IMIS

At all sampling stations and during all campaigns, sediment samples were collected by scuba divers with cut-off plastic syringes (2.6 cm i.d.). At each station and each sampling period, four sediment cores were sampled from which one was dedicated to the grain-size analysis and the remaining three to the microphytobenthic abundance (ABU) and community composition. Samplings were carried out during four sampling campaigns in the following periods: June and October 2012, May 2013 and May 2014. A total of five stations were sampled: three near Basiluzzo Island, characterised by different sediment colour and gas emission, and two NE of Panarea Island on a sampling site referred as “Hot-Cold” where the sediments are subjected to very different temperatures at a distance of approximately 1 m one from another. For the quantitative determination of microphytobenthos (MPB), the uppermost sediment layer obtained from three distinct sediment cores was fixed with 10 mL of 4% formaldehyde buffered solution (CaMg(CO3)2) in pre-filtered seawater (0.2 μm filter) and stored at 4 °C until processing. For each sampling, three replicates were counted for the Basiluzzo samples and two for the Hot-Cold stations as for the latter the standard deviation did not exceed 15%. After manual stirring, 125 μL aliquots of the sediment suspension were drawn off and placed in a counting chamber. To obtain an estimate of the total viable cells, only cells containing pigments and not empty frustules were counted under a Leitz inverted light microscope (Leica Microsystems AG) using a 32× objective (320× final magnification) (Utermöhl, 1958). Microphytobenthos community was divided into major taxonomic groups and within the group of diatoms more precise species identification was attempted. Diatoms frustule cleaning was performed by oxidizing subsamples with two time volume of hydrogen peroxide (35%) and ≈1 mL of sulphuric acid (95–97%) (CEN, 2003). Between single treatments, the subsamples were heated over a Bunsen burner until boiling and left under the hood for 24 h. After that, they were washed with Milli-Q water and left to settle for at least 24 h between single washings, until reaching a pH close to 7. All samples were then mounted on permanent slides in Naphrax™ (Northern Biological Supplies Ltd. Ipswich, UK) and observed under 1000× final magnification using a light microscope (LM) (Leica DM2500, Germany) equipped with a digital camera (Leica DFC490, Germany). Detailed information on: https://hdl.handle.net/10.1016/j.marenvres.2016.04.009