Inorganic carbon, pH and alkalinity in the Canary Current Large Marine Ecosystem

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Authors González-Dávila, Melchor; Santana-Casiano, J. Magdalena

Publisher IOC-UNESCO

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Link to Item http://hdl.handle.net/1834/9184 4.2. Inorganic carbon, pH and alkalinity in the Canary Current Large Marine Ecosystem

For bibliographic purposes, this article should be cited as: González‐Dávila, M. and Santana‐Casiano, J. M. 2015. Inorganic carbon, pH and alkalinity in the Canary Current Large Marine Ecosystem. In: Oceanographic and biological features in the Canary Current Large Marine Ecosystem. Valdés, L. and Déniz‐González, I. (eds). IOC‐UNESCO, Paris. IOC Technical Series, No. 115, pp. 143‐150. URI: http://hdl.handle.net/1834/9184.

The publication should be cited as follows: Valdés, L. and Déniz‐González, I. (eds). 2015. Oceanographic and biological features in the Canary Current Large Marine Ecosystem. IOC‐UNESCO, Paris. IOC Technical Series, No. 115: 383 pp. URI: http://hdl.handle.net/1834/9135.

The report Oceanographic and biological features in the Canary Current Large Marine Ecosystem and its separate parts are available on‐line at: http://www.unesco.org/new/en/ioc/ts115. The bibliography of the entire publication is listed in alphabetical order on pages 351‐379. The bibliography cited in this particular article was extracted from the full bibliography and is listed in alphabetical order at the end of this offprint, in unnumbered pages.

ABSTRACT

The vertical distribution of the carbon dioxide (CO2) variables in the Canary Current Large Marine Ecosystem (CCLME) along the last fifteen years have clearly indicated significant changes over, at least, the first 1000 m affecting the inorganic carbon content and the acidity of the seawater. In the surface, the normalized total dissolved inorganic carbon increased at a rate of 0.9 mol kg‐1, the pH in total scale decreased each year on average 0.0019 units, while the normalized total alkalinity keeps constant at a ‐1 value of 2292 mol kg . This increase in total dissolved inorganic carbon (CT) is controlling the total column ‐2 inventory of anthropogenic CO2 that has reached a value of 66 ± 3 mol m for the reference year 2000. It has been shown that upwelled waters in the Mauritanian upwelling area provide high contents of inorganic carbon that lead to low calcium carbonate saturation states. The uptake of carbon by phytoplankton acts by decreasing CT and consequently increasing saturation states. The Eastern North Atlantic Ocean at the ‐2 ‐1 CCLME is increasing its storage capacity for excess CO2 by 0.85 mol m yr . Model results indicate the importance of physical and environmental conditions in shaping the sensitivity of CCLME to potential climate change induced upwelling‐favorable wind intensification. Keywords: Alkalinity · Total dissolved inorganic carbon · Anthropogenic carbon · ESTOC · Interannual trends · Canary Current Large Marine Ecosystem · Northwest Africa IOC TECHNICAL SERIES, No. 115, pp. 143‐150. URI: http://hdl.handle.net/1834/9184. 2015

INORGANIC CARBON, pH AND ALKALINITY IN THE CANARY CURRENT LARGE MARINE ECOSYSTEM

Melchor GONZÁLEZ‐DÁVILA and J. Magdalena SANTANA‐CASIANO

Instituto de Oceanografía y Cambio Global (IOCAG), Universidad de Las Palmas de Gran Canaria. Spain

4.2.1. INTRODUCTION

Atmospheric carbon dioxide (CO2) levels have increased by 40% since pre‐industrial times, from 280 ppmv (parts per million volume) to 400 ppmv, reached for the first time in 2014. One third of the anthropogenic carbon added to the atmosphere is taken up by the ocean (Le Queré et al., 2014; Sabine et al., 2004). This oceanic CO2 uptake alters the chemistry of seawater, including pH, carbonate ion concentration as well as calcite and aragonite saturation, which together are commonly referred to as ‘ocean acidification’. As the ocean absorbs more anthropogenic CO2, the CO2 reacts with the seawater to form carbonic acid (H2CO3). - + This then dissociates to form a bicarbonate ion (HCO3 ) and a hydrogen ion (H ), which can react with ‐ - carbonate ions (CO3² ) to form bicarbonate (HCO3 ).This process on its own is independent of any climatic change and the reductions in CO2 emissions will not reduce ocean acidification of immediate form. Although all CO2 emissions stopped nowadays, it will take thousands of years for the ocean to recover

