Trace metal behavior along fluvio-marine gradients in the Samborombَn Bay, outer Rيo de la Plata estuary, Argentina

Water and sediment samples were collected in the Samborombón Bay, outer Río de la Plata estuary to evaluate the dynamics of trace metals along the fluvial–marine transition. This estuarine environment presented strong salinity (0.11–31.4) and turbidity (21 to >999 mg L−1) gradients with a consistent turbidity maximum zone (TMZ:>1 g suspended solids L−1) in the low salinity region. Suspended particulate metals showed different spatial patterns: homogeneous distributions (Fe, Zn) and progressive enrichment from the TMZ to the maritime area (Cu, Ni). The influence of the TMZ is clearly reflected by the enrichment of flocculated clays which in turns control the distribution of sedimentary TOC and trace metals resulting in a prevailing decreasing land–sea gradient. Mn has a particular behavior along the salinity gradient denoting intense dissolved-particulated phase changes both in the water column and bottom sediments. An average 36% drop of particulate Mn observed in parallel with a strong dissolved peak suggests a rapid Mn desorption/dissolution in the water column whereas bottom sediments display an average 72% Mn enrichment in the TMZ relative to the riverine sector reflecting the diffusive escape from anoxic sediments and reprecipitation in oxic surface layers.