO2 and SST
(r2=0.37, Fig. 5a). This relationship becomes even more significant
when it is obtained from the mean values of pCO2 and SST of each
cruise (r2=0.71, Fig. 5b). The slope, 4.80 µatm ∘C-1, is lower than the thermal effect on pCO2
described by Takahashi et al. (1993) and indicates the influence of other
non-thermal processes on the distribution of pCO2 in this zone of the
Gulf of Cádiz.
Dependence of pCO2 with sea surface temperature (SST) for the
complete underway database during all the cruises (a) and for the mean
values of pCO2 and SST for each cruise showing their standard
deviations (b). The solid line shows the linear correlation.
There are previous studies in which the seasonal variations in pCO2 in
more coastal zones of the Gulf of Cádiz (depth < 100 m) are
described (Table 4). Ribas-Ribas et al. (2011) found in the north eastern
shelf during June 2006 and May 2007 a dependence of pCO2 with
temperature similar to that found in this study (5.03 µatm ∘C-1, r2=0.42) and a pCO2 that ranged
between 338 and 397 µatm. In 2003, Huertas et al. (2006) found
variations in 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. In addition, de la Paz et al. (2009)
established a variation in pCO2 between 387 µatm in September
2005 and 329 µatm in March 2006 in the Strait of Gibraltar, a deeper
zone situated at the south eastern limit of the Gulf of Cádiz. This
dependence of pCO2 with temperature has also been determined in other
studies of continental shelves, such as in the East China Sea (Wang et al.,
2000), in the northern East China Sea (Shim et al., 2007) and in the
northern Yellow Sea (Xue et al., 2012).
Range, mean and standard deviation of pCO2, air–sea
CO2 fluxes (FCO2) and T/B ratio found in different areas of the
Gulf of Cádiz.
Site∘ E∘ NDatepCO2FCO2T/BReference(µatm)(mmol m-2 d-1)aStrait of Gibraltar-5.5 to -5.235.6 to 36.0September 1997352.8±2.03±8b–Santana-Casiano et al. (2002)339–381Gulf of Cádiz-7.0 to -6.536.3 to 36.7February 1998360.2±27.9-19.5±3.5b–González-Dávila et al. (2003)334–416Gulf of Cádiz-8.3 to -6.033.5 to 37.0July 2002–18.6±4b–Aït-Ameur and Goyet (2006)300–450Northeastern shelf of-7.5 to -6.336.6 to 37.3March 2003 to March 2004–-2.5–1.0b–Huertas et al. (2006)the Gulf of Cádiz130–650Strait of Gibraltar-6.0 to -5.235.8 to 36.1September, December 2005;–-1.9–1.9b2.4de la Paz et al. (2009)March, May 2006320–387Northeastern shelf of-6.8 to -6.336.4 to 36.9June, November 2006;360.6±18.2-2.2–3.6b1.3Ribas-Ribas et al. (2011)the Gulf of CádizFebruary, May 2007338–397Gulf of Cádiz-6.0 to -7.235.4 to 36.7March, June, October, December 2014;398.9±15.5-2.3–1.5c1.15This workMarch, June, September 2015;321–514March 2016
a Gas transfer coefficient (k): b Wanninkhof (1992). c Wanninkhof et al. (2014).
When comparing the data given in previous studies of the Gulf of Cádiz with
the mean value found in this study (398.9±15.5µatm), it is
evident that there has been an increase in pCO2 during the last decade,
even when taking into account the uncertainty associated with the different
measurement techniques employed. When we compare this mean value with the
value found in the shallower and deeper zones of the Gulf of Cádiz
studied by Ribas-Ribas et al. (2011) (360.6±18.2µatm), who
used the same methodology, there has been an increase in pCO2 of 38.3±16.9µatm 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±1.7µatm yr-1) exceeds the rate
of increase in pCO2 in the atmosphere (2.3 µatm yr-1 for
the last 10 years in Izaña (Earth System Research Laboratory;
https://www.esrl.noaa.gov/gmd/dv/data/index.php, last access: 9 January
2019)). The cause of this increase could be a greater input of anthropogenic
nutrients and inorganic carbon from land (Mackenzie et al., 2004) since the
direction and magnitude of estuarine and continental shelf CO2 exchange
with the atmosphere is highly dependent on the terrestrial organic budget
and nutrient supplies to the coastal ocean (Borges and Abril, 2011; Cai,
2011). However, we do not have any additional evidence to confirm this
effect in our area of study currently.
Non-thermal factors controlling pCO2
In accordance with Olsen et al. (2008), Fig. 6 shows the decomposition of
the variations in pCO2 between cruises due to changes in SST, in
air–sea CO2 exchange, in SSS, in combined mixing and biology, and in
distal and coastal areas. In general, the variations are greater than those
found in other works (Olsen et al., 2008; Omar et al., 2010) because this
study considers seasonal changes against the monthly change analysed in
previous applications. The average time between cruises is 86±8 d, with the exception of the last period (between September 2015 and
February 2016) that was 140 d. dpCO2sw
presents a similar variation between deep and coastal areas but with small
differences in the mean values between the distal zones (dpCO2sw=-3.4±28.9µatm) and the
shallower areas (dpCO2sw=0.2±22.7µatm). The high standard deviations associated with this variable
are due to the spatio-temporal variability in the database. In distal
areas (Fig. 6), pCO2 changes are mainly brought about by SST (-58.4–106.2 µatm) together with mixing and biological processes (-90.8–36.2 µatm). An inverse coupling is observed between dSSTpCO2sw and dMBpCO2sw since with the increase in the system SST
(increase dSSTpCO2sw) there is
greater biological uptake of CO2 (decrease dMBpCO2sw). As reported in the studies of Olsen et al. (2008) and Omar et al. (2010), the changes produced by the air–sea CO2
exchange are relatively small. Instead, in coastal areas (Fig. 6), the
dominant effects on pCO2 changes are produced by air–sea CO2
exchange (-196.2 to 103.4 µatm) and mixing plus biology (-101.1 to 198.5 µatm). In regions with shallower mixed layers, the effect of
air–sea exchange on the pCO2 variation is larger (Olsen et al., 2008).
A relative inverse coupling between the two factors was also observed;
outgassing is produced (decrease dASpCO2sw,i) when the system receives greater
inputs or production of CO2 (increase dMBpCO2sw). There is a different behaviour between the
transition from spring to summer of 2014 (ST1 and ST2) and 2015 (ST5 and
ST6) for dMBpCO2sw, which may be due
to a greater quantity of continental inputs, as reflected in the
Guadalquivir river flow rate in these periods (85.1±75.4 and 25.3±10.2 m3 s-1, respectively). Changes
in SSS do not have a substantial effect on pCO2 during the whole period
in both areas with a range of variation in dSSSpCO2sw,i between -11.3 and 11.0 µatm. This
behaviour was also described by Olsen et al. (2008) in the subpolar North
Atlantic, except for an area influenced by continental runoff where
pCO2 decreases.
Observed changes in pCO2 (first row) and pCO2 changes broken down due to SST changes (second row), air–sea CO2 exchange (third row), SSS changes (fourth row), and biology plus mixing (last row) in the distal (left column) and coastal areas (right column) between the periods of each cruise: ST1 (March 2014), ST2 (June 2014), ST3 (October 2014), ST4 (December 2014), ST5 (March 2015), ST6 (June 2015), ST7 (September 2015) and ST8 (February 2016).
In relation to the factors that affect the pCO2 changes brought
about by mixing and biological processes, a dependence between the mean
values of pCO2 and pH, AOU and the concentration of chlorophyll a has
been observed at the fixed stations (n=126, Fig. 7). AOU and pCO2
show a positive relationship (pCO2 (µatm) = 410 + 1.1 AOU
(µmol L-1), r2=0.21) with a slope close to what would
be obtained taking into account the processes of formation or oxidation of the
organic matter phytoplankton considering a Redfield-type relationship.
