Chemical dynamics of the Changjiang estuary

A reconnaissance of the chemical dynamics of the estuary and plume of the Changjiang was carried out on cruises in the summer of 1980 and the winter of 1981. In summer vigorous turbulence in the main channel of the inner estuary maintains high concentrations of suspended material in the surface layers which suppresses biological activity. Plankton blooms occur only on the inner shelf at salinities greater than about 20 ppt. In winter there is no significant photosynthetic activity over the entire mixing zone. Therefore a wide variety of inorganic processes can be studied in detail. Of the nutrients, nitrate is present in the river in very high concentrations and suffers only minor depletion in the biologically active areas. The distribution of silica is similar. Phosphate shows major release from the suspended particles and complete depletion in the plankton blooms. Among the trace elements, iron decreases rapidly to 5 ppt, beyond which mixing is conservative; comparison with the nutrient trends indicates that the element is chemically adsorbed onto the particulates. In contrast manganese undergoes desorption out to about 12 ppt beyond which it mixes conservatively. Copper and beryllium behave conservatively over the entire mixing zone while nickel and barium are desorbed rapidly at low salinity. Cadmium is undetectable in the river waters (< 10pmol kg ^-1) but displays a broad desorptive maximum at intermediate salinities in the mixing zone. These data allow fluxes to be calculated for the net transport of dissolved material from the Changjiang Basin to the surface waters of the East China Sea and their comparison with those of other large rivers of the world.