Variability and downward fluxes of particulate matter in the Otranto Strait area

The space-time distribution and vertical fluxes of particulate matter in the Otranto channel area have been studied. A three-layered structure was observed: (i) a surface layer with abundance of suspended matter, characterised by the presence of two water masses (the Adriatic Surface Water and the Ionian Surface Water); (ii) an intermediate layer (Levantine Intermediate Water) with very low particle content; and (iii) a bottom nepheloid layer. In winter, the particulate matter concentration at the surface revealed an east-west gradient that was not evident in summer, when the water exchange through the strait was reduced and local recirculation phenomena prevailed. Sediment traps and current meters were moored on the Italian shelf and on the shelf slope. Total vertical flux values of particulate matter, which were by one-tenth lower in the slope area with respect to the shelf, were in good agreement with the distribution of suspended matter in the water column observed during the seasonal cruises. Flux values measured at the shelf station appear to be related both to biological and advective processes, whereas the main source in the slope area appears to be the vertical transfer of biogenic material. (C) 1999 Elsevier Science B.V. All rights reserved.