Inverse relationships between pCO2 and dissolved oxygen were also found
in other studies of a continental shelf (Zhai et al., 2009; de la Paz et al.,
2010; Xue et al., 2012, 2016). The pCO2 and pH dependence presents an inverse
relationship (pCO2 (µatm) =1710-162.8 pH, r2=0.34) due to the effect of the uptake or production of CO2 on the pH
(Tsunogai et al., 1997; Shaw et al., 2014). The variation in pCO2 with
chlorophyll a (pCO2 (µatm) =413-20.8 [chlorophyll a]
(µg L-1), r2=0.14) also shows the influence of the
processes of photosynthesis and respiration (e.g. Cai et al., 2011; Clargo
et al., 2015) with a slope value similar to that obtained in the study of
Huertas et al. (2005) (pCO2 (µatm) =274-19.6
[chlorophyll a] (µg L-1), r2=0.32; n=28). Other
authors have also 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).
Relationships between the surface values of pCO2 and apparent
oxygen utilization (AOU), pH and chlorophyll a (Chl a) at the 16 discrete
stations during the eight cruises. pCO2 presents the standard deviation
associated with the mean value obtained from the underway measurements.
Something that could affect the distribution of pCO2 in the Gulf of
Cádiz (and could be considered to be part of mixing and biology; sensu Olsen et al., 2008) is the vertical and lateral transport. For example, there are
two upwelling systems in our study zone: one more permanent situated in the
coastal zone (depth between 50 and 100 m) of the Trafalgar section (Prieto
et al., 1999; Vargas-Yáñez et al., 2002) and the other located
between the Cape Santa María and the Guadalquivir river and more
sensitive to meteorological forcing (Criado-Aldeanueva et al., 2006). In our
database, experimental evidence of the upwelling was found only in the TF
transect. A local decrease in the mean values of SST (17.4 ∘C) and pCO2 (399.1 µatm) was observed in this coastal area of
TF with respect to the deeper areas (18.8 ∘C and 405.1 µatm, respectively) for the whole period. This input of colder waters
could cause higher or lower concentrations of CO2 (e.g. Liu et al.,
2010; Xue et al., 2015; González-Dávila et al., 2017). There is a
progressive increase in SST and pCO2 with increasing depth of the
system measured below 100–200 m (Fig. 4); this 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
on the variability in pCO2 has been observed in other studies, such as
the Gulf Stream in the southeastern continental shelf of the United States
(Wang et al., 2005; Jiang et al., 2008) and the Kuroshio Current in the
northern East China Sea (Shim et al., 2007).
Additionally, related to the lateral transport on the distribution of
pCO2 in surface waters, several authors have described the influence of
the continental inputs. In general, the continental shelf as a whole acts as
a sink of atmospheric CO2 (e.g. Rabouille et al., 2001; Chen and
Borges, 2009), whereas the coastal zone is usually oversaturated with
CO2 (Fig. 4). This behaviour has been described in other systems,
including the southern part of the Yellow Sea (Qu et al., 2014), the
southwestern part of the Atlantic Ocean (Arruda et al., 2015), the North Sea
(Clargo et al., 2015) and on the continental shelf of Maranhense
(Lefèvre et al., 2017).
The principal continental inputs in the northeast zone of the Gulf of
Cádiz derive 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 internal part of the
estuary of the Guadalquivir, and Ribas-Ribas et al. (2013) established that
this estuary acts as an exporter system of inorganic carbon, 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. 2b. The highest values of pCO2 (up to 500 µatm) were observed
during March 2014 in the zone close to the Guadalquivir river mouth, a
consequence of the river's high flow rate (between 192.7 and 299.2 m3 s-1; Confederación Hidrográfica del Guadalquivir;
http://www.chguadalquivir.es/saih/DatosHistoricos.aspx, last access: 19 July 2018). In contrast, the lowest values of pCO2 were recorded in spring
of 2015 in this zone (as low as 320 µatm) in a period of drought
(flow rate 20 m3 s-1) and subject to intense biological activity
associated with the highest value found for the concentration of
chlorophyll a (2.4 µg L-1). The Bay of Cádiz occupies an
area of 38 km2 and receives urban effluents from a population of
640 000 inhabitants. This shallow zone is oversaturated with CO2
(Ribas-Ribas et al., 2011) due largely to the inputs of inorganic carbon,
organic matter and nutrients that are received from the Guadalete River,
Sancti Petri Channel and the Río San Pedro tidal creeks (de la Paz et
al., 2008a, b; Burgos et al., 2018).
Moreover, in the coastal zone another source of CO2 results from the
net production of inorganic carbon derived from the processes of
remineralization of the organic matter in the surface sediments originating
from the continuous deposition of organic matter through the water column
(de Haas et al., 2002; Jahnke et al., 2005). The intensity of this effect
decreases towards offshore areas since the influence of primary
production and the continental supplies on the deposition of the particulate
organic matter are less (Friedl et al., 1998; Burdige, 2007; Al Azhar et
al., 2017), which could be related to the greater effect determined by the
mixing and biology processes in the coastal areas using the Olsen et al. (2008) method. 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 C m-2 d-1 for stations with a
mean depth of 23 m. This flux of DIC is equivalent to a CO2 flux of 198±80µmol C m-2 d-1 through the sediment–water
interface when considering a well-mixed water column, a pH of 8, the
conditions of mean temperature and salinity in the Gulf of Cádiz (18.8 ∘C and 36.19, respectively), and using the K1 and K2 acidity
constants proposed by Lueker et al. (2000) in the total pH scale through the
program CO2SYS (Lewis et al., 1998). Moreover, this estimated CO2
benthic flux would produce an increase in pCO2 of 0.25±0.10µatm d-1 in the water column.
T/B ratio
In this study, the total T/B ratio is 1.15, which indicates that the thermal
effect is an important factor controlling intra-annual variation in
pCO2. This value is similar to that determined by Ribas-Ribas et al. (2011) (see date and study zone in Table 4) in the northeast zone of the
shelf of the Gulf of Cádiz with a ratio of 1.3. De la Paz et al. (2009)
(see date and study zone in Table 4) propose a T/B ratio of 2.4 in the
Strait of Gibraltar, indicating very significant thermal control in this
relatively deep zone situated to the east of the Gulf of Cádiz.
Figure 8 presents the values of the T/B ratio grouped in different
bottom-depth intervals of the water column in the system. The variations found in
non-thermal ΔpCO2 and thermal ΔpCO2 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 a value of
1.1 is found. Qu et al. (2014) also reported the variation in the values of
the T/B ratio with the distance from the coast in the southern Yellow Sea:
between 0.4 and 0.6 in the nearshore area (depth < 50 m) to more than
1 (up to 2.4) in the offshore area (depth > 50 m).
Variation of the T/B ratio (blue bar), non-thermal ΔpCO2 (green point) and thermal ΔpCO2 (red point) at
different bottom-depth ranges of the water column (metres) for the eight cruises.
This variation in the T/B ratio is largely caused by the variations in
ΔpCO2 non-thermal effects, which are observed to decrease from the coast
to the deeper zone regardless of which method is used (Takahashi et al., 2002;
Olsen et al., 2008). High values of non-thermal ΔpCO2 close
to the coast were observed (120.2 µatm), affected by continental
inputs, processes of remineralization in the sediment and biological
utilization of CO2. The increase in the T/B ratio and the decrease in
non-thermal ΔpCO2 (75 µatm) from the coastal zone to
the central part of the Gulf of Cádiz are associated with the variations
in 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, respectively, than the concentrations found close to the
coast. However, the concentrations of chlorophyll a and nutrients are
relatively constant in waters with bottom depth greater than 200 m and do
not explain the decrease in the T/B ratio and the increase in non-thermal ΔpCO2 (90.7 µatm) in waters with bottom depth
greater than 400 m. These variations have been associated with the change in
the origin of the surface water masses. In the central zone of the Gulf of
Cádiz, the origin of the surface waters is a branch of the larger-scale
Portuguese-Canaries eastern boundary current that circulates around a
cyclonic eddy off Cape St. Vincent and veers eastward into the Gulf of
Cádiz (García-Lafuente et al., 2006). The deepest zone is under the
influence of a branch of the Azores Current, which is a warmer stream that
could lead to an increase in primary production; in addition it is the
northern border of the subtropical gyre (Klein and Siedler, 1989); these two
factors favour the accumulation of CO2 in this area as a convergence
zone (Ríos et al., 2005).
Variation of the T/B ratio (blue bar), the T/B ratio at depths
< 100 m (green bar), the T/B ratio at depths > 100 m (red
bar), and ΔpCO2 non-thermal effects (green point) and ΔpCO2
thermal effects (red point) on the three transects of the study (Guadalquivir, Sancti
Petri and Trafalgar) during the eight cruises.
