Decadal Study: What are the Factors Influencing the Carbon Cycle in the Western European Coastal Ecosystems?

The constraint of air–sea CO₂ fluxes and their variability at various time and spatial levels remain a central task in global carbon and climate studies. In the past decade time-series measurements allowed to estimate the dynamics of air-sea CO₂ fluxes in costal ecosystems from daily to inter-annual scales. Further, these measurements provided essential data to monitor the long-term modification of coastal ecosystems under ocean acidification (OA).

The French Coastal Monitoring Network SOMLIT performs time-series in 12 coastal ecosystems off the French coast since the late 90’s. Since 2008, we sampled Dissolved Inorganic Carbon (DIC) and Total Alkalinity (TA) in addition to 12 other biogeochemical parameters at 3 coastal sites in the Western English Channel: Roscoff-Pier, Roscoff-Offshore, and Bay of Brest.

Monthly anomaly trends applied to our data showed an increase of temperature and pCO₂, and a strong acidification at the 3 sites. These trends were significantly different from one site to another, with a notable acidification in the Bay of Brest of -0.01 pH unit yr^-1 and an important increase of pCO₂ at 4 µatm yr^-1.

We established trends for the physical (pCO_2term) and biological (pCO_2non-term) processes influencing the pCO₂ signal. In the nearshore ecosystem (Roscoff-Pier), the pCO₂ trend was strongly influenced (up to 80%) by the biological signal whereas at the Roscoff-Offshore site the pCO_2term and pCO_2non-term balanced each other. With a physical variability stable in the study sites, these results showed the importance of the biological signal on the different pCO₂ decadal trends.

The separation of the anthropogenic and natural contributions by the calculation of anthropogenic carbon (C_ant) on the DIC monthly trend anomalies revealed an influence around 75% of C_ant on the DIC variations, constant in every site. The natural contribution was explained by a natural climatic factor: the NAO, which promoted an increase of CO₂ in our area.