Abstract
Dissolved and particulate vanadium and chromium concentrations along a transect between Cherbourg and the Isle of Wight were investigated in the English Channel. Seawater samples were collected at two different depths (surface and bottom) at five sampling stations during five cruises carried out between September 1994 and September 1995. Calculated mass flows through this Channel section were 7600 T yr −1 for vanadium (about 66% was in the dissolved phase) and 1650 T yr −1 for chromium (about 50% was in the dissolved phase). Dissolved chromium concentrations do not vary significantly along the transect. Seasonal variations in chromium distribution linked to biotic parameters were noted during the September 1994 cruise, when a significant relationship between particulate chromium and algal organic carbon was found. In addition, dissolved Cr (III) and Cr (VI) were well correlated with both algal and terrestrial organic matter. In the May cruise, during phytoplanktonic decay, particulate chromium was correlated with the detritic and bacterial organic fractions. These observations suggest interactions between chromium and biotic material. During the winter period, no relationship between chromium and POC was found. Dissolved and particulate vanadium concentrations varied, respectively, between 15 and 28 nmol l −1 and 2–32 nmol l −1. Values of dissolved vanadium showed depletion in the Channel with respect to oceanic waters. This loss in dissolved vanadium was higher near the English coast, but was compensated for by the increase in the particulate vanadium. There was no clear relationship between dissolved vanadium and algal organic carbon and it is inferred that the vanadium transfer cannot be explained by trapping with biotic material. On the other hand, the dissolved vanadium depletion could be attributed to the presence of ferric oxyhydroxide phases in particles, which have strong adsorbing properties for a range of dissolved metal ions.
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