The T/B ratios have also been calculated for the different transects at
right angles to the coast, as shown in Fig. 9. The T/B ratio increases with
the distance from the coast for the three transects and the temperature
generally has a greater influence on the distribution of pCO2 than the
non-thermal effects. The T/B ratio varies to the east with values between 1.0 in the zone of the GD and 1.4 in SP and an intermediate value of 1.2 in
the TF zone. These variations are related to changes in the biological
activity and the presence of coastal upwelling. The Guadalquivir zone
receives substantial continental supplies that lead to high relative
concentrations of chlorophyll a and nutrients; these give rise to high
values of non-thermal ΔpCO2. In particular, coastal waters
near the mouth of the Guadalquivir river show 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 emerging water masses could
be related to the relatively low values of thermal ΔpCO2
found in this zone; in fact, the mean temperature in this area 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 thermal ΔpCO2 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
In the Gulf of Cádiz, the air–sea flux of CO2 exhibits 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 different areas of the Gulf of
Cádiz (Table 4). As can be seen in Fig. 10, seasonal and spatial
variations were observed in the air–sea fluxes 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
(González-Dávila et al., 2003; Aït-Ameur and Goyet, 2006;
Ribas-Ribas et al., 2011).
Spatial distribution of mean values of air–sea CO2 fluxes in
the eastern shelf of the Gulf of Cádiz at the 16 discrete stations
during spring (ST1, ST5), summer (ST2, ST6), autumn (ST3, ST7) and winter
(ST4, ST8).
Correlations between the mean values of air–sea CO2 fluxes
and sea surface temperature (SST) for the underway database (a) and the
CO2 fluxes and chlorophyll a (Chl-a) at the 16 discrete surface
stations (b) for each cruise and showing the standard deviations.
As discussed above for pCO2, temperature change is one of the principal
factors that controls the fluxes of CO2. In fact, for each cruise, a
linear and positive relationship was found between the mean values of
the CO2 fluxes and SST (r2=0.72, 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 (r2=0.74, Fig. 11) as a consequence of the
biological utilization of the CO2 and the subsequent tendency for
CO2 undersaturation (Qin et al., 2014). Such 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 air–sea 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 air–sea 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 greater than 600 m.
However, these differences are not statistically significant because of the high
standard deviations associated with the seasonal variations. This dependence
of the air–sea CO2 fluxes with distance from the coast has also been
reported in other systems, such as in the South Atlantic Bight of the United
States (Jiang et al., 2008), in the southwestern 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 in influence of the continental supplies
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 air–sea 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 source (1.39 mmol m-2 d-1) and the zone comprising the rest of the shelf acts as a sink
(-0.44 mmol m-2 d-1).
In addition, on both the GD and SP transects a decrease in the air–sea
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),
although these variations are not statistically significant due to the
seasonal variability associated with the values. This finding can be explained
by the presence of an upwelling close to the coast that is likely to be
causing an increase in 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±0.43µg L-1 and 1.09±0.77µmol L-1, respectively) and in deeper zones (0.17±0.12µg L-1 and 0.32±0.33µmol L-1,
respectively).
The Gulf of Cádiz carbon flux, during the sampling period, shows a mean rate of
-0.18±1.32 mmol m-2 d-1 even though it is necessary to
consider the intrinsic variability in the database that generates a high
standard deviation. With the total surface of the study area
(52.8×102 km2) and the mean annual flux during the eight
cruises, the uptake capacity estimated for the Gulf of Cádiz will be 4.1 Gg C 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.
Conclusions
A high variability in pCO2 in the Gulf of Cádiz was observed which
is associated with its location as a transition zone between coastal and
shelf areas, superimposed on the usual seasonal variation due to thermal and
biological effects. The mean value of pCO2 found in this study (398.9±15.5µatm) indicates that the Gulf of Cádiz could be
slightly undersaturated in CO2 with respect to the atmosphere (402.1±3.9µatm). The spatio-temporal variation in pCO2 found
responds to the influence of different factors that usually affect its
distribution in the littoral oceans. The temporal variability in pCO2
is principally explained by two factors, considering the mean values of the
eight cruises: SST (pCO2 (µatm) =302.0+5.16 SST
(∘C), r2=0.71) and biological activity,
represented by chlorophyll a (pCO2 (µatm) =425.0-59.15
[chlorophyll a] (µg L-1), r2=0.76). Over and above
these general tendencies, there are spatial variations associated
fundamentally with other processes. Firstly, the dominant effects in the
shallower areas are also due to the continental inputs, the biological
activity and the air–sea CO2 exchange. Then pCO2 values diminish
progressively in line with increasing distance from the coast, out as far as
an approximate depth of some 400 m. There is a relative increase in SST and
pCO2 as a consequence of a change in the origin of the surface water,
with the arrival of waters in a warm branch of the Azores Current and the
change produced by the biological activity.
The total T/B ratio (1.15) of the region suggests that the distribution is
principally controlled by temperature changes. However, the situation is
more complicated if the ratio is considered a function of bottom depth,
which is associated with the existence of non-thermal processes. In the
proximity of the Guadalquivir estuary the ratio takes a value of 0.93 due to
the continental inputs of C and nutrients, and in the zone around the
coastal upwelling off Cape Trafalgar the ratio is 1.09. Furthermore, the
actual characteristics of the surface water mass that originates under the
influence of a branch of the Azores Current also produce a decrease in the
T/B ratio in the deeper zone studied (1.05 for depths > 600 m).
In contrast, the highest T/B ratio values have been found in the SP
transect, where values of up to 1.54 are obtained for depths greater than
100 m, probably related to the greater effect of thermal processes.
The annual uptake capacity of CO2 by the surface waters in our study
area is 4.1 Gg C yr-1. The air–sea CO2 fluxes present seasonal
variation: these waters act as a source of CO2 to the atmosphere in
summer and autumn and as a sink in winter and spring. Based on the
information available in the zone, there seems to have been a decrease in
the capacity for CO2 capture in the zone in recent decades since the
pCO2 has increased from 360.6±18.2µatm in a study
realized between 2006 and 2007 (Ribas-Ribas et al., 2011) to 398.9±15.5µatm in actuality and this exceeds the rate of increase in
pCO2 in the atmosphere (2.3 µatm yr-1 for the last 10 years).
Data availability
All data used in this study are compiled in Tables 1, 2 and 3.
Research data are not yet deposited in a public data repository.
Author contributions
DJL wrote the article with contributions from AS, TO and JF.
DJL and JF processed the experimental data. DJL, TO and JF
conceived the original idea. All authors contributed to collecting the data.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
Dolores Jiménez-López was financed by the University of Cádiz with a
FPI fellowship (FPI-UCA) and Ana Sierra was financed by the Spanish Ministry
of Education with a FPU fellowship (FPU2014-04048). The authors gratefully
acknowledge the Spanish Institute of Oceanography (IEO) for giving us the
opportunity to participate in the STOCA cruises. We thank the crews of the
R/Vs Angeles Alvariño and Ramón Margalef for their assistance during
field work. We are also grateful to Xose A. Padin and Fiz F. Pérez
(IIM-CSIC) for collaboration on the calibration of the substandards of
CO2. We also thank the three anonymous reviewers and to the editor
for their comments provided, which helped substantially to improve this
article.
Financial support
This work was supported by
the Spanish Program for Science and Technology
(grant nos. CTM2014-59244-C3 and RTI2018-100865-B-C21).
Review statement
This paper was edited by Mario Hoppema and reviewed by three anonymous referees.
ReferencesAït-Ameur, N. and Goyet, C.: Distribution and transport of natural and
anthropogenic CO2 in the Gulf of Cádiz, Deep-Sea Res. Pt II., 53, 1329–1343, 10.1016/j.dsr2.2006.04.003,
2006.Al Azhar, M., Lachkar, Z., Lévy, M., and Smith, S.: Oxygen minimum zone
contrasts between the Arabian Sea and the Bay of Bengal implied by
differences in remineralization depth, Geophys. Res. Lett., 44, 106–114,
10.1002/2017GL075157, 2017.Alvarez, I., Ospina-Alvarez, N., Pazos, Y., deCastro, M., Bernardez, P.,
Campos, M. J., Gomez-Gesteira, J. L., Alvarez-Ossorio, M. T., Varela, M.,
Gomez-Gesteira, M., and Prego, R.: A winter upwelling event in the Northern
Galician Rias: Frequency and oceanographic implications, Estuar. Coast.
