DIURNAL CHANGES IN SPECIATION OF TRACE ELEMENTS (CD, CU) IN THE SURFACE FRESHWATER REACHES OF THE GIRONDE ESTUARY (FRANCE)

Speciation and reactivity of Cd and Cu in the surface freshwater reaches of the Gironde Estuary (south-west France) were studied by high frequency sampling (every 30 min) during 25 h covering two consecutive tides in July 2008. Particulate and dissolved metal concentrations were measured in different fractions obtained by filtration (<0.45 µm, <0.2 µm and <0.02 µm), solid/liquid ion exchange (Digisep®, SCP-Science) and in-situ voltammetry (free and dynamic fractions, i.e. labile and mobile metal complexes). Environmental parameters (temperature, conductivity, dissolved oxygen, pH, SPM concentration, current velocity, water depth) were measured continuously and organic matter was characterized by several techniques (DOC, POC, chlorophyll-a, pheopigment and humic/fulvic acid measurements). Cadmium and Cu concentrations of all measured fractions were compared to environmental parameters and organic matter characteristics. Moreover, experimental metal speciation data were compared with theoretical values computed using an equilibrium-based model.

Diurnal variations in O₂ saturation, pH, chlorophyll-a, pheopigment and POC concentrations suggested intense primary production during the daylight. Particulate and dissolved (<0.2 µm and <0.02 µm) Cd and Cu fractions seemed to reflect physical processes (tidal oscillation and mixing of water masses). In contrast, free and dynamic fractions of Cd and Cu (i.e. potentially bioavailable fractions) clearly responded to diurnal biological processes (e.g. photo-reduction, photosynthesis), suggesting that in the low to moderate -turbidity freshwater reaches both physical and biogeochemical processes (e.g. complexation processes with autochthonous organic molecules) may control metal reactivity and speciation at various scales. This study highlights the potential of high temporal resolution speciation measurements using multi-analytical approach for the understanding of the biogeochemical diurnal processes controlling trace element behavior in complex aquatic environments.