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.
Highlights
CO2 emissions from fossil fuels burning and land use change since the industrial revolution have caused a considerable increase in atmospheric CO2 concentrations[1]
Distinct ocean acidification (OA) rates are provided depending on the degree of anthropogenic carbon accumulation by a particular sub-basin, with regional pH decreases oscillating between − 0.055 to − 0.156 pH units with respect to the preindustrial levels[12]
Our work provides the first rates of pH decrease in the Mediterranean Outflow Water (MOW) and in its forming water masses separately, which can be considered indicators of OA in the basin, confirming previous evidence[12,13,15]
Summary
The regression statistics were significant and resulted in a Δ pH of − 0.0044 units per year in the MOW This rate of pH decline is of the same order of magnitude than acidification rates reported by sustained observations of different carbon system parameters collected in several oceanic seasurface time series[21], it is two or three fold higher than those (depending on the site). It still falls within the range of pH change in Mediterranean deep waters estimated recently through a modelling approach (− 0.005 to − 0.06 pH units[13]). The data presented here may well serve as a base line to assess sensitiviy and evolution of the Mediterranean waters to the impact of increasing CO2 emmissions
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