Ocean acidification processes in coastal and offshore ecosystems

Since the beginning of Industrial Revolution a massive amount of atmospheric carbon dioxide, produced by human activity, has been absorbed by the World's Oceans. This process has led to an acidification of marine waters on a global scale and is one of the most serious threats facing marine ecosystems in this century. The negative impacts of ocean acidification could be much more relevant in coastal ecosystems, where marine life is concentrated and biogeochemical processes are more active. However, future projections of pH reduction in these areas are difficult to estimate because result from multiple physical and biological drivers, including watershed weathering, river-born nutrient inputs, and changes in ecosystem structure and metabolism. In order to assess the sensibility of the Gulf of Trieste to the ocean acidification, high quality determination of the marine carbonate system (pHT, total alkalinity, dissolved inorganic carbon-DIC, buffer capacity) and other related biogeochemical parameters were carried out along a transect from the Isonzo River mouth to the centre of the gulf and at the coastal Long Term Ecological Research station C1. At the same time the biological influence of organic matter production and decomposition on the marine CO2 system was estimated using 14C primary production and heterotrophic prokaryote production (by 3H-leucine incorporation). The two years long measurements revealed a complex dynamic of the marine carbonate system, due to the combined effects of local freshwater inputs, biological processes, and air-sea CO2 exchange. However, it was possible to estimate the influence of the different drivers on a seasonal time scale. In winter the very low seawater temperature (minima = 2.88 °C) and strong Bora events determined a marked dissolution of atmospheric CO2 and elevated DIC concentration. During warm seasons the DIC concentration gradually decreased in the surface layer, due to biological drawdown (primary production) and thermodynamic equilibria (CO2 degassing), whereas under the pycnocline the respiration and remineralisation of organic matter prevailed, causing a temporary acidification of bottom waters. The winter seawater invasion of atmospheric CO2 was balanced by high riverine AT input (maxima ? 2933 µmol kg-1), derived mainly from chemical weathering of carbonate rocks of the surrounding karstic area, which increased the buffer capacity of this system and probably could mitigate the effect of ocean acidification. The marine carbonate system was also analysed in the Middle and Southern Adriatic Sea, in order to estimate the concentration of anthropogenic carbon dioxide currently present in this area. The results suggested that the entire water column was contaminated by a large amount of anthropogenic CO2 and very high concentration was detected near the bottom, in correspondence of the North Adriatic Dense Waters. This finding supported the hypothesis that during dense water formation events the very low seawater temperature can favour the physical dissolution of atmospheric carbon dioxide, and also revealed the active role of the North Adriatic Sea in sequestering and storing anthropogenic CO2 into the deep layers of Mediterranean Sea.