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 m agnitude and the mineralogical composition of atmospheric dust inputs indic ate that eolian deposition is an important (50%) or even dominant (>80%) co ntribution to sediments in the offshore waters of the entire Mediterranean basin. Model data for trace metals and nutrients indicate that the atmosphe re delivers more than half the lead and nitrogen, one-third of total phosph orus, and 10% of the zinc entering the entire basin. Measured data in sub-b asins, such as the north-western Mediterranean and northern Adriatic indica te an even greater proportions of atmospheric versus riverine inputs. When dissolved fluxes are compared (the form most likely to impinge on surface w ater biogeochemical cycles), the atmosphere is found to be 5 to 50 times mo re important than rivers for dissolved zinc and 15 to 30 times more importa nt for lend fluxes. Neglecting colimitation by other nutrients, new product ion supported by atmospheric nitrogen deposition ranges from 2-4 g C m(-2) yr(-1), whereas atmospheric phosphorus deposition appears to support less t han 1 g C m(-2) yr(-1). In spite of the apparently small contribution of at mospheric deposition to overall production in the basin it has been suggest ed that certain episodic phytoplankton blooms are triggered by atmospheric deposition of N, P or Fe. Future studies are needed to clarify the extent a nd 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 di rect sampling of the ocean and atmosphere before and after such events is t herefore highly recommended. (C) 1999 Elsevier Science Ltd. All rights rese rved.