River versus atmospheric input of material to the mediterranean sea: an overview

River input has long been considered as the major source of chemical elements to the ocean system. However, in recent years, it has become increasingly apparent that atmospheric transport plays a central role in the supply of material to the ocean. This atmospheric input is especially important in shelf seas and semi-enclosed seas such as the Mediterranean, which is close to potential pollution sources in Northern Europe and subjected to the well-known influx of Sahara red dust. This paper summarizes the work performed by the Land-Sea Interactions Group of CNRS (1984-1987) and the EROS-2000 programme (1987-1988) on a variety of elements and compounds in this area, where a combined study of river and atmospheric fluxes has been carried out. It is shown that the atmospheric input of red dust is of the same order of magnitude as the annual downstream flow of rivers discharging to the Western Mediterranean. As far as trace metals are concerned, the atmospheric flux of Cu, Pb and Cd exceeds river input by one to two orders of magnitude. However, the incoming flux through the straits of Sicily and (mainly) Gibraltar might be as important as the atmospheric flux; the residence times of these three metals are either shorter than the water renewal time (Pb, Cu) or equivalent to it, so that their accumulation in the water column is unlikely. As far as artificial radionuclides are concerned, atmospheric input is predominant for ²³⁹Pu + ²⁴⁰Pu and ¹³⁷Cs, whereas ²³⁸ Pu is mainly of riverine origin. The fluxes of nitrogen supplied by rivers and rain are approximately equivalent. The biological significance of this nitrogen input is discussed; it is estimated that the atmospheric nitrogen corresponds to 10% of the average 'new' production and may reach values as high as 50% during oligotrophic periods.