Trace metal behavior in surface waters: emphasis on dynamic speciation, sorption processes and bioavailability

Trace metals (TMs) play a central role in the functioning of aquatic systems. Both essential and toxic trace metals are involved in different chemical, biological and physical processes that determine their concentration and chemical forms. The various TM chemical form include free hydrated ions ; metal complexes with various small inorganic and organic ligands as well as with ill-defined dissolved organic matter ; metals adsorbed on the surface of natural or anthropogenic colloids and particles. TMs reactivity, fate and impact on the organisms depend on the proportion of these different forms which in turn is influenced by the biophysicochemical conditions of the media. In this overview, dedicated to the father of the modern limnology Francois-Alphonse Forel, we present current concepts of chemical speciation, sorption processes and biological availability that play a key role in metal cycling and regulation occurring in the aquatic systems. These concepts are illustrated with examples of field case studies performed in the Riou-Mort stream in France and the plume of the Po River in the Northern Adriatic Sea. The necessity of more environmental realism as well as the exciting gaps and challenges in TM cycling in the aquatic systems are also discussed.