Benthic remineralization and nutrient recycling in central and southern Adriatic and Ionian seas

Assessing processes affecting the carbon cycle and solute exchanges between sediments and overlying water at sediment-water interface in Adriatic and Ionian seas were the aims of The CaDi Cruise (PITAGEM Project). In particular, the objectives were to study the mechanisms responsible of the recycle of carbon and nutrients at the sediment-water interface and to understand the role of sediments to the nutrient mass balance in Central and Southern Adriatic and Ionian seas. The study has been carried out in 7 stations located in environments characterized by different bathymetry, hydrological setting and trophic conditions. Northern stations, located in central and southern Adriatic depressions, are characterized by shallower depths, higher sedimentation rates and higher organic matter inputs. Southern stations, located in the Northern and Central Ionian Sea, are characterized by increasing depths, different provenances of terrigenous sediments, lower productivity, lower sedimentation rates and organic matter inputs. In each stations sediment cores were collected to analyze pore waters (pH, Eh, O2, NO3, NO2, NH4, PO4, alkalinity, DIC, Si(OH)4, Ca, Mg, Fe and Mn) and solid phase (grain size, organic C) and to calculate diffusive benthic fluxes. Pore water profiles highlighted regional differences between the Adriatic and Ionian basins. The first one is characterized by oxyc and suboxic-non sulphidic organic matter degradation. In this area the most intensive degradation processes occur in the Otranto Channel station where oxyc and sub-oxyc reactions are stronger and confined at lesser depths respect to the Mid-Adriatic and Southern Adriatic depressions. This suggests higher inputs of reactive organic matter along the western side of the Otranto Channel respect to the mid and Southern Adriatic depressions, where already degraded organic matter arrive at the sediment. In the Ionian basin the remineralization processes takes place mainly by means of oxyc reactions (the depth of oxygen penetration is higher, the denitrification processes reach greater depths and produce lower organic matter degradation products (DIC and NH4)). This means that the inputs of reactive organic matter in these area are lower for the lower productivity of this basin, for the greater water column depth and for the higher distance from fluvial inputs. In both basins nitrification processes, stronger in the Ionian basin, take place in the uppermost layers. Benthic diffusive fluxes reflect the early diagenesis processes recorded in the Adriatic and Ionian pore water profiles showing a northern-southern and shallow-deep trend characterized by lowering of remineralization processes, this generate weaker oxygen fluxes into the sediments and DIC fluxes outside the sediments. Ammonium and nitrate fluxes are complicated by the nitrification/denitrification processes occurring in the oxyc zone.