The role of atmospheric deposition in the biogeochemistry of the Mediterranean Sea

Estimates of atmospheric inputs to the Mediterranean and some coastal areas are reviewed, and uncertainities in these estimates considered. Both the magnitude and the mineralogical composition of atmospheric dust inputs indicate that eolian deposition is an important (50%) or even dominant (>80%) contribution to sediments in the offshore waters of the entire Mediterranean basin. Model data for trace metals and nutrients indicate that the atmosphere delivers more than half the lead and nitrogen, one-third of total phosphorus, and 10% of the zinc entering the entire basin. Measured data in sub-basins, such as the north-western Mediterranean and northern Adriatic indicate an even greater proportions of atmospheric versus riverine inputs. When dissolved fluxes are compared (the form most likely to impinge on surface water biogeochemical cycles), the atmosphere is found to be 5 to 50 times more important than rivers for dissolved zinc and 15 to 30 times more important for lead fluxes. Neglecting colimitation by other nutrients, new production supported by atmospheric nitrogen deposition ranges from 2-4 g C m^-2 yr^-1, whereas atmospheric phosphorus deposition appears to support less than 1 g C m^-2 yr^-1. In spite of the apparently small contribution of atmospheric deposition to overall production in the basin it has been suggested that certain episodic phytoplankton blooms are triggered by atmospheric deposition of N, P or Fe. Future studies are needed to clarify the extent and causal links between these episodic blooms and atmospheric/oceanographic forcing functions. A scientific program aimed at elucidating the possible biogeochemical effects of Saharan outbreaks in the Mediterranean through direct sampling of the ocean and atmosphere before and after such events is therefore highly recommended.