Trace Metals in the Mediterranean Sea

Abstract

The major aspects of the impact of trace metal fluxes on oceanic areas, their involvement in biogeochemical cycles and, for some of them, their biologically active roles are reviewed. The land-based atmospheric input of trace metals is presented, the atmosphere being the main transport route for metals to offshore Mediterranean waters. Two antagonistic inputs (crustal and anthropogenic) prevail: the Mediterranean coastal atmosphere is characterised by a European background anthropogenic signature upon which natural or exceptional anthropogenic episodes are superimposed. Partitioned (dissolved+particulate) atmospheric fluxes are provided for a variety of metals (Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn). The dissolved phase roughly increases with the anthropogenic character. Insofar as unmodified atmospheric particles do not sink in seawater, the trace metal transfer off the euphotic layer is driven by biological activity and hydrological processes, and therefore exhibits a marked seasonal pattern. Removal and release of trace metals during their sinking to the sea floor are discussed, with emphasis on their residence time in surface waters, i.e. their mineralisation with depth. Since many trace metals are involved in phytoplankton uptake, their interaction with nutrients is discussed, although data are very scarce for the Mediterranean Sea. Chemical speciation and isotopic fractionation are key processes that determine the biological uptake of metals by phytoplankton and their further transfer up the food chain and bioaccumulation. Trace metal fluxes to deep waters are evaluated upon from NW Mediterranean data at 1000 m depth. Temporal trends at the decennial scale are discussed through time-series data and future evolutions are predicted tentatively.