(Caldeira and Wickett, 2003). CO2 is being absorbed by the ocean so rapidly that the buffering capacity of the surface waters of the oceans will not be able to prevent a substantial lowering of ocean pH (Raven et al., 2005). Increasing temperatures of surface waters reduce the solubility of CO2 in seawater, consequently the amount of CO2 the ocean can absorb from the atmosphere decreases. The concern about this, called “ the other CO2 problem” (Turley, 2005), has led us to re‐examine the fundamental processes controlling the distributions of dissolved inorganic carbon (CT) and total alkalinity (AT) in oceans.

Long‐term time series are a powerful tool to investigate any change in ocean bio‐geochemistry and its effects on the carbon cycle. They are also the most direct way of estimating the accumulation of anthropogenic CO2 in the oceans. The ESTOC site (European Station for Time series in the Ocean, the Canary Islands), located off the Canary Islands in the NE Atlantic Ocean (e.g. González‐Dávila et al., 2003, 2007, 2010; Santana‐Casiano et al., 2001, 2007; Cianca et al., 2013; Bates et al., 2014) has provided evidence about the seasonal and inter-annual changes in the carbonate system for the Canary Island region. Similar role plays the Hawaiian ocean Time Series (HOTS) in the Pacific Ocean and the Bermuda Time Series (BATS) in the West Subtropical Atlantic Ocean (Bates et al., 2014). Physical processes such as turbulent mixing, subduction, advection, and transport of anthropogenic CO2 from the seasonally mixed layer into deeper water masses will alter the biogeochemical properties there over time. The characterization of these changes requires accurate highly resolved vertical long‐term data to be able to adequately identify any observed variation (González‐Dávila et al., 2010; Bates et al., 2014). The concentration of CO2 within the ocean is also critical to the pH of the water and the concentration of carbonate ions. Carbonate ions are needed to build calcium carbonate or limestone structures like coral skeletons and shells for many marine organisms, including shellfish, and marine plankton via calcification (Feely et al., 2004). Therefore, knowledge of the changes in ocean total dissolved inorganic carbon and its speciation, is of utmost importance in understanding and predicting changes in ocean chemistry properties.

The Canary Current Large Marine Ecosystem (CCLME) region, including the adjacent waters of Cape Verde and Guinea Conakry, also includes the Canary Upwelling Ecosystem (CUE) situated Northwest (NW) of 143 González‐Dávila, M. and Santana‐Casiano, J. M. Carbon dioxide system in the CCLME

Africa (11°–35°N) with the most active Mauritanian upwelling system (18°‐24°N). Several studies have improved our understanding of the Canary Current (Stramma and Siedler, 1988; Lozier et al., 1995; Siedler and Onken, 1996), and have confirmed the existence of a water inflow from the open ocean towards the coastal upwelling region north of the Canary Islands. Therefore the coastal upwelling region has been identified as the eastern boundary condition for the subtropical gyre (Pelegrí et al., 2005).

The coastal upwelling system transfers both nutrients and dissolved inorganic carbon to the sun‐lit surface ocean (Pelegrí et al., 2005). While the first stimulates phytoplankton growth, the second causes low calcium carbonate saturation states in the upper water column, with potentially adverse effects on marine calcifiers. The upwelled seawater also results in high natural variations of CO2 concentrations and pH reduction in surface seawater compared to most marine environments. As soon as the deeper water with high levels of CO2 reaches the surface, the temperature increases, and the newly introduced inorganic carbon can be partially or totally consumed through photosynthesis by phytoplankton, causing extensive blooms. This leads to the rapid drawdown of CO2 and an increase in pH in the more acidic upwelled waters (Santana‐Casiano and González‐Dávila, 6.5 this book). Phytoplankton can deplete seawater carbon dioxide concentrations in these regions far below current atmospheric levels of ≈400 ppm (Loucaides et al., 2012; Lachkar and Gruber, 2013).

