Chemical composition of downward fluxes in the Cretan Sea (Eastern Mediterranean) and possible link to atmospheric deposition: A 7 year survey

To better assess the role of atmospheric deposition on seawater productivity in the Eastern Mediterranean 200 sinking particulate matter samples were collected at two different depths in the water column (500 and 1715 m from the surface) during a seven-year period (1999–2005). The samples were analysed for various nutrients and elements of both natural and anthropogenic origin such as nitrogen (N), phosphorus (P), organic matter, CaCO3, Si, Al, Ca, V, Cr, Mn, Fe, Ti, Ni, Cu, Zn, Cd and Pb. Moreover, sediment traps data were compared with atmospheric deposition to assess possible transfer mechanisms of material from the atmosphere to the water column. The amount of dissolved inorganic nitrogen (DIN) deposited via the atmosphere was higher up to a factor of 6 compared to the dissolved organic nitrogen (DON) collected with the sediment traps, indicating significant external N deposition. On the other hand, atmospheric dissolved inorganic phosphorus (DIP) deposition accounted for only 60% of the P measured at upper sediment traps indicating that the atmosphere plays a less significant role of the atmosphere for P compared to N. Atmospheric dust was found to be transferred almost quantitatively from the atmosphere to the deeper water layers, indicating significant impact of vertical mass transfer. The rapid vertical mass transfer during spring can be attributed to the formation of big aggregates around atmospheric dust particles, which settle quite fast. During summer and autumn mass transfer is hindered due to the stratification of the water column and low primary productivity.