The iron binding capacities (IBC) of fulvic acid (FA) and humic acid (HA) were determined in the salinity range from 5 to 40. The results indicated that IBC decreased while salinity increased. In addition, dissolved iron (dFe), FA and HA were also determined along the Yangtze River estuary’s increasing salinity gradient from 0.14 to 33. The loss rates of dFe, FA and HA in the Yangtze River estuary were up to 96%, 74%, and 67%, respectively. The decreases in dFe, FA and HA, as well as the change in IBC of humic substances (HS) along the salinity gradient in the Yangtze River estuary were all well described by a first-order exponential attenuation model: y(dFe/FA/HA, S) = a0 × exp(kS) + y0. These results indicate that flocculation of FA and HA along the salinity gradient resulted in removal of dFe. Furthermore, the exponential attenuation model described in this paper can be applied in the major estuaries of the world where most of the removal of dFe and HS occurs where freshwater and seawater mix.

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Basing on the data from four times surveys, the spatio-temporal distribution patterns and the variation mechanism of nutrients in the Yangtze estuary were analyzed. The nitrate and silicate concentration exhibited an eminent "double tongue" pattern in summer and autumn, however, it moved southward along the shore in spring and winter. Phosphate concentration showed the similar "double tongue" pattern in summer and autumn, but the northeast frontal zone was more west comparing to that of the nitrate and silicate. The nitrate and total dissolved nitrogen (TDN) were positively correlated in all seasons; similarly the phosphate was positively correlated with total dissolved phosphorus (TDP) and total phosphorus (TP) in all seasons. It was also found that nitrate, silicate and phosphorus all had high negative correlation with salinity, which meant that the nutrient dynamics in the estuary was predominantly governed by the freshwater discharge. The Redfield ratio analyses revealed that SiO3-Si/DIN was obviously higher in the far-shore area than the near-shore area. The DIN/PO4-P was higher than the Redfield ratio 16 in the entire study area, and it gradually decreased from near-shore to far-shore.