SIGNIFICANCE OF COLLOIDS IN THE BIOGEOCHEMICAL CYCLING OF ORGANIC-CARBON AND TRACE-METALS IN THE VENICE LAGOON (ITALY)

Colloidal organic C and trace metals from the waters df a highly produ ctive coastal environment (the Venice Lagoon, Italy) have been separat ed by a cross-flow ultrafiltration device. On average, 18% of organic C, 34% of Cd, 46% of Cu, 87% of Fe, 18% of Ni, 58% of Pb, and 54% of M n which previously would have been considered in the dissolved phase a re actually associated with colloidal material, Thus, past studies ove restimate the dissolved trace-metal concentration in the nearshore env ironment. Compared to total concentration, the proportion of the collo idal fraction represents on average 15% of organic C, 18% of Cd, 28% o f Cu, 11% of Fe, 11% of Ni, 29% of Pb, and 12% of Mn. This fraction ac ts differently from the truly dissolved and macroparticulate phases. T he behavior of organic C and trace elements during mixing between fres hwater and seawater is more complicated than expected when a colloidal fraction is involved. The flocculation of colloids, encountered norma lly during estuarine mixing, is not very significant on the time scale of mixing in the lagoon. Conversely, the interaction between colloida l and truly dissolved phases seems important. The partitioning of trac e metals between different fractions of organic C appears variable, Fe and Mn are preferentially tied to macroparticulate organic matter, an d Cu and Cd are preferentially tied to colloidal organic matter in sea water. Truly dissolved organic C appears to be important for Ni. Pb is mainly associated with microparticulate organic matter at most statio ns except in the highly productive region where Pb prefers colloidal o rganic C.