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

14 páginas, 4 figuras.-- This work is licensed under a Creative Commons Attribution 4.0 International License.-- Proyecto CarbochangeA 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 GibraltarFunding for this work was provided by the European Commission through the projects CARBOCHANGE (FP7- 264879), and PERSEUS (FP7-287600) and by the Spanish Ministry of Economy and Competitiveness (CTM2010-21229). SF was supported by a JAE PREDOCTORAL scholarship, part-funded by the European Commission (European Social Fund, ESF2007-2013) and the Spanish Ministry for Economy and Competitiveness.Peer reviewe