Shelf Sci., 82, 573–582, 10.1016/j.ecss.2009.02.023, 2009.Anfuso, E., Ponce, R., Castro, C. G., and Forja, J. M.: Coupling between the
thermohaline, chemical and biological fields during summer 2006 in the
northeast continental shelf of the Gulf of Cádiz (SW Iberian Peninsula),
47–56, Sci. Mar., 10.3989/scimar.2010.74s1047, 2010.Arístegui, J., Barton, E. D., Álvarez-Salgado, X. A., Santos,
A. M. P., Figueiras, F. G., Kifani, S., Hernández-León, S., Mason, E.,
Machú, E., and Demarcq, H.: Sub-regional ecosystem variability in the
Canary Current upwelling, Prog. Oceanogr, 83, 33–48,
10.1016/j.pocean.2009.07.031, 2009.Armi, L. and Farmer, D. M.: The flow of Mediterranean water through the
Strait of Gibraltar, Prog. Oceanogr., 21, 1–105,
10.1016/0079-6611(88)90055-9, 1988.Arnone, V., González-Dávila, M., and Santana-Casiano, J. M.:
CO2 fluxes in the South African coastal region, Mar. Chem., 195,
41–49, 10.1016/j.marchem.2017.07.008, 2017.Arruda, R., Calil, P. H. R., Bianchi, A. A., Doney, S. C., Gruber, N., Lima, I., and Turi, G.: Air-sea CO2 fluxes and the controls on ocean surface pCO2 seasonal variability in the coastal and open-ocean southwestern Atlantic Ocean: a modeling study, Biogeosciences, 12, 5793–5809, 10.5194/bg-12-5793-2015, 2015.Astor, Y. M., Scranton, M. I., Muller-Karger, F., Bohrer, R., and Garcia,
J.: CO2 variability at the CARIACO tropical coastal upwelling time
series station, Mar. Chem., 97, 245–261,
10.1016/j.marchem.2005.04.001, 2005.Baringer, M. O. N. and Price, J. F.: A review of the physical oceanography
of the Mediterranean outflow, Mar. Geol., 155, 63–82,
10.1016/S0025-3227(98)00141-8, 1999.Bates, N. R., Merlivat, L., Beaumont, L., and Pequignet, A. C.:
Intercomparison of shipboard and moored CARIOCA buoy seawater fCO2
measurements in the Sargasso Sea, Mar. Chem., 72, 239–255,
10.1016/S0304-4203(00)00084-0, 2000.Bauer, J. E., Cai, W. J., Raymond, P. A., Bianchi, T. S., Hopkinson, C. S.,
and Regnier, P. A.: The changing carbon cycle of the coastal ocean, Nature,
504, 61–70, 10.1038/nature12857, 2013.Bellanco, M. J. and Sánchez-Leal, R. F.: Spatial distribution and
intra-annual variability of water masses on the Eastern Gulf of Cádiz
seabed, Cont. Shelf Res., 128, 26–35,
10.1016/j.csr.2016.09.001, 2016.Borges, A. V. and Frankignoulle, M.: Daily and seasonal variations of the
partial pressure of CO2 in surface seawater along Belgian and southern
Dutch coastal areas, J. Mar. Syst., 19, 251–266,
10.1016/S0924-7963(98)00093-1, 1999.Borges, A. V. and Frankignoulle, M.: Distribution of surface carbon dioxide
and air-sea exchange in the upwelling system off the Galician coast, Global
Biogeochem. Cy., 16, 1020, 10.1029/2000GB001385, 2002.
Borges, A. V. and Abril., G.: Treatise on Estuarine and Coastal Science,
Elsevier, Waltham, 328 pp., 2011.Borges, A. V., Delille, B., and Frankignoulle, M.: Budgeting sinks and
sources of CO2 in the coastal ocean: Diversity of ecosystems counts,
Geophys. Res. Lett., 32, L14601, 10.1029/2005GL023053, 2005.Borges, A. V., Schiettecatte, L. S., Abril, G., Delille, B., and Gazeau, F.:
Carbon dioxide in European coastal waters, Estuar. Coast. Shelf Sci., 70,
375–387, 10.1016/j.ecss.2006.05.046, 2006.Burdige, D. J.: Preservation of Organic Matter in Marine Sediments?:
Controls, Mechanisms, and an Imbalance in Sediment Organic Carbon Budgets?,
Chem. Rev., 107, 467–485, 10.1021/cr050347q, 2007.Burgos, M., Ortega, T., and Forja, J.: Carbon Dioxide and Methane Dynamics
in Three Coastal Systems of Cádiz Bay (SW Spain), Estuar. Coast.,
41, 1069–1088, 10.1007/s12237-017-0330-2, 2018.Cai, W. J.: Estuarine and coastal ocean carbon paradox: CO2 sinks or
sites of terrestrial carbon incineration?, Ann. Rev. Mar.
Sci., 3, 123–145, 10.1146/annurev-marine-120709-142723,
2011.Cai, W. J., Wang, Z. A., and Wang, Y.: The role of marsh-dominated
heterotrophic continental margins in transport of CO2 between the
atmosphere, the land-sea interface and the ocean, Geophys. Res. Lett., 30,
1–4, 10.1029/2003GL017633, 2003.Cai, W. J., Dai, M., and Wang, Y.: Air-sea exchange of carbon dioxide in
ocean margins: A province-based synthesis, Geophys. Res. Lett., 33, 2–5,
10.1029/2006GL026219, 2006.Cai, W. J., Hu, X., Huang, W. J., Murrell, M. C., Lehrter, J. C.,
Lohrenz, S. E., Chou, W. C., Zhai, W., Hollibaugh, J. T., Wang, Y., Zhao,
P., Guo, X., Gunderser, K., Dai, M., and Gong, G. C.: Acidification of
subsurface coastal waters enhanced by eutrophication, Nat. Geosci., 4, 766–770,
10.1038/ngeo1297, 2011.Carvalho, A. C. O., Marins, R. V., Dias, F. J. S., Rezende, C. E.,
Lefèvre, N., Cavalcante, M. S., and Eschrique, S. A.: Air-sea CO2
fluxes for the Brazilian northeast continental shelf in a climatic
transition region, J. Mar. Syst., 173, 70–80,
10.1016/j.jmarsys.2017.04.009, 2017.Chen, C. T. A. and Borges, A. V.: Reconciling opposing views on carbon
cycling in the coastal ocean: Continental shelves as sinks and near-shore
ecosystems as sources of atmospheric CO2, Deep. Res. Part II Top. Stud.
Oceanogr., 56, 578–590, 10.1016/j.dsr2.2009.01.001, 2009.Chen, C.-T. A., Huang, T.-H., Chen, Y.-C., Bai, Y., He, X., and Kang, Y.: Air–sea exchanges of CO2 in the world's coastal seas, Biogeosciences, 10, 6509–6544, 10.5194/bg-10-6509-2013, 2013.Clargo, N. M., Salt, L. A., Thomas, H., and de Baar, H. J. W.: Rapid
increase of observed DIC and pCO2 in the surface waters of the North
Sea in the 2001–2011 decade ascribed to climate change superimposed by
biological processes, Mar. Chem., 177, 566–581,
10.1016/j.marchem.2015.08.010, 2015.Cohen, J. E., Small, C., Mellinger, A., Gallup, J., and Sachs, J.: Estimates
of coastal populations, Science, 278, 1209–1213,
10.1126/science.278.5341.1209c, 1997.Criado-Aldeanueva, F., García-Lafuente, J., Vargas, J. M., Del
Río, J., Vázquez, A., Reul, A., and Sánchez, A.: Distribution
and circulation of water masses in the Gulf of Cádiz from in situ
observations, Deep-Sea Res. Pt II, 53, 1144–1160,
10.1016/j.dsr2.2006.04.012, 2006.Dafner, E. V., González-Dávila, M., Santana-Casiano, J. M., and
Sempere, R.: Total organic and inorganic carbon exchange through the Strait
of Gibraltar in September 1997, Deep-Sea Res. Pt I,
48, 1217–1235, 10.1016/S0967-0637(00)00064-9, 2001.de Haas, H., van Weering, T. C. E., and de Stieger, H.: Organic carbon in
shelf seas: sinks or sources, processes and products, Cont. Shelf Res., 22,
691–717, 10.1016/S0278-4343(01)00093-0, 2002.de la Paz, M., Gómez-Parra, A., and Forja, J.: Inorganic carbon dynamic
and air-water CO2 exchange in the Guadalquivir Estuary (SW Iberian
Península), J. Mar. Syst., 68, 265–277,
10.1016/j.jmarsys.2006.11.011, 2007.de la Paz, M., Debelius, B., Macías, D., Vázquez, A.,
Gómez-Parra, A., and Forja, J. M.: Tidal-induced inorganic carbon
dynamics in the Strait of Gibraltar, Cont. Shelf Res., 28, 1827–1837,
10.1016/j.csr.2008.04.012, 2008a.de la Paz, M., Gómez-Parra, A., and Forja, J.: Tidal-to-seasonal
variability in the parameters of the carbonate system in a shallow tidal
creek influenced by anthropogenic inputs, Rio San Pedro (SW Iberian
Península), Cont. Shelf Res., 28, 1394–1404,
10.1016/j.csr.2008.04.002, 2008b.de la Paz, M., Gómez-Parra, A., and Forja, J. M.: Seasonal variability
of surface fCO2 in the Strait of Gibraltar, Aquat. Sci., 71, 55–64,
10.1007/s00027-008-8060-y, 2009.de la Paz, M., Padín, X. A., Ríos, A.F., and Pérez, F. F.:
Surface fCO2 variability in the Loire plume and adjacent shelf waters:
High spatio-temporal resolution study using ships of opportunity, Mar.