The CUE is an important socio‐economic, oceanographic and climatological region. The Canary upwelling area is in one of the major coastal upwelling regions of the world. Others include the Benguela (Hagen et al., 2001), Humboldt (Thiel et al., 2007) and California (Pérez‐Brunius et al., 2007) eastern boundary upwelling ecosystems (EBUE). These EBUEs cover approximately 1% of the total ocean surface. Upwelled water is typically denser, cooler and richer in nutrients and in carbon dioxide than surface waters and has significant impacts on coastal climates and marine biology (Miranda et al., 2012). The Upwelling Intensification Hypothesis’ (Bakun, 1990), later taken up by Diffenbaugh et al. (2004) and Bakun et al. (2010), suggested that, in a globally warming world, an increase in greenhouse gases will cause reduced night‐time continental cooling and increased in day time heating during the warm seasons of the year (spring and summer for the northern Hemisphere subtropics). Consequently the temperature gradient between land and ocean would increase, which would intensify the continental–oceanic pressure gradient. Correspondingly, the along shore wind stress would be strengthened, leading to enhanced coastal upwelling. A considerable amount of literature has emerged since the original hypothesis, showing data and analyses for and against the upwelling intensification mechanism across the four main EBUEs and around the NW African coastline (Barton et al., 2013, Cropper et al., 2014).

4.2.2. DATA SOURCES AND METHODS

4.2.2.1. Research vessels: Time Series Stations

The European Station for Time series in the ocean (ESTOC) (29°10’N, 15°30’ W) is located north of the islands of Gran Canaria and Tenerife, and has a depth of 3600 m. The time series station was inaugurated in February 1994 and has continued its monthly operations through to the year 2004. Since 2004, the ESTOC site is been visited seasonally. This station is intended to be representative of the eastern boundary of the northeast Atlantic Ocean, in the Subtropical region. In the eastern region of the subtropical gyre, the water masses are modified by the upwelling processes. The ESTOC website can be found at: http://estoc.plocan.eu, accessed on 20 February 2015.

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4.2.2.2. Mooring: Fixed point Open Ocean Observatories

ESTOC and CVOO‐ (Cape Verde Ocean Observatory, formerly TENATSO Time series station) are both part of the EuroSITES network (http://www.eurosites.info/, accessed on 20 February 2015) which contributes to the global OceanSITES network and the FixO3 network (The Fixed point Open Ocean Observatory network), as a part of the European open ocean fixed point observatories.

4.2.2.3. Cruises

Data for the carbon dioxide variables at the Canary Island region from cruises carried out by single countries or by collaborative projects as a part of national, European or International programs can be found in the Carbon Dioxide Information Analysis Center (CDIAC) web page (http://cdiac.ornl.gov/, accessed on 21 February 2015) and in the websites of the European funded projects CARBOOCEAN (http://www.carboocean.org, accessed on 21 February 2015) and CARBOCHANGE (http://www.carbochange.b.uib.no, accessed on 20 February 2015).

In the Mauritanian upwelling region, between 20.5°N to 21.5°N and 17°W to 19°W, a cruise designed to track an upwelling patch was carried out between April‐May 2009. The CT and AT values were measured on board as part of the UK SOLAS (Surface Ocean Lower Atmosphere Study).

4.2.2.4. Methodology

Total Alkalinity

Samples for AT measurements were potentiometrically titrated with HCl to the carbonic acid end point until 2004 (Mintrop et al., 2000). Afterwards, a VINDTA 3C system (Mintrop et al., 2000), which determines both total potentiometric alkalinity and total dissolved inorganic carbon was used. The titration of different certified reference materials (CRM) has been used to test the performance of the titration system. The standard deviation was ± 2 µmol kg‐1.

