Speciation of organic carbon, Cu and Mn in the River Marne (France): the role of colloids

The distribution of organic carbon (OC) and of some metals (Cu, Mn) amongst the particulate (>0.2 mu m), colloidal (10 kDa-0.2 mu m) and the truly dis solved (<10 kDa) fractions of the River Marne was investigated during the p hytoplankton spring bloom. A tangential ultrafiltration (UF) device was uti lized to separate the colloidal fraction. On average, 22% of the OC, 31% of the Cu and 53% of the Mn, usually assigned to the so-called dissolved frac tion, were found in the colloidal fraction. The colloidal fraction exhibite d a behaviour different from that of the particulate and truly dissolved fr actions. Autochthonous production led to enrichment in the colloidal and pa rticulate OC pools: up to 47% of the total dissolved OC was in the colloida l fraction. An increase in the colloidal metal fraction, concomitant with a fall in the truly dissolved fraction, coincided with peaks in phytoplankto n during the bloom. These phenomena might be related either to an increase in pH associated with photosynthetic activity, resulting in the precipitati on of truly dissolved forms into the colloidal fraction, or to scavenging o f the truly dissolved metals by the algal species of colloidal size. The interaction between the colloidal and the truly dissolved phases was ve ry important. The partition coefficients of the Cu and Mn between the collo idal and truly dissolved fractions were higher than between the particulate and the truly dissolved fractions. This pattern is consistent with a great er specific surface area of colloids than macroparticles. Consequently, the adsorption and complexation capacities are enhanced in the colloidal fract ion of the particulate matter. The extraction of hydrophobic complexes with Cu using C-18 Sep-Pak columns, showed that the Cu occurring in colloidal, total dissolved or truly dissol ved forms was significantly complexed by the organic matter. The truly diss olved fraction might be complexed up to 100% during a phytoplankton bloom. Copyright (C) 1999 John Wiley & Sons, Ltd.