Chem., 118, 108–118, 10.1016/j.marchem.2009.11.004, 2010.DOE: in: Guide to best practices for ocean CO2 measurement, edited by:
Dickson, A. G., Sabine, C. L. and Christian, J. R.,
North Pacific Marine Science Organization Sidney, British Columbia, 191 pp., 2007.Echevarría, F., García-Lafuente, J., Bruno, M., Gorsky, G., Goutx,
M., González, N., García, C. M., Gómez, F., Vargas, J. M.,
Picheral, M., Striby, L., Varela, M., Alonso, J. J., Reul, A., Cózar,
A., Prieto, L., Sarhan, T., Plaza, F., and Jiménez-González, F.:
Physical-biological coupling in the Strait of Gibraltar, Deep-Sea Res. Pt.
II, 49, 4115–4130, 10.1016/S0967-0645(02)00145-5, 2002.Feely, R. A., Boutin, J., Cosca, C. E., Dandonneau, Y., Etcheto, J., Inoue,
H. Y., Ishii, M., Quéré, C. L., Mackey, D. J., McPhaden, M., Metzl,
N., Poisson, A., and Wanninkhof, R.: Seasonal and interannual variability of
CO2 in the equatorial Pacific, Deep-Sea Res. Pt. II,
49, 2443–2469, 10.1016/S0967-0645(02)00044-9, 2002.Fennel, K. and Wilkin, J.: Quantifying biological carbon export for the
northwest North Atlantic continental shelves, Geophys. Res. Lett., 36, 2–5,
10.1029/2009GL039818, 2009.Ferrón, S., Alonso-Pérez, F., Anfuso, E., Murillo, F. J., Ortega,
T., Castro, C. G., and Forja, J. M.: Benthic nutrient recycling on the
northeastern shelf of the Gulf of Cádiz (SW Iberian Península),
Mar. Ecol. Prog. Ser., 390, 79–95, 10.3354/meps08199, 2009.
Fiúza, A. F., de Macedo, M., and Guerreiro, M.: Climatological space and
time variation of the Portuguese coastal upwelling, Oceanol. Acta, 5,
31–40, 1982.Frankignoulle, M. and Borges, A. V.: European continental shelf as a
significant sink for atmospheric carbon dioxide, Global Biogeochem. Cy.,
15, 569–576, 10.1029/2000GB001307, 2001.Friederich, G. E., Walz, P. M., Burczynski, M. G., and Chavez, F. P.:
Inorganic carbon in the central California upwelling system during the
1997–1999 El Niño-La Niña event, Prog. Oceanogr., 54, 185–203,
10.1016/S0079-6611(02)00049-6, 2002.Friederich, G. E., Ledesma, J., Ulloa, O., and Chavez, F. P.: Air-sea carbon
dioxide fluxes in the coastal southeastern tropical Pacific, Prog.
Oceanogr., 79, 156–166, 10.1016/j.pocean.2008.10.001, 2008.Friedl, G., Dinkel, C., and Wehrli, B.: Benthic fluxes of nutrients in the
northwestern Black Sea, Mar. Chem., 62, 77–88,
10.1016/S0304-4203(98)00029-2, 1998.García Lafuente, J. and Ruiz, J.: The Gulf of Cádiz pelagic
ecosystem: A review, Prog. Oceanogr., 74, 228–251,
10.1016/j.pocean.2007.04.001, 2007.García, C. M., Prieto, L., Vargas, M., Echevarría, F.,
García-Lafuente, J., Ruiz, J., and Rubín, J. P.: Hydrodynamics and
the spatial distribution of plankton and TEP in the Gulf of Cádiz (SW
Iberian Península), J. Plankton Res., 24, 817–833,
10.1093/plankt/24.8.817, 2002.Garcia-Lafuente, J., Delgado, J., Criado-Aldeanueva, F., Bruno, M., del Rio,
J., and Vargas, J. M.: Water mass circulation on the continental shelf of
the Gulf of Cádiz, Deep-Sea Res. Pt. II, 53,
1182–1197, 10.1016/j.dsr2.2006.04.011, 2006.González-Dávila, M., Santana-Casiano, J. M., and Dafner, E. V.:
Winter mesoscale variations of carbonate system parameters and estimates of
CO2 fluxes in the Gulf of Cádiz, northeast Atlantic Ocean (February
1998), J. Geophys. Res., 108, 1–11, 10.1029/2001JC001243,
2003.González-Dávila, M., Santana-Casiano, J. M., and Ucha, I. R.: Seasonal
variability of fCO2 in the Angola-Benguela region, Prog. Oceanogr., 83,
124–133, 10.1016/j.pocean.2009.07.033, 2009.González-Dávila, M., Santana Casiano, J. M., and Machín, F.: Changes in the partial pressure of carbon dioxide in the Mauritanian–Cap Vert upwelling region between 2005 and 2012, Biogeosciences, 14, 3859–3871, 10.5194/bg-14-3859-2017, 2017.González-García, C., Forja, J., González-Cabrera, M. C.,
Jiménez, M. P., and Lubián, L. M.: Annual variations of total and
fractionated chlorophyll and phytoplankton groups in the Gulf of Cádiz,
Sci. Total Environ., 613, 1551–1565,
10.1016/j.scitotenv.2017.08.292, 2018.Gould, W. J.: Physical oceanography of the Azores Front, Prog. Oceanogr.,
14, 167–190, 10.1016/0079-6611(85)90010-2, 1985.
Grasshoff, K., Erhardt, M., and Kremiling, K.: Methods of Seawater Analysis,
Verlag Chemie, New York, 419 pp., 1983.Gypens, N., Lacroix, G., Lancelot, C., and Borges, A. V.: Seasonal and
inter-annual variability of air–sea CO2 fluxes and seawater carbonate
chemistry in the Southern North Sea, Prog. Oceanogr., 88, 59–77,
10.1016/j.pocean.2010.11.004, 2011.Hales, B., Takahashi, T., and Bandstra, L.: Atmospheric CO2 uptake by a
coastal upwelling system, Global Biogeochem. Cy., 19, 1–11,
10.1029/2004GB002295, 2005.Hofmann, E. E., Cahill, B., Fennel, K., Friedrichs, M. A. M., Hyde, K., Lee,
C., Mannino, A., Najjar, R. G., O'Reilly, J. E., Wilkin, J., and
Xue, J.: Modeling the Dynamics of Continental Shelf Carbon, Annu. Rev. Mar.