Total Dissolved Inorganic Carbon

CT has been computed from experimental values of pHT (Santana‐Casiano and González‐Dávila, 6.5 this book) and AT using the carbonic acid dissociation constants by Mehrbach et al. (1973) as in Dickson and

Millero (1987). The CT (pHT-AT) values were corrected using certified CRMs with previously determined pHT ‐1 and AT, resulting in a CT(pHT‐AT) residual of ± 3 µmol kg (n = 90). After 2004, a VINDTA 3C system (Mintrop et al., 2000) with coulometer determination has been used for most of the research groups with a substantially increased precision of ± 1.0 µmol kg‐1 (www.MARIANDA.com, accessed 21 February 2015) in all cruises in the area.

The CT and total alkalinity, alongside temperature, salinity, pressure and macronutrient concentrations from all discrete samples, are used to calculate the remaining carbonate chemistry parameters, including saturation state  aragonite (A) and calcite (C). This was carried out using the CO2Sys programme with thermodynamic dissociation constants for K1 and K2 from Mehrbach et al. (1974) and re‐fit by Dickson and Millero (1987).

145 González‐Dávila, M. and Santana‐Casiano, J. M. Carbon dioxide system in the CCLME

4.2.3. RESULTS

4.2.3.1. ESTOC Site

The ESTOC site is about 1000 km west of the NW African coast and 100 km north of the Canary Islands. It is far away from the coastal upwelling zone but the effects of upwelling filaments reaching the area on CO2 system have been described previously (Pelegrí et al., 2005; Santana‐Casiano et al., 2007; González‐Dávila et al., 2007). The ESTOC site is located in the Sub‐tropical Gyre and exhibits oligotrophic characteristics (Neuer et al., 2007). A description of the water masses for the Canary region where ESTOC is located has been provided by Pérez et al. (2001), Llinás et al. (2002) where the focus was to describe the distribution of salinity anomalies, and Cianca et al. (2007) and Machín et al. (2006), presenting mass fluxes. The oxygen dynamics at ESTOC has been discussed by Cianca et al. (2013). The CT values normalized to a constant salinity of 35 in the surface waters at ESTOC has increased in the last 15 years by 0.9 ± 0.1 µmol kg‐1 yr‐1

(Figure 4.2.1) linked to the fCO2 increase in both the atmosphere and the ocean (Santana‐Casiano and

González‐Dávila, 6.5 this book). The NAT at the surface remains constant over the years with a value of 2292 ± 2 µmol kg‐1 (Figure 4.2.1), a similar effect than that observed in the full profile (Figure 4.2.2) (González‐Dávila et al., 2010). The stratification of water masses affects the profiles of the carbonate system parameters. Gonzalez‐Dávila et al. (2010) provided a description of profiles and trends for the area for each variable. The pHT at in situ conditions decreases with depth to a minimum near 900‐1000 m, partly overlapping with the oxygen minimum layer at 800‐900 m. After normalization to a constant temperature of 25°C, the pHT also decreases with depth from a value of around 7.95‐7.97 at 200 m to 7.71‐7.74 at

1000 m. The CT and NCT (the normalized CT to a constant salinity of 35) distribution increase with depth (Figure 4.2.3), reaching a maximum near 1000‐1100 m, with values in the range of 2090 µmol kg‐1 and 2010 ‐1 ‐1 ‐1 µmol kg for the CT and 2160 µmol kg and 2170 µmol kg for the NCT, reflecting the impact of organic matter remineralization. AT decreases with depth (Figure 4.2.2), but with a distribution strongly related to salinity over the first 600 m in the Eastern North Atlantic Central Water (ENACW) region.

Figure 4.2.1. Long‐term trends of surface (10 m) total inorganic carbon and total Alkalinity both of them normalized to a constant salinity of 35, NCT and NAT, respectively, at ESTOC. The slope of the regression line ‐1 ‐1 ‐1 for NCT was 0.9 ± 0.1 µmol kg yr , while NAT remains constant at 2292 ± 2 µmol kg .

