Distributions of dissolved and particulate elements in the Yangtze estuary in 1997–2002: Background data before the closure of the Three Gorges Dam

Concentrations of 31 particulate elements (Al, As, Ba, Ca, Cd, Ce, Co, Cr, Cs, Cu, Eu, Fe, Hf, La, Lu, Mg, Mn, Ni, P, Pb, Rb, Sb, Sc, Sm, Sr, Th, Ti, V, Y, Yb, and Zn) and 6 dissolved elements (Al, Cd, Cu, Fe, Mn, and Ni) in the Yangtze estuary were determined during the period from 1997 to 2002. The salinity increased monotonously, and suspended particulate matter (SPM) peaked initially and then decreased rapidly in the mixing zone, which lies between 121.5° E and 122.5° E. The elemental composition of SPM was almost constant throughout the estuary, except for an increase of P, Cd, Pb, Sr, Cu, Mg, Cr, Zn, Ni, Ca, and Ba at some stations on the continental shelf. On the other hand, dissolved metal concentrations changed as the salinity increased in the mixing zone: dissolved Cu was diluted conservatively, dissolved Cd and Ni were released from SPM, and dissolved (colloidal) Fe, Al, and Mn increased when SPM increased. By mixing experiments using unfiltered river water and seawater, the conservative mixing of dissolved Cu and the release of dissolved Cd and Ni from SPM were reproduced in the laboratory. Effective dissolved metal concentrations (including the metals released in the mixing zone) in the river were Al (330 nM), Cd (0.39 nM), Cu (23 nM), Fe (72 nM), Mn (8 nM), and Ni (7 nM), values significantly lower than the concentrations of particulate metals, that is, Al (880 μM), Cd (1.7 nM), Cu (300 nM), Fe (230 μM), Mn (5.5 μM), and Ni (240 nM). Riverine metals were transported to the sea mainly in particulate form. The metal input through the Yangtze River has not changed significantly since the 1980s, when previous investigations in the same area (U.S.–China joint cruises in 1980 and 1981, and France–China joint cruises in 1986) were conducted.