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Trends of pH decrease in the Mediterranean Sea through high frequency observational data: indication of ocean acidification in the basin.

Flecha S, Pérez FF, García-Lafuente J, Sammartino S, Ríos AF, Huertas IE - Sci Rep (2015)

Bottom Line: Assessing the impact of OA on marine ecosystems requires the accurate detection of the rate of seawater pH change.This work reports the results of nearly 3 years of continuous pH measurements in the Mediterranean Sea at the Strait of Gibraltar GIFT time series station.Both water masses also exhibited a decline in pH with time, particularly the WMDW, which can be related to their different biogeochemical nature and processes occurring during transit time from formation sites to the Strait of Gibraltar.

View Article: PubMed Central - PubMed

Affiliation: Instituto de Ciencias Marinas de Andalucía, (CSIC), Polígono Río San Pedro, s/n, 11519, Puerto Real, Cádiz, Spain.

ABSTRACT
A significant fraction of anthropogenic carbon dioxide (CO2) released to the atmosphere is absorbed by the oceans, leading to a range of chemical changes and causing ocean acidification (OA). Assessing the impact of OA on marine ecosystems requires the accurate detection of the rate of seawater pH change. This work reports the results of nearly 3 years of continuous pH measurements in the Mediterranean Sea at the Strait of Gibraltar GIFT time series station. We document a remarkable decreasing annual trend of -0.0044 ± 0.00006 in the Mediterranean pH, which can be interpreted as an indicator of acidification in the basin based on high frequency records. Modeling pH data of the Mediterranean outflow allowed to discriminate between the pH values of its two main constituent water masses, the Levantine Intermediate Water (LIW) and the Western Mediterranean Deep Water (WMDW). Both water masses also exhibited a decline in pH with time, particularly the WMDW, which can be related to their different biogeochemical nature and processes occurring during transit time from formation sites to the Strait of Gibraltar.

No MeSH data available.


Related in: MedlinePlus

Linear fitting of pH with time (see SItext) of the MOW and its forming water masses during the monitoring period: (a) MOW, (b) LIW and (c) WMDW. Blue and red lines represent the 95% confidence and prediction bands, respectively. Equations are shown in the SI text. Note the different scales for “y” axes in figures (a–c).
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f4: Linear fitting of pH with time (see SItext) of the MOW and its forming water masses during the monitoring period: (a) MOW, (b) LIW and (c) WMDW. Blue and red lines represent the 95% confidence and prediction bands, respectively. Equations are shown in the SI text. Note the different scales for “y” axes in figures (a–c).

Mentions: As each water mass is characterized by a distinctive salinity and potential temperature, pH can also act as a tracer to define water masses. The OMP and MLR analysis allowed determining that LIW and WDMW in the SG were characterized by average pHT25 and standard error values of 7.8897 ± 0.0003 and 7.9077 ± 0.0004, respectively (see Fig. 4). Those values faithfully correspond to recently reported measures18 equal to ~7.89 for the LIW in the SG and 7.9–7.91 for the WMDW in the Western Mediterranean basin. Differences in pH values betwen both water masses can be explained on the basis of the transit times from their respective formation sites to the SG; the LIW takes around 8 years to complete the distance from the Levantine basin (LB, Fig. S1) to the Strait of Sicile (SS, Fig. S1)32. An active remineralization of organic matter can take place during such period, which implies the rise of dissolved inorganic carbon concentration, the decrease of dissolved oxygen and the consequent pH decrease1618. In contrast, the WMDW takes roughly 1.8 years to travel from the Gulf of Lions to the Alboran Sea (GL and AS respectively in Fig. S1), a much shorter transit time33. Although the ventilation time of LIW and WMDW is an issue that has not been solved yet and there are still large discrepancies regarding the age of each water mass34353637, estimated ages are around 80–120 ± 20 yr and 20–40 ± 40 yr for the bottom waters of the eastern and western basins, respectively, when they arrive at the SG35. Therefore, considering the circulation pattern in the MS38 and regardless of a particular transit time or age for each water mass, when they both arrive at the SG the WMDW is much younger than the LIW formed in the distant Levantine Basin (LB, Fig. S1). Accordingly, the LIW is more stable than the WMDW from a biogechemical point of view, as the former lost contact with the atmosphere a longer ago. Therefore, in the SG, the WMDW,which has been exposed to the atmosphere more recently, exhibits higher pH values than the eastern-originated LIW (Fig. 4). This is confirmed when the monthly variability of pH in both water masses is analysed (see Fig. S2). Although a little seasonality can be detected, over the summer months, the two water masses showed stable pH values whereas winter conditions resulted in the highest pH variability, which was especially remarkable in February. During this month, pH in the WMDW presented noticeable oscillations, ranging from 7.7789 to 8.0003 whereas the pH of the LIW varied slightly, with values changing from 7.8195 to 7.9634. This seasonal variability could be again attributed to the ventilation pattern of the WMDW. A previous work25 demostrated that a lag of few weeks can be found between the events of WMDW fomation in winter in the Gulf of Lions and the detection of cold pulses of old WMDW at the monitoring site in Espartel Sill. Renovation of the old resident WMDW in the SG depends on the volume of water formed during winter by deep convection in the Gulf of Lions and the intensification of the WAG that subsequently uplifts the ancient WMDW at the SG. During our study period, the appearance of colder (and older) WDMW pulses in February 2013, 2014 and 2015 (Figs 1a and 2b) possibly resulted in the lowest pH values detected in the WMDW during the months of February (see Fig. S2b), as the older WMDW residing in the eastern side of the Strait will be characterized by lower pH values, due to the active remineralization processes occurring in the Alboran basin39.


