Drivers of the carbonate system seasonal variations in a Mediterranean gulf

The effects of different physical and biogeochemical drivers on the carbonate system were investigated in a semi-enclosed coastal area characterized by high alkalinity riverine discharge (Gulf of Trieste, Northern Adriatic Sea, Mediterranean Sea). Our 2-year time-series showed that a large part of the seasonal carbonate chemistry variation was controlled by the large seasonal change of seawater temperature, though air-sea CO2 exchange, biological activity (primary production-respiration), and riverine inputs also exerted a significant influence.

With the exception of summer, the Gulf of Trieste was a sink of atmospheric carbon dioxide, showing a very strong CO2 fluxes from atmosphere into the sea (−16.10 mmol m−2 day−1) during high wind speed event of north easterly Bora wind. The CO2 influx was particularly evident in winter, when the biological activity was at minimum and the low seawater temperature enhanced CO2 solubility. During spring, the drawdown of CO2 by primary production overwhelmed the CO2 physical pump, driving a significant decrease of dissolved inorganic carbon (DIC), [CO2], and increase of pHT25 °C. In summer the primary production in surface waters occurred with the same intensity as respiration in the bottom layer, so the net biological effect on the carbonate system was very low and the further reduction of seawater CO2 concentration observed was mainly due to carbon dioxide degassing induced by high seawater temperature. Finally, during autumn the respiration was the predominant process, which determined an overall increase of DIC, [CO2], and decrease of pHT25 °C. This was particularly evident when the breakdown of summer stratification occurred and a large amount of CO2, generated by respiration and segregated below the pycnocline, was released back to the whole water column. Local rivers also significantly affected the carbonate system by direct input of total alkalinity (AT) coming from the chemical weathering of carbonate rocks, which dominate the river watershed. Our finding clearly demonstrates a high AT concentration in low salinity surface waters (AT max = 2742.8 μmol kg−1) and a negative AT-salinity correlation. As a result the Gulf of Trieste revealed a low Revelle factor (10.1) and one of the highest buffer capacities of the Mediterranean Sea (ßDIC = 0.31 mmol kg−1), which allows the system to store a significant amount of atmospheric CO2 with a small decrease of seawater pH.

Ingrosso G., Giani M., Comici C., Kralj M., Piacentino S., De Vittor C. & Del Negro P., 2016. Drivers of the carbonate system seasonal variations in a Mediterranean gulf. Estuarine, Coastal and Shelf Science 168: 58–70. Article (subscription required).

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