Size and composition of colloidal organic matter and trace elements in the Mississippi River, Pearl River and the northern Gulf of Mexico, as characterized by flow field-flow fractionation

The continuous colloidal size spectra (0.5–40 nm) of chromophoric and fluorescent organic matter, Fe, P, Mn, Cu, Zn, Pb, and U, were determined by on-line coupling of flow field-flow fractionation (FFF) to detectors including UV-absorbance, fluorescence, and ICP-MS, in samples from the lower Mississippi River, the Atchafalaya River, the Pearl River, and from marine stations in the northern Gulf of Mexico. The colloidal size spectra showed the presence of 3–4 colloid populations; 0.5–4 nm CDOM-colloids, binding most elements, 3–8 nm protein-like colloids, binding P in seawater, and 5–40 nm Fe-rich colloids, binding P, Mn, Zn, and Pb. Moreover, protein-like colloidal matter, Fe, P, Mn and Pb were largely found in the > 40 nm fraction. We hypothesize that the CDOM-colloids represent terrestrial fulvic acid, and that the protein-like colloids are mostly derived from in situ biological production, while the iron-rich colloids are largely inorganic and contain Fe(III)-hydroxide/oxyhydroxide. The colloidal concentrations, determined by both FFF and ultrafiltration, were generally much higher in the Pearl River than in the other rivers, and decreased seaward in the Gulf of Mexico. The colloidal size distribution of protein-like organic matter, Fe-rich colloids and associated elements were shifted to larger sizes in the Mississippi and Atchafalaya Rivers compared with the Pearl River.