Trends of pH decrease in the Mediterranean Sea through high frequency observational data: indication of ocean acidification in the basin.

Flecha S, Pérez FF, García-Lafuente J, Sammartino S, Ríos AF, Huertas IE - Sci Rep (2015)

Linear fitting of pH with time (see SItext) of the MOW and its forming water masses during the monitoring period: (a) MOW, (b) LIW and (c) WMDW. Blue and red lines represent the 95% confidence and prediction bands, respectively. Equations are shown in the SI text. Note the different scales for “y” axes in figures (a–c).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4660394&req=5

f4: Linear fitting of pH with time (see SItext) of the MOW and its forming water masses during the monitoring period: (a) MOW, (b) LIW and (c) WMDW. Blue and red lines represent the 95% confidence and prediction bands, respectively. Equations are shown in the SI text. Note the different scales for “y” axes in figures (a–c).
Mentions: As each water mass is characterized by a distinctive salinity and potential temperature, pH can also act as a tracer to define water masses. The OMP and MLR analysis allowed determining that LIW and WDMW in the SG were characterized by average pHT25 and standard error values of 7.8897 ± 0.0003 and 7.9077 ± 0.0004, respectively (see Fig. 4). Those values faithfully correspond to recently reported measures18 equal to ~7.89 for the LIW in the SG and 7.9–7.91 for the WMDW in the Western Mediterranean basin. Differences in pH values betwen both water masses can be explained on the basis of the transit times from their respective formation sites to the SG; the LIW takes around 8 years to complete the distance from the Levantine basin (LB, Fig. S1) to the Strait of Sicile (SS, Fig. S1)32. An active remineralization of organic matter can take place during such period, which implies the rise of dissolved inorganic carbon concentration, the decrease of dissolved oxygen and the consequent pH decrease1618. In contrast, the WMDW takes roughly 1.8 years to travel from the Gulf of Lions to the Alboran Sea (GL and AS respectively in Fig. S1), a much shorter transit time33. Although the ventilation time of LIW and WMDW is an issue that has not been solved yet and there are still large discrepancies regarding the age of each water mass34353637, estimated ages are around 80–120 ± 20 yr and 20–40 ± 40 yr for the bottom waters of the eastern and western basins, respectively, when they arrive at the SG35. Therefore, considering the circulation pattern in the MS38 and regardless of a particular transit time or age for each water mass, when they both arrive at the SG the WMDW is much younger than the LIW formed in the distant Levantine Basin (LB, Fig. S1). Accordingly, the LIW is more stable than the WMDW from a biogechemical point of view, as the former lost contact with the atmosphere a longer ago. Therefore, in the SG, the WMDW,which has been exposed to the atmosphere more recently, exhibits higher pH values than the eastern-originated LIW (Fig. 4). This is confirmed when the monthly variability of pH in both water masses is analysed (see Fig. S2). Although a little seasonality can be detected, over the summer months, the two water masses showed stable pH values whereas winter conditions resulted in the highest pH variability, which was especially remarkable in February. During this month, pH in the WMDW presented noticeable oscillations, ranging from 7.7789 to 8.0003 whereas the pH of the LIW varied slightly, with values changing from 7.8195 to 7.9634. This seasonal variability could be again attributed to the ventilation pattern of the WMDW. A previous work25 demostrated that a lag of few weeks can be found between the events of WMDW fomation in winter in the Gulf of Lions and the detection of cold pulses of old WMDW at the monitoring site in Espartel Sill. Renovation of the old resident WMDW in the SG depends on the volume of water formed during winter by deep convection in the Gulf of Lions and the intensification of the WAG that subsequently uplifts the ancient WMDW at the SG. During our study period, the appearance of colder (and older) WDMW pulses in February 2013, 2014 and 2015 (Figs 1a and 2b) possibly resulted in the lowest pH values detected in the WMDW during the months of February (see Fig. S2b), as the older WMDW residing in the eastern side of the Strait will be characterized by lower pH values, due to the active remineralization processes occurring in the Alboran basin39.

Bottom Line: Assessing the impact of OA on marine ecosystems requires the accurate detection of the rate of seawater pH change.This work reports the results of nearly 3 years of continuous pH measurements in the Mediterranean Sea at the Strait of Gibraltar GIFT time series station.Both water masses also exhibited a decline in pH with time, particularly the WMDW, which can be related to their different biogeochemical nature and processes occurring during transit time from formation sites to the Strait of Gibraltar.

View Article: PubMed Central - PubMed

Affiliation: Instituto de Ciencias Marinas de Andalucía, (CSIC), Polígono Río San Pedro, s/n, 11519, Puerto Real, Cádiz, Spain.

ABSTRACT
A significant fraction of anthropogenic carbon dioxide (CO2) released to the atmosphere is absorbed by the oceans, leading to a range of chemical changes and causing ocean acidification (OA). Assessing the impact of OA on marine ecosystems requires the accurate detection of the rate of seawater pH change. This work reports the results of nearly 3 years of continuous pH measurements in the Mediterranean Sea at the Strait of Gibraltar GIFT time series station. We document a remarkable decreasing annual trend of -0.0044 ± 0.00006 in the Mediterranean pH, which can be interpreted as an indicator of acidification in the basin based on high frequency records. Modeling pH data of the Mediterranean outflow allowed to discriminate between the pH values of its two main constituent water masses, the Levantine Intermediate Water (LIW) and the Western Mediterranean Deep Water (WMDW). Both water masses also exhibited a decline in pH with time, particularly the WMDW, which can be related to their different biogeochemical nature and processes occurring during transit time from formation sites to the Strait of Gibraltar.

No MeSH data available.


Related in: MedlinePlus