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Over the past 15 years the distribution of the carbonate system parameters at the ESTOC station indicated changes in the chemical, biological and physical characteristics of the water column in this area (Figure 4.2.3). A total of 12 depth values were used to define the inter‐annual variability for the carbonate system parameters at ESTOC, 7 of which were located over the first 1000 m. In the mixed layer, the NCT content increased by 0.95 ± 0.6 μmol kg−1 yr−1, a similar rate than that indicated above for the surface seawater. At −1 −1 intermediate depths of 300 m, 600 m, and 1000 m, annual increase in the NCT of 0.69 μmol kg yr , 0.61 μmol kg−1 yr−1 and 0.48 μmol kg−1 yr−1, respectively, were defined (González‐Dávila et al., 2010). The

NAT (normalized AT to a constant salinity of 35) remains constant over the years at the different water depths. The addition of CO2 acidifies seawater and lowers its pH. The values for the pHT,25 over the first 100 meters decrease at a rate of around 0.0018. The values are statistically significant at 95% confidence level ‐1 over the first 1000 m, where the pHT,25 decreases at a rate of 0.0008 ± 0.0003 units yr .

These data were used to compute anthropogenic carbon, CANT, penetration and carbon storage in the

ESTOC area (González‐Dávila et al., 2010). The concentration of anthropogenic CO2 in the 150–200 m depth −1 was 57 ± 4 μmol kg , for the year 2000. The yearly average for CANT was found to increase at a rate of −1 −1 0.85 ± 0.6 μmol kg yr , which is in the same range as the observed increase in CT. The total column ‐2 inventory of anthropogenic CO2 was estimated to be 66 ± 3 mol m at the ESTOC site for the period 1995‐ 2004.

Figure 4.2.2. Hovmöller diagram of the water column AT evolution for the years 1995 to 2012 (Schlitzer, 2015).

147 González‐Dávila, M. and Santana‐Casiano, J. M. Carbon dioxide system in the CCLME

Figure 4.2.3. Hovmöller diagram of the water column NCT evolution for the years 1995 to 2012 (Schlitzer, 2015).

4.2.3.2. Mauritanian upwelling

The number of CO2 sutides carried out in the Mauritanean upwelling area is limited. Loucaides et al. (2012) measured CT and AT changes in an upwelling filament and detected that average CT concentrations in the mixed layer decreased during a Lagrangian study. They found the highest CT levels shortly after upwelling (2168 mol kg‐1) and lowest levels during the last days of the study (2131 mol kg‐1). Average mixed layer

AT concentrations increased during the study. The pCO2 of the upwelled water decreased by 100‐150 atm after 8 to 9 days. As expected, pHtot values were at their lowest initially and increased by 0.11 units 6 days after. Moreover, the saturation levels of calcite and aragonite increased gradually by 0.8 and 0.6, respectively. Throughout this process, it was observed that CT in the mixed layer was primarily controlled by biological incorporation into soft tissues and not significantly affected by calcification or dissolution of

CaCO3, or vertical exchanges. The observed increase in AT was caused by nitrate uptake and water mass mixing. In deep waters, the production of CT was tied to the consumption of oxygen through microbial respiration with a ratio of 0.51, similar to the ratio reported for remineralization in the shallower subsurface layers of the North Atlantic Ocean (Omta et al., 2011]).

The results of the studies on directions in upwelling across the NW African coastline based in satellite wind and sea surface temperature data (Barton et al., 2013; Cropper et al., 2014), that will affect the levels of the different carbon system parameters, are inconsistent with results for and against the upwelling intensification mechanism. Some results support the theory that changes towards upwelling intensification might due to natural inter‐annual variability (Cropper et al. 2014 and reference herein). Lachkar and Gruber (2013) have shown that model predicts doubling of the wind‐stress doubles net primary production (NPP) north of Cape Blanc (21°N), in the central and northern CUE, which will lead to an enhanced net biological uptake of the upwelled CO2 and a small change in the CO2 atmospheric‐ocean interchange. 148 IOC TECHNICAL SERIES, No. 115, pp. 143‐150. URI: http://hdl.handle.net/1834/9184. 2015

