The main objective of this PhD thesis is to elucidate the physical, chemical and biological processes affecting the partitioning of Fe and Mn between the dissolved and particulate phases during the estuarine mixing on the Scheldt (Belgium). To do so, the concentrations of these two metals in the dissolved (< 0.45 micro-m) and particulate (> 0.45 micro-m) phases were determined during axial transects and with discrete samples collected between 1992 to 1995 under various climatic conditions. In addition, the temporal variation of the Fe and Mn concentrations in these two phases was also investigated at the upstream boundary of the Scheldt estuary (Hemiksem) in 1994-1995 for the dissolved phase and in 1996-1997 for both the dissolved and particulate phases.
A particular emphasis was put on the study of the speciation of these two metals in the dissolved phase. A physical separation, bases on the size of the particles, allowed us to isolate a fraction (called colloidal, between 0.05 and 0.45 micro-m) and a fraction whose size is lower than 0.5 micro-m and which is considered as dissolved. If the dissolved manganese is mostly present in this last form, the iron case is different, with a high proportion of colloidal iron present in the upstream zone of the estuary. Several processes such as flocculation or precipitation following the reoxygenation of the water column have been show to happen in the Scheldt estuary.
In addition to the study of the partitioning of Fe and Mn between the different phases, a study of teh process responsible for the transfer of these two metals between the dissolved and particulate phases was done. To do so, Fe and Mn radioactive isotopes were added to water samples from the Scheldt and subjected then to different incubation conditions. The kinetic study of the transfer of the radioisotope and its modelling allowed to separate two different processes: the isotopic exchange and the active transfer of the metal considered. The incubation under various conditions, elaborated during this work, allowed to distinguish between the abiotic (chemical and photochemical transfer) and biotic (bacterial and phytoplanktonic) processes responsible for the Fe and Mn transfer between the dissolved and particulate phases.