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Volume 13, issue 3 | Copyright
Ocean Sci., 13, 411-426, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 15 May 2017

Research article | 15 May 2017

Coastal ocean acidification and increasing total alkalinity in the northwestern Mediterranean Sea

Lydia Kapsenberg1, Samir Alliouane1, Frédéric Gazeau1, Laure Mousseau1, and Jean-Pierre Gattuso1,2 Lydia Kapsenberg et al.
  • 1Sorbonne Universités, Université Pierre et Marie Curie-Paris 6, CNRS-INSU, Laboratoire d'Océanographie de Villefranche, 06230 Villefranche-sur-Mer, France
  • 2Institute for Sustainable Development and International Relations, Sciences Po, 27 rue Saint Guillaume, 75007 Paris, France

Abstract. Coastal time series of ocean carbonate chemistry are critical for understanding how global anthropogenic change manifests in near-shore ecosystems. Yet, they are few and have low temporal resolution. At the time series station Point B in the northwestern Mediterranean Sea, seawater was sampled weekly from 2007 through 2015, at 1 and 50m, and analyzed for total dissolved inorganic carbon (CT) and total alkalinity (AT). Parameters of the carbonate system such as pH (pHT, total hydrogen ion scale) were calculated and a deconvolution analysis was performed to identify drivers of change. The rate of surface ocean acidification was −0.0028±0.0003unitspHTyr−1. This rate is larger than previously identified open-ocean trends due to rapid warming that occurred over the study period (0.072±0.022°Cyr−1). The total pHT change over the study period was of similar magnitude as the diel pHT variability at this site. The acidification trend can be attributed to atmospheric carbon dioxide (CO2) forcing (59%, 2.08±0.01ppmCO2yr−1) and warming (41%). Similar trends were observed at 50m but rates were generally slower. At 1m depth, the increase in atmospheric CO2 accounted for approximately 40% of the observed increase in CT (2.97±0.20µmolkg−1yr−1). The remaining increase in CT may have been driven by the same unidentified process that caused an increase in AT (2.08±0.19µmolkg−1yr−1). Based on the analysis of monthly trends, synchronous increases in CT and AT were fastest in the spring–summer transition. The driving process of the interannual increase in AT has a seasonal and shallow component, which may indicate riverine or groundwater influence. This study exemplifies the importance of understanding changes in coastal carbonate chemistry through the lens of biogeochemical cycling at the land–sea interface. This is the first coastal acidification time series providing multiyear data at high temporal resolution. The data confirm rapid warming in the Mediterranean Sea and demonstrate coastal acidification with a synchronous increase in total alkalinity.

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Short summary
In the interest of global ocean change, weekly water samples were collected at a coastal site in the northwestern Mediterranean Sea (2007–2015). Seawater pH declined faster than expected from anthropogenic carbon dioxide increase. Total alkalinity increased, but the driver could not be identified, and it may be linked to changes in freshwater chemistry of watersheds. This is the first coastal acidification time-series providing multiyear data at high temporal resolution.
In the interest of global ocean change, weekly water samples were collected at a coastal site in...