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Variations of dissolved silicate (DSi) flux in the Yangtze River have caused great concern among scientists. Analysis of spatial and temporal variations of DSi indicates that the distribution of DSi concentration (DSiC) is closely related to the occurrence of bedrocks in the river cathchment. On average, the upper Yangtze River and Dongting and Poyang Lake of the middle Yangtze basin serve as the major DSi sinks, retaining 3.39 × 104 t y−1, 5.59 × 104 and 2.51 × 104 t y−1. The middle and low Yangtze River remains DSi sources, emitting 2.85 × 104 and 2.48 × 104 t y−1, respectively. Geochemical data at Datong hydrological station recorded the flux of nutrients entering into the sea during the flood season, i.e. 74% for DSi, 73% for DIN (Dissolved Inorganic Nitrate) and 68% DIP (Dissolved Inorganic Phosphate). The yearly DSiC and flux show a sharp decrease since 1950s. The mean DSiC was 109.47, 91.09, and 77.56 μmol l−1 in the 1960s, 1970s and 1980s, respectively. The mean DSi fluxes for the same time period were 2.72, 2.23, and 2.13 × 106 t. A PP (primary productivity) model estimates 3.14 × 105 t of DSi (13.08% of the annual mean flux entering into the sea) was fixated within the 162 reservoirs in 2002. On the other hand, DIN and DIP concentration and flux have increased greatly since the 1950s. Fertilizer application peaked after the 1980s can interpret these high DIN and DIP. A sharply decreasing DSi flux and quickly increasing DIN and DIP fluxes into the sea have enhanced eutrophication and caused frequent harmful algal blooms in coastal waters. On average, red-tide frequency was from 0.04 during 1933–1979 to 7.0 during 2000–2002. The Skeletonema costatum (siliceous alga), the red-tide-predominant species that is in positive proportion to DSi flux, decreased from 33% during the 1980s to 24% during 2000–2002. The present data evidence the increase of Prorocentrum dentatum (non-siliceous alga) from 12.5% in the 1980s to 36% in 2000–2002, which tends to be dominant species of the red tides off the Yangtze estuary. The number of big dams in the Yangtze River basin will double in the next 30–50 years. This will significantly influence the variations of nutrient fluxes in the river basin and estuary, in relation to health management of river-coast ecosystem.

We studied the influence of the water-soluble fraction (WSF) of diesel fuel oil on bacterial and photosynthetic producers and on carbon transfer to higher trophic organisms in an aquatic plankton ecosystem by means of a mesocosm experiment in the Yangtze estuary, China. In the oil-enriched mesocosm, average bacterial production increased 3.9 times compared with that in the control mesocosm. WSF addition also increased the abundance of bacterivorous heterotrophic nanoflagellates, but it decreased the abundance of other higher trophic organisms (micro- and mesozooplankton) due to the toxic effects of the WSF. The amount of carbon transfer from bacterial production to mesozooplankton was of nearly similar magnitude in the oil-enriched and control mesocosms, though bacterial production was much larger in the former. The relative reachable efficiency of bacterial production to mesozooplankton in the oil-enriched mesocosm was only about 36% of that in the control. This lower efficiency in the oil-enriched mesocosm was attributed to decreased mesozooplankton abundance rather than decreased abundance of trophic intermediates in microbial food webs. We found only a slight difference between photosynthetic production in the oil-enriched and control mesocosms. A complementary batch-incubation experiment showed that the WSF may have considerably decreased the photosynthetic activity of an algal assemblage, however, so the photosynthetic production in mesocosms was regulated by nutrient-limitation rather than by the toxic effect of WSF. The carbon transfer from photosynthetic production to mesozooplankton in the oil-enriched mesocosm was comparable to that in the control, though WSF addition decreased the abundance of mesozooplankton in the oil-enriched mesocosm. This may have resulted from the occurrence of the diatom Skeletonema costatum, which is much preferred by mesozooplankton (copepods) as prey, in the oil-enriched mesocosm, whereas the dinoflagellate Prorocentrum dentatum was overwhelmingly dominant in the control mesocosm.

A sea survey was conducted in May 2001 using CTD together with direct-reading current meter and water sampling to investigate the sediment dynamics off the Yangtze River mouth. Data obtained from five observational sites reveal density flows characterized by the halocline, thermocline and associated sharp current velocity gradients that prevail in the study area, especially at the freshwater plume front. The interaction of the Yangtze freshwater plume, littoral current and tidal current has generated these density flows. Particularly, the anomalous high speed (>140 cm s −1 ) current recorded at the freshwater plume front during the early flood stage leads to the resuspension of the sediment (SSC reaching 0.5 g l −1 ) off the river mouth during that time period. The suspended sediment flux measured at B2, the ideal site for recording frontal processes in the estuary, indicates the main trend of southward transport of fine-grained sediment up to 60 kg m −2 s −1 at early flood stage and 1.32 kg m −2 s −1 as net flux during two tidal periods. It is inferred that the density flow at the plume front plays key roles in dispersing the Yangtze fine suspended sediments southward during the non-flood season.

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.