Ocean Acidification studies in the Baia di Levante (Vulcano island, Italy). Advantages and disadvantages of the “in situ” approach.

Years of scientific research have shown that acidification of oceans (OA) is an undisputed fact. Why is it so important to increase knowledge about OA? Because many animals and plants in the ocean have calcium carbonate skeletons or shells, and a decreasing in pH can affect their population health state and the marine-ecosystem structure. Another point of view about OA which must to be considered is that it may alter the behavior of sediment-bound metals, modifying their bioavailability and thus toxicity. The toxic free-ion concentration of metals such as copper may increase by as much as 115% in coastal waters in the next 100 years due to reduced pH. Since increasing atmospheric CO2 over the next 200 years will cause a pH decrease in ocean water, and consequently change the organic and inorganic speciation of metals in surface ocean waters, and it will effect on their interaction with marine species. Most of the intense submersed hydrothermal seeps are located near the isthmus of the Baia di Levante along the beach (38°25’01.44”N, 14°57’36.29”E), where dispersed underwater leaks cover a 130 × 35m shallow water area (<1m depth). Here we aim at studying the geochemical characteristics of seawater in the Baia di Levante area. We studied the most prominent geochemical parameters across the whole bay and focused on the spatio-temporal variability of pH/CO2 along a stretch of coast in the northeastern part of the bay in the Vulcanello area. Eh and pH were greatly affected by the main vents. Eh values range from -152 to 170 mV in April and from -23 to 171 mV in September 2011 and from -39.7 to 181.3 in May 2012, while pH values range between 5.70 and 8.05 in April and from 6.05 to 8.03 in September 2011 and from 5.85 to 8.03 in May 2012. These is the reason why a pH and Eh gradient (from bubbles to offshore) affect seawater chemistry and biota in the Bay. About 3.6 tonnes of CO2 bubble into Baia di Levante per day which strongly influences the seawater chemistry of the area. The pH displayed a clear gradient from 5.65 at the main gas vents increasing to 8.1 and may represent suitable sites for ocean acidification studies. The ternary diagram of CH4-(N2+O2)-CO2 confirms the atmospheric contribution to dissolved gases and shows variable ratios between CH4 and CO2. Dissolved gases samples were analysed with gas chromatography. Calcite and aragonite saturation in the bay is achieved only in the northern part where pH values exceed respectively 7.5 and 7.6. According to projections, 7.8 is the predicted average global sea surface pH value for the year 2100, and it is considered an ecological tipping point at which most subtidal calcifiers disappear in the Mediterranean. Vulcano’s seawater composition in terms of the major elements is close to that of Mediterranean surface waters even if salinity is few higher than oceans and greater variability is recorded for dissolved Fe concentrations and trace metals distribution along the bay. Major elements were analyzed with IC (Ionic Chromatography). Calculated enrichment factors (EF) for trace metals in general show that in general we can considered V as not so enriched, Ni, Al, Cu, Fe and Zn as medium enriched and Mn as very enriched, furthermore, EF show that, except for Cu, trace metals are enriched in Baia di Levante probably because of the hydrothermal input. Average seawater concentration for each metals are; 5.1 μg/l for Al, 0.8 μg/l for V, 11.4 μg/l for Mn, 3.7 μg/l for Fe, 0.3 μg/l for Ni, 0.5 μg/l for Cu, 2.3 μg/l for Zn and 5.2-3 μg/l for La. Seawater samples were pre-concentrated with chelex-100 resin and analyzed with ICP-MS. A transplant mussel experiment in acidic condition (natural seawater respectivly) was conducted and, after one month exposure mussel (Mytilus gallorpovincialis) had accumulated Fe, Zn, Cu and V. Soft body mussel metals concentration are 0.7 μg/g for Ni, 2.7 μg/g for V, 4.5 μg/g for Cu, 7.2 μg/g for Mn, 23 μg/g for Al, 78.6 μg/g for Zn and 336.4 μg/g for Fe. Mussels are confirmed to be good bioaccumulator of heavy metals, although, their study in situ in the context larger of OA studies is complicated despite the fact have a lot of vantages. Geochemical approach is fundamental in this field, and biota accumulation should always be matched with geochemical survey in their habitat in order to better understand the bio-accumulation dynamics.