Sci., 3, 93–122, 10.1146/annurev-marine-120709-142740,
2011.Huertas, E., Navarro, G., Rodríguez-Gálvez, S., and Prieto, L.: The
influence of phytoplankton biomass on the spatial distribution of carbon
dioxide in surface sea water of a coastal area of the Gulf of Cádiz
(southwestern Spain), Can. J. Bot., 83, 929–940,
10.1139/b05-082, 2005.Huertas, I. E., Navarro, G., Rodríguez-Gálvez, S., and Lubián,
L. M.: Temporal patterns of carbon dioxide in relation to hydrological
conditions and primary production in the northeastern shelf of the Gulf of
Cádiz (SW Spain), Deep-Sea Res. Pt II., 53,
1344–1362, 10.1016/j.dsr2.2006.03.010, 2006.Ito, R. G., Garcia, C. A. E., and Tavano, V. M.: Net sea-air CO2 fluxes
and modelled pCO2 in the southwestern subtropical Atlantic continental
shelf during spring 2010 and summer 2011, Cont. Shelf Res., 119, 68–84,
10.1016/j.csr.2016.03.013, 2016.Jahnke, R., Richards, M., Nelson, J., Robertson, C., Rao, A., and Jahnke,
D.: Organic matter remineralization and porewater exchange rates in
permeable South Atlantic Bight continental shelf sediments, Cont. Shelf
Res., 25, 1433–1452, 10.1016/j.csr.2005.04.002, 2005.Jiang, L. Q., Cai, W. J., Wanninkhof, R., Wang, Y., and Lüger, H.:
Air-sea CO2 fluxes on the U.S. South Atlantic Bight: Spatial and
seasonal variability, J. Geophys. Res., 113, C07019,
10.1029/2007JC004366, 2008.Jiang, L.-Q., Cai, W.-J., Wang, Y., and Bauer, J. E.: Influence of terrestrial inputs on continental shelf carbon dioxide, Biogeosciences, 10, 839–849, 10.5194/bg-10-839-2013, 2013.Johnson, J. and Stevens, I.: A fine resolution model of the eastern North
Atlantic between the Azores, the Canary Islands and the Gibraltar Strait,
Deep-Sea Res. Pt. I, 47, 875–899,
10.1016/S0967-0637(99)00073-4, 2000.Kahl, L. C., Bianchi, A. A., Osiroff, A. P., Pino, D. R., and Piola, A. R.:
Distribution of sea-air CO2 fluxes in the Patagonian Sea: seasonal,
biological and thermal effects, Cont. Shelf Res., 143, 18–28,
10.1016/j.csr.2017.05.011, 2017.Käse, R. H., Zenk, W., Sanford, T. B., and Hiller, W.: Currents, Fronts
and Eddy Fluxes in the Canary Basin, Progr. Oceanogr., 14, 231–257,
10.1016/0079-6611(85)90013-8, 1985.Klein, B. and Siedler, G.: On the origin of the Azores Current, J. Geophys.
Res., 94, 6159–6168, 10.1029/JC094iC05p06159, 1989.Körtzinger, A., Thomas, H., Schneider, B., Gronau, N., Mintrop, L., and
Duinker, J. C.: At-sea intercomparison of two newly designed underway
pCO2 systems encouraging results, Mar. Chem., 52, 133–145,
10.1016/0304-4203(95)00083-6, 1996.Landschützer, P., Gruber, N., Haumann, F. A., Rödenbeck, C., Bakker,
D. c. E., van Heuven, S., Hoppema, M., Metzl, N., Sweeney, C., Tkahashi, T.,
Tilbrook, B., and Wanninkhof, R.: The reinvigration of the Southern Ocean carbon
sink, Science, 349, 1221–1224, 10.1126/science.aab2620,
2015.Laruelle, G. G., Dürr, H. H., Slomp, C. P., and Borges, A. V.:
Evaluation of sinks and sources of CO2 in the global coastal ocean
using a spatially-explicit typology of estuaries and continental shelves,
Geophys. Res. Lett., 37, L15607, 10.1029/2010GL043691, 2010.Laruelle, G. G., Lauerwald, R., Pfeil, B., and Regnier, P.: Regionalized
global budget of the CO2 exchange at the air-water interface in
continental shelf seas, Global Biogeochem. Cy., 28, 1199–1214,
10.1002/2014GB004832, 2014.Laruelle, G. G., Landschützer, P., Gruber, N., Tison, J.-L., Delille, B., and Regnier, P.: Global high-resolution monthly pCO2 climatology for the coastal ocean derived from neural network interpolation, Biogeosciences, 14, 4545–4561, 10.5194/bg-14-4545-2017, 2017.Le Quéré, C., Andrew, R. M., Friedlingstein, P., Sitch, S., Pongratz, J., Manning, A. C., Korsbakken, J. I., Peters, G. P., Canadell, J. G., Jackson, R. B., Boden, T. A., Tans, P. P., Andrews, O. D., Arora, V. K., Bakker, D. C. E., Barbero, L., Becker, M., Betts, R. A., Bopp, L., Chevallier, F., Chini, L. P., Ciais, P., Cosca, C. E., Cross, J., Currie, K., Gasser, T., Harris, I., Hauck, J., Haverd, V., Houghton, R. A., Hunt, C. W., Hurtt, G., Ilyina, T., Jain, A. K., Kato, E., Kautz, M., Keeling, R. F., Klein Goldewijk, K., Körtzinger, A., Landschützer, P., Lefèvre, N., Lenton, A., Lienert, S., Lima, I., Lombardozzi, D., Metzl, N., Millero, F., Monteiro, P. M. S., Munro, D. R., Nabel, J. E. M. S., Nakaoka, S., Nojiri, Y., Padin, X. A., Peregon, A., Pfeil, B., Pierrot, D., Poulter, B., Rehder, G., Reimer, J., Rödenbeck, C., Schwinger, J., Séférian, R., Skjelvan, I., Stocker, B. D., Tian, H., Tilbrook, B., Tubiello, F. N., van der Laan-Luijkx, I. T., van der Werf, G. R., van Heuven, S., Viovy, N., Vuichard, N., Walker, A. P., Watson, A. J., Wiltshire, A. J., Zaehle, S., and Zhu, D.: Global Carbon Budget 2017, Earth Syst. Sci. Data, 10, 405–448, 10.5194/essd-10-405-2018, 2018.Lefèvre, N., da Silva Dias, F. J., de Torres, A. R., Noriega, C.,
Araujo, M., de Castro, A. C. L., Rocha, C., Jiang, S., and Ibánhez, J.
S. P.: A source of CO2 to the atmosphere throughout the year in the
Maranhense continental shelf (2∘30′ S, Brazil), Cont. Shelf Res.,
141, 38–50, 10.1016/j.csr.2017.05.004, 2017.Lewis, E., Wallace, D., and Allison, L. J.: Program developed for CO2
system calculations. Carbon Dioxide Information Analysis Center, managed by
Lockheed Martin Energy Research Corporation for the US Department of Energy
Tennessee, 1998.Litt, E. J., Hardman-Mountford, N. J., Blackford, J. C., and
Mitchelson-Jacob, G. A. Y.: Biological control of pCO2 at station L4 in
the Western English Channel over 3 years, J. Plank. Res, 32, 621–629,
10.1093/plankt/fbp133, 2018.Liu, S. M., Zhu, B. D., Zhang, J., Wu, Y., Liu, G. S., Deng, B., Zhao, M.
X., Liu, G. Q., Du, J. Z., Ren, J. L., and Zhang, G. L.: Environmental
change in Jiaozhou Bay recorded by nutrient components in sediments, Mar.
Pollut. Bull., 60, 1591–1599,
10.1016/j.marpolbul.2010.04.003, 2010.Lueker, T. J., Dickson, A. G., and Keeling, C. D.: Ocean pCO2
calculated from dissolved inorganic carbon alkalinity, and equations
for K1 and K2: validation based on laboratory measurements of
CO2 in gas and seawater at equilibrium, Mar. Chem., 70, 105–119,
10.1016/S0304-4203(00)00022-0, 2000.
Mackenzie, F. T., Bewers, J. M., Charlson, R. J., Hofmann, E. E., Knauer, G.