4.2.4. DISCUSSION

The carbonate system in the North East Atlantic Ocean is well represented by measurements conducted at the ESTOC site (Pérez et al., 2001; González‐Dávila et al., 2010). The vertical distribution of the carbonate system parameters is affected by the water mass structure. Moreover, the biological/biogeochemical processes related with the production/decomposition of organic matter, the formation/dissolution of calcium carbonates and the differences in the respective carbonate values when each water masses were formed control the final vertical distribution (González‐Dávila et al., 2010). In order to identify and quantify the mechanisms controlling the vertical variations of AT and the CT distribution, the study took into consideration the changes due to organic matter decomposition and changes due to the carbonate pump caused by carbonate dissolution. The region is characterized by dissolution of surface‐produced calcite and aragonite in the upper 1000 m. The dissolution of hard material predominates in deeper water as a result of increased pressure, decreased temperature and longer residence time. The remineralization of organic matter increased with depth. In the first 500 m, 51% of the increase in the CT can be attributed to remineralization of organic matter. Maximum contribution was obtained at around 900–1000 m, with a value of 80 ± 7 μmol kg−1. It has been determined that the potential contribution of the formation and/or dissolution of CaCO3 and the consumption and/or remineralization of nitrate remain constant with the observed NAT variability at ESTOC. When the water column CT was decomposed, the Eastern North Atlantic Ocean, at the area where the ESTOC station is located, showed an increasing storage capacity for excess ‐2 ‐1 CO2 of 0.85 mol m yr .

Though carbonate system parameter data are scarce for the Mauritanian upwelling region, it is known that the upwelling of waters along the NW African shelf can transport high levels of CO2 to the sea surface near shore (Copin‐Montégut and Avril, 1995; González Dávila et al., 2014). As these waters age and are advected offshore, CO2 levels decrease, falling well below atmospheric concentrations at the continental shelf break.

The initial upwelling leads to low saturation states of CaCO3 in surface waters (Loucaides et al., 2012;

Lachkar and Gruber, 2013). However, the uptake of carbon by phytoplankton leads to reduced CT and upwelling driven CO2 outgassing. This process, modelled for the northern part of the CUE (Lachkar and

Gruber, 2013) consequently raises the saturation states of CaCO3. In the Cape Blanc area, the model applied by Lachkar and Gruber (2013) showed the reduced biological efficiency should produce an enhancement of the CO2 outgassing. In both regions, the remineralization of sinking organic matter in sub‐ surface waters would generate CO2, which causes an increase of CT and lowers the saturation states of

CaCO3.

4.2.5. CONCLUSIONS AND RECOMMENDATIONS

The vertical distribution of the carbon dioxide variables in the Canary Islands region along the last fifteen years have clearly indicated significant changes over, at least, the first 1000 m affecting the inorganic ‐1 carbon content and the acidity of the seawater. In the surface, the NCT increased at a rate of 0.9 mol kg , the pHT decreased each year on average 0.0019 units, while the NAT keeps constant at a value of ‐1 2292 mol kg . This increase in CT is controlling the total column inventory of anthropogenic CO2 that has reached a value of 66 ± 3 mol m‐2 for the reference year 2000. Considering the importance of EBUE in the coastal ocean from a socio‐economic, oceanographic and climatological point of view, few studies have been undertaken about the role of this region in inorganic carbon cycling. It has been showed that upwelled waters in the Mauritanian upwelling area provided high contents of inorganic carbon which lead to low calcium carbonate saturation states. The uptake of carbon by phytoplankton acts to decrease CT and 149 González‐Dávila, M. and Santana‐Casiano, J. M. Carbon dioxide system in the CCLME

consequently increase saturation states. Our knowledge about whether coastal upwelling will intensify, or whether upper‐ocean warming and the increased stratification will prevail and how these processes will affect the nutrient inputs, carbon exported out of the surface ocean, iron supply, acidification, hypoxia, rainfall patterns and anthropogenic source terms are lacking. Model results indicate the importance of physics and environmental conditions in shaping the sensitivity of CCLME to potential climate change induced upwelling‐favorable wind intensification. However, experimental data should be gathered in order to sustain model results.

Acknowledgements

This work has been supported by the European Project CARBOCHANGE 2011‐2015 (Changes in carbon uptake and emissions by oceans in a changing climate), GA no: 264879.

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