The colloidal speciation of some trace metal elements (Cu, Pb and Cd) was studied in the Penzé estuary (Western Channel, Brittany) over a seasonal cycle. This study was performed in order to understand the metal transfer mechanisms between the different fractions (particulate, colloidal and truly-dissolved). Two methods were used: (i) frontal ultrafiltration (FU) allowing the partition in 7 size fractions and (ii) stripping chronopotentiometry (SCP) allowing the determination of metal concentrations in the various fractions. FU was chosen for its easy implementation and because it is a non-destructive technique. The detection limits of SCP were improved in order to measure the low-metal concentrations. Ours results have shown an important occurrence of Cu, Pb and Cd in the colloidal phase. If Cd is rather found in the lower part of the colloidal size-spectrum (300 kDa); Cu having an intermediate distribution. Over the seasonal cycle, two major phenomena were identified: (i) a strong mobilisation of humic substances from soils during autumn and (ii) the degradation of particulate organic matter in the benthic compartment during spring. These processes affect not only the colloidal distribution of the studied elements but also play an important role in the quantities and the forms exported toward the coastal zone.