A., Kraft, J. C., Nöthig, E. M., Quack, B., Walsh, J. J., Whitfield, M.,
and Wollast, R.: What is the importance of ocean margin processes in global
change?, in: Ocean Margin Processes in Global Change, edited by: Mantoura,
R. F. C., Martin, J. M., Wollast, R., Dahlem workshop reports, J. Wiley &
Sons, Chichester, 433–454, 1991.Mackenzie, F. T., Lerman, A., and Andersson, A. J.: Past and present of sediment and carbon biogeochemical cycling models, Biogeosciences, 1, 11–32, 10.5194/bg-1-11-2004, 2004.Michaels, A. F., Karl, D. M., and Capone, D. G.: Element stoichiometry, new
production and nitrogen fixation, Oceanography, 14, 68–77,
10.5670/oceanog.2001.08, 2001.Millero, F. J.: Thermodynamics of the carbon dioxide system in the
oceans, Geochi. Cosmo. Acta, 59, 661–677,
10.1016/0016-7037(94)00354-O, 1995.Muller-Karger, F. E., Varela, R., Thunell, R., Luerssen, R., Hu, C., and
Walsh, J. J.: The importance of continental margins in the global carbon
cycle, Geophys. Res. Lett, 32, 1–4, 10.1029/2004GL021346,
2005.Navarro, G. and Ruiz, J.: Spatial and temporal variability of phytoplankton
in the Gulf of Cádiz through remote sensing images, Deep-Sea Res. Pt II., 53, 11–13, 10.1016/j.dsr2.2006.04.014, 2006.Olsen, A., Brown, K. R., Chierici, M., Johannessen, T., and Neill, C.: Sea-surface CO2 fugacity in the subpolar North Atlantic, Biogeosciences, 5, 535–547, 10.5194/bg-5-535-2008, 2008.Omar, A. M., Olsen, A., Johannessen, T., Hoppema, M., Thomas, H., and Borges, A. V.: Spatiotemporal variations of fCO2 in the North Sea, Ocean Sci., 6, 77–89, 10.5194/os-6-77-2010, 2010.Padin, X. A., Navarro, G., Gilcoto, M., Rios, A. F., and Pérez, F. F.:
Estimation of air-sea CO2 fluxes in the Bay of Biscay based on
empirical relationships and remotely sensed observations, J. Mar. Syst., 75,
280–289, 10.1016/j.jmarsys.2008.10.008, 2009.Padin, X. A., Vázquez-Rodríguez, M., Castaño, M., Velo, A., Alonso-Pérez, F., Gago, J., Gilcoto, M., Álvarez, M., Pardo, P. C., de la Paz, M., Ríos, A. F., and Pérez, F. F.: Air-Sea CO2 fluxes in the Atlantic as measured during boreal spring and autumn, Biogeosciences, 7, 1587–1606, 10.5194/bg-7-1587-2010, 2010.
Parsons, T. R., Maita, Y., and Lalli, C. M.: A Manual Of Chemical And
Biological Methods For Seawater Analysis, Pergamon Press, Oxford, 172 pp.,
1984.Peliz, A., Dubert, J., Marchesiello, P., and Teles-Machado, A.: Surface
circulation in the Gulf of Cádiz: Model and mean flow structure, J.
Geophys. Res.-Oceans, 112, 1–20, 10.1029/2007JC004159,
2007.Peliz, A., Marchesiello, P., Santos, A. M. P., Dubert, J., Teles-Machado,
A., Marta-Almeida, M., and Le Cann, B.: Surface circulation in the Gulf of
Cádiz: 2. Inflow-outflow coupling and the Gulf of Cádiz slope
current, J. Geophys. Res.-Oceans, 114, 1–16,
10.1029/2008JC004771, 2009.
Prieto, L., Garcia, C. M., Corzo, A., Ruiz Segura, J., and Echevarria, F.:
Phytoplankton, bacterioplankton and nitrate reductase activity distribution
in relation to physical structure in the northern Alboran Sea and Gulf of
Cádiz (southern Iberian Península), Bol. Inst. Esp. Oceanogr., 15,
401–411, 1999.Qin, B. Y., Tao, Z., Li, Z. W., and Yang, X. F.: Seasonal changes and
controlling factors of sea surface pCO2 in the Yellow Sea, in: IOP Conf.
Ser.: Earth Environ. Sci., 17, 012025,
10.1088/1755-1315/17/1/012025, 2014.Qu, B., Song, J., Yuan, H., Li, X., and Li, N.: Air-sea CO2 exchange
process in the southern Yellow Sea in April of 2011, and June, July, October
of 2012, Cont. Shelf Res., 80, 8–19,
10.1016/j.csr.2014.02.001, 2014.Rabouille, C., Mackenzie, F. T., and Ver, L. M.: Influence of the human
perturbation on carbon, nitrogen, and oxygen biogeochemical cycles in the
global coastal ocean, Geochim. Cosmo. Acta, 65, 3615–3641,
10.1016/S0016-7037(01)00760-8, 2001.
Redfield, A. C., Ketchum, B. H., and Richards, F. A.: The influence of organisms
on the composition of sea-water, in: , The sea, edited by: Hill, M. N., 2,
Interscience, New York, 26–77, 1963.Reimer, J. J., Cai, W.-J., Xue, L., Vargas, R., Noakes, S., Hu, X.,
Signorini, S. R., Mathis, J. T., Feely, R. A., Sutton, A. J., Sabine, C.,
Musielewicz, S., Chen, B., and Wanninkhof, R.: Time series of pCO2 at a
coastal mooring: Internat consistency, seasonal cycles, and interannual
variaiblity, Cont. Shelf Res., 145, 95–108,
10.1016/j.csr.2017.06.022, 2017.Ribas-Ribas, M., Gómez-Parra, A., and Forja, J. M.: Air-sea CO2
fluxes in the north-eastern shelf of the Gulf of Cádiz (southwest
Iberian Península), Mar. Chem., 123, 56–66,
10.1016/j.marchem.2010.09.005, 2011.Ribas-Ribas, M., Sobrino, C., Debelius, B., Lubián, L.M., Ponce, R.,
Gómez-Parra, A., and Forja, J. M.: Picophytoplankton and carbon cycle on
the northeastern shelf of the Gulf of Cádiz (SW Iberian Península),
Sci. Mar., 77, 49–62, 10.3989/scimar.03732.27D, 2013.Ríos, A. F., Pérez, F. F., Álvarez, M. A., Mintrop, L.,
González-Dávila, M., Santana-Casiano, J. M., Lefèvre, N., and
Watson, A. J.: Seasonal sea-surface carbon dioxide in the Azores area, Mar.
Chem., 96, 35–51, 10.1016/j.marchem.2004.11.001, 2005.Sala, I., Caldeira, R. M. A., Estrada-Allis, S. N., Froufe, E., and
Couvelard, X.: Lagrangian transport pathways in the northeast Atlantic and
their environmental impact, Limnol. Oceanogr. Fluids Environ., 3, 40–60,
10.1215/21573689-2152611, 2013.Sala, I., Navarro, G., Bolado-Penagos, M., Echevarría, F., and
García, C. M.: High-Chlorophyll-Area Assessment Based on Remote Sensing
Observations: The Case Study of Cape Trafalgar, Remote Sensing, 10, 165,
10.3390/rs10020165, 2018.Sánchez, R. F. and Relvas, P.: Spring-summer climatological circulation
in the upper layer in the region of Cape St. Vincent, Southwest
Portugal, ICES J. Mar. Sci., 60, 1232–1250,
10.1016/S1054-3139(03)00137-1, 2003.Sánchez, R. F., Relvas, P., Martinho, A., and Miller, P.: Physical
description of an upwelling filament west of Cape St. Vincent in late
October 2004, J. Geophys. Res.-Oceans, 113, C07044,
10.1029/2007JC004430, 2008.Sánchez-Leal, R. F., Bellanco, M. J., Fernández-Salas, L. M.,
García-Lafuente, J., Gasser-Rubinat, M., González-Pola, C.,
Hernández-Molina, F. J., Pelegrí, J. L., Peliz, A., Relvas, P.,
Roque, D., Ruiz-Villarreal, M., Sammartino, S., and Sánchez-Garrido, J.
