Carbon cycling and acidification
The issues related to organic particulate matter are discussed in the previous paragraph but dissolved organic carbon is also an important pool in the carbon cycling. The available historical data set does not allow to draw conclusions on the impact of anthropogenic perturbation on the cycling of DOC. However, it is obvious that the magnitude of the sources, the nature and the mode of supply to the surface ocean, the mode of transfer from coastal areas to offshore or from surface to deep waters are not enough constrained.
Does the seawater Cant level increase with time in parallel with that in the atmosphere?
Increasing atmospheric concentration of CO2 results in increased seawater CO2 concentrations and consequently modified the carbonate equilibrium decreasing the pH. In a general context calcifying taxa are largely exposed to the acidification although the real effect of high level CO2 seawater is still debated. In the case of the Mediterranean Sea, in addition to the higher carbon input from the riverine waters, the Mediterranean Sea takes up a large part of the anthropogenic CO2 (Cant) injected into the atmosphere through air-sea interactions. Unfortunately, today, data of the carbonate system properties (total dissolved inorganic carbon, CT; total alkalinity, AT; pH; and CO2 partial pressure, pCO2) in the Mediterranean Sea are very scarce compared with other ocean areas.
As indicated by the levels of CFC-12 (Schlitzer et al., 1991), the ventilation of the deep Mediterranean waters is fast (less than 50 years). Consequently, it is not surprising that the recent estimates indicate that the deep layers are already contaminated by Cant (Aït-Ameur and Goyet, 2005; Touratier and Goyet, 2008).
Today, the scientific community is still unable to answer this question mainly because of the lack of data of the carbonate system properties (CT, AT, pH, pCO2). In particular, time-series measurements are necessary to determine and then forecast any temporal evolution of any ocean property. In the Mediterranean Sea there was the time-series station DYFAMED (DYnamique des Flux Atmosphériques en MEDiterranée; http://www.obs-vlfr.fr/sodyf/home.htm) located in the central part of the Ligurian Sea (43°25 N, 7°52 E). Although, there were relatively few and discontinued measurements of the carbonate system properties (CT, AT, pH, pCO2) at this station, it was possible via interpolations and use of the simple TrOCA approach (Touratier and Goyet, 2004a, 2004b, Touratier et al., 2007) to estimate the distribution of Cant over more than a decade (1993-2005) at the DYFAMED site (Touratier and Goyet, 2008).
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Variabilité et tendances de AT et CT ainsi que les estimations de pCO2 et pH à 2000m dans le bassin Méditerranéen nord-occidental de 1998 à 2016 (Coppola et al., in prep.)
Surprisingly, the results indicated that in the intermediate and the deep waters, the concentrations of Cant decreased with time. The Cant and O2 signatures suggest that an invasion of one or several older water mass did occur during this decadal times-series. The older water masses may have originated either from the eastern basin as a consequence of the transients LT (Levantine Transient; Theocharis et al., 2002) and EMT (Eastern Mediterranean Transient event; Roether et al., 1998), or from the western basin via the transient WMT (Western Mediterranean Transient; Schroeder et al., 2008).
Nevertheless, the concentrations of Cant in the Mediterranean Sea are high, much higher than in the open oceans. Thus the Mediterranean Sea is a source of Cant for the Atlantic Ocean via the Gibraltar Detroit. A direct impact of this high accumulation of Cant in the Mediterranean Sea is a decrease of its pH by 0.15 since the pre-industrial era. However, in the case of the Mediterranean Sea, the actual high degree of supersaturation of carbonate indicates that the dissolution should not be a matter of concern in the coming years. But other effects of high CO2 waters are possible. For example at the phytoplankton level, the increase in CO2 could change the stoechiometry of C:N (Riebesell et al. 2007) or impact N2 fixation. It is also predicted that the whole ecosystem could be impacted.