C.: The Mediterranean Overflow in the Gulf of Cádiz: A rugged journey,
Sci. Adv., 3, eaao0609, 10.1126/sciadv.aao0609, 2017.Santana-Casiano, J. M., Gonzalez-Davila, M., and Laglera, L. M.: The carbon
dioxide system in the Strait of Gibraltar, Deep-Sea Res. Pt II., 49, 4145–4161, 10.1016/S0967-0645(02)00147-9,
2002.Santana-Casiano, J., González-Dávila, M., and Ucha, I.: Carbon
dioxide fluxes in the Benguela upwelling system during winter and spring: A
comparison between 2005 and 2006, Deep-Sea Res. Pt. II, 56,
533–541, 10.1016/j.dsr2.2008.12.010, 2009.Schiettecatte, L. S., Thomas, H., Bozec, Y., and Borges, A. V.: High
temporal coverage of carbon dioxide measurements in the Southern Bight of
the North Sea, Mar. Chem., 106, 161–173,
10.1016/j.marchem.2007.01.001, 2007.Shaw, E. C. and McNeil, B. I.: Seasonal variability in carbonate chemistry
and air-sea CO2 fluxes in the southern Great Barrier Reef, Mar. Chem.,
158, 49–58, 10.1016/j.marchem.2013.11.007, 2014.Shim, J. H., Kim, D., Kang, Y. C., Lee, J. H., Jang, S. T., and Kim, C. H.:
Seasonal variations in pCO2 and its controlling factors in surface
seawater of the northern East China Sea, Cont. Shelf Res., 27, 2623–2636,
10.1016/j.csr.2007.07.005, 2007.Smith, S. V. and Hollibaugh, J. T.: Coastal metabolism and the oceanic
organic carbon balance, Rev. Geophys, 31, 75–89,
10.1029/92RG02584, 1993.Takahashi, T., Olafsson, J., Goddard, J. G., Chipman, D. W., and Sutherland,
S. C.: Seasonal variations of CO2 and nutrients in the high-latitude
surface oceans: A comparative study, Global Biogeochem. Cy., 7, 843–878,
10.1029/93GB02263, 1993.Takahashi, T., Sutherland, S. C., Sweeney, C., Poisson, A., Metzl, N.,
Tilbrook, B., Bates, N., Wanninkhof, R., Feely, R. A., Sabine, C., Olafsson,
J., and Nojiri, Y.: Global sea-air CO2 flux based on climatological
surface ocean pCO2, and seasonal biological and temperature effects,
Deep-Sea Res. Pt. II, 49, 1601–1622,
10.1016/S0967-0645(02)00003-6, 2002.Tseng, C. M., Liu, K. K., Gong, G. C., Shen, P. Y., and Cai, W. J.: CO2
uptake in the East China Sea relying on Changjiang runoff is prone to
change, Geophys. Res. Lett., 38, 1–6, 10.1029/2011GL049774,
2011.Tsunogai, S., Watanabe, S., Nakamura, J., Ono, T., and Sato, T.: A
preliminary study of carbon system in the East China Sea, J. Oceanogr., 53,
9–17, 10.1007/BF02700744, 1997.Vandemark, D., Salisbury, J. E., Hunt, C. W., Shellito, S. M., Irish, J. D.,
McGillis, W. R., Sabine, C. L., and Maenner, S. M.: Temporal and spatial
dynamics of CO2 air–sea flux in the Gulf of Maine, J. Geophys. Res.-Oceans, 116, C01012, 10.1029/2010JC006408, 2011.van Geen, A., Takesue, R. K., Goddard, J., Takahashi, T., Barth, J. A., and
Smith, R. L.: Carbon and nutrient dynamics during coastal upwelling off Cape
Blanco, Oregon, Deep-Sea Res. Pt. II, 47, 975–1002,
10.1016/S0967-0645(99)00133-2, 2000.Vargas-Yáñez, M., Viola, T. S., Jorge, F. P., Rubín, J. P., and
García, M. C.: The influence of tide-topography interaction on
low-frequency heat and nutrient fluxes, Application to Cape Trafalgar, Cont.
Shelf Res., 22, 115–139, 10.1016/S0278-4343(01)00063-2,
2002.Volk, T. and Hoffert, M. I.: Ocean carbon pumps: Analysis of relative
strengths and efficiencies in ocean-driven atmospheric CO2 changes in
The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to
Present, Geophys. Monogr. Ser., 32, 99–110, 10.1029/GM032p0099,
1985.
Walsh, J. J.: On the Nature of Continental Shelves, Academic Press, New
York, 510 pp., 1988Walsh, J. J.: Importance of continental margins in the marine biogeochemical
cycling of carbon and nitrogen, Nature, 350, 53–55,
10.1038/350053a0, 1991.Wang, S. L., Arthur Chen, C. T., Hong, G. H., and Chung, C. S.: Carbon
dioxide and related parameters in the East China Sea, Cont. Shelf Res., 20,
525–544, 10.1016/S0278-4343(99)00084-9, 2000.Wang, Z. A., Cai, W. J., Wang, Y., and Ji, H.: The southeastern continental
shelf of the United States as an atmospheric CO2 source and an exporter
of inorganic carbon to the ocean, Cont. Shelf Res., 25, 1917–1941,
10.1016/j.csr.2005.04.004, 2005.Wanninkhof, R.: Relationship between wind speed and gas exchange, J.
Geophys. Res., 97, 7373–7382, 10.1029/92JC00188, 1992.Wanninkhof, R.: Relationship between wind speed and gas exchange over the
ocean revisited. Limnol. Oceanogr. Methods, 12, 351–362,
10.4319/lom.2014.12.351, 2014.Weiss, R.: Carbon dioxide in water and seawater: the solubility of a
non-ideal gas, Mar. Chem., 2, 203–215,
10.1016/0304-4203(74)90015-2, 1974.
Wollast, R.: The Coastal Carbon Cycle: Fluxes, Sources and Sinks, in: Ocean Margin Processes in Global Change, edited by: Mantoura, R. F. C., Martin, J. M., and Wollast, R., J.Wiley & Sons
Chichester, 365–382, 1991.Wollast, R.: Interactions of Carbon and Nitrogen cycles in the Coastal Zone,
in: Interactions of C, N, P, and S biogeochemical cycles and global change,
edited by: Wollast, R., Mackenzie, F. T., and Chou, L., Springer, Berlin,
NATOASI Series, 14, 195–210,
10.1007/978-3-642-76064-8_7, 1993.Woolf, D. K., Land, P. E., Shutler, J. D., Goddijn-Murphy, L. M., and
Donlon, C. J.: On the calculation of air-sea fluxes of CO2 in the
presence of temperature and salinity gradients, J. Geophys. Res.-Oceans,
121, 1229–1248, 10.1002/2015JC011427, 2016.Xue, L., Xue, M., Zhang, L., Sun, T., Guo, Z., and Wang, J.: Surface partial
pressure of CO2 and air-sea exchange in the northern Yellow Sea, J.
Mar. Syst., 105–108, 194–206,
10.1016/j.jmarsys.2012.08.006, 2012.Xue, L., Gao, L., Cai, W. J., Yu, W., and Wei, M.: Response of sea surface
fugacity of CO2 to the SAM shift south of Tasmania: Regional
differences, Geophys. Res. Lett., 42, 3973–3979,
10.1002/2015GL063926, 2015.Xue, L., Cai, W. J., Hu, X., Sabine, C., Jones, S., Sutton, A. J., Jiang, L.
Q., and Reimer, J. J.: Sea surface carbon dioxide at the Georgia time series
site (2006–2007): Air-sea flux and controlling processes, Prog. Oceanogr.,
140, 14–26, 10.1016/j.pocean.2015.09.008, 2016.Yentsch, C. S. and Menzel, D. W.: A method for the determination of
phytoplankton chlorophyll and pheophytin by fluorescence, Deep-Sea Res.
Oceanogr. Abstracts, 10, 221–231,
10.1016/0011-7471(63)90358-9, 1963.Zeebe, R. E. and Wolf-Gladrow, D. A.: CO2 in seawater: equilibrium,
kinetics, isotopes, Elsevier Oceanography Series, Amsterdam, 347 pp., 2001.Zhai, W. D., Dai, M., and Cai, W.-J.: Coupling of surface pCO2 and dissolved oxygen in the northern South China Sea: impacts of contrasting coastal processes, Biogeosciences, 6, 2589–2598, 10.5194/bg-6-2589-2009, 2009.Zhang, L., Xue, L., Song, M., and Jiang, C.: Distribution of the surface
partial pressure of CO2 in the southern Yellow Sea and its controls,
Cont. Shelf Res., 30, 293–304, 10.1016/j.csr.2009.11.009,
2010.Zhang, L., Xue, M., and Liu, Q.: Distribution and seasonal variation in the
partial pressure of CO2 during autumn and winter in Jiaozhou Bay, a
region of high urbanization, Mar. Pollut. Bull., 64, 56–65,
10.1016/j.marpolbul.2011.10.023, 2012.