| The distribution characteristic of nutrients, chemical oxygen demand, suspended particulate matter and chlorophyll a and status of eutrophication in the Changjiang Estuary were analyzed. A modified box budget approach was adopted to estimate nutrient fluxes and processes in the Changjiang Estuary and its adjacent waters, including the amount from river, the deep open sea, atmosphere, the amount of release in alongshore mixing zone and deep open sea water, and the amount of consuming in outer sea, as well as primary production and denitrification rate. The seasonal distribution characteristics and influencing factors of dissolved oxygen were also analyzed, and the formation mechanism of low oxygen area in the Changjiang Estuary was discussed in this paper.The main results are as follows:The distribution of nutrients, chemical oxygen demand, suspended particulate matter and chlorophyll a had obvious seasonal characters, and it was affected by dilute water. The Changjiang Estuary was characterized by high dissolved inorganic nitrogen (DIN) and dissolved silicate (DSi) concentrations, with low dissolved inorganic phosphate (DIP) concentrations. The mean concentrations of nutrients were high in inner estuary and Turbidity Maximum zone and they become small with the increasing distance to land. Nitrate was main component of dissolved inorganic nitrogen. The values of N:P and Si:P were high and the value of Si:N was currently above 1. The distribution of nitrate and silicate was conservative in the water of mid and low salinity, since they were influenced by physical mixing. The maximum Chl a concentration existed in the open sea in spring and summer and the observed maximum was 26.22 mg m-3. Excess nutrient and large phytoplankton in spring and summer accelerated the eutrophication in the Changjiang Estuary.The majority of the nutrient input to the estuary came from the river, for annual 1.53×1011 mmol DIN day-1, 2.22×109 mmol DIP day-1 and 2.97×1011 mmol DSi day-1. The budgets from the open sea were 9.05×1010 mmol DIN day-1, 3.57×109 mmol DIP day-1 and 1.65×1011 mmol DSi day-1, while the budgets from atmosphere were 4.06×108 mmol DIN day-1, 6.45×105 mmol DIP day-1 and 2.52×109 mmol DSi day-1 respectively. The dissolved inorganic phosphorus from the deep open sea served as a major source of support for primary production over the outer sea area. The alongshore mixing zone served as a source that supplies 40% of DIN, 51% of DIP and 30%of DSi for the amount of export during their transport along the estuary to the coastal water. The alongshore water system was heterotrophic for light-limited turbid condition and low phytoplankton biomass. The outer sea zone served as nutrient trap and the rate of consumption was about 894μmol DIN m-3 day-1, 18.0μmol DIP m-3 day-1 and 1418μmol DSi m-3 day-1. The net ecosystem metabolism was 38.15 mmol m-2 day-1. In the bottom oxygen-deficient water, the denitrification process was significant.The distribution of dissolved oxygen (DO) in the bottom water had obvious seasonal characters. The values of DO were high in winter and low in summer. Low values of DO might appear in spring and autumn for the temporal physical and biological status. The concentrations of DO in the bottom water in the trough were lower than those in the surrounding water. The low values of DO might appear in the north water tongue of the Changjiang Estuary in the summer of some years. The distribution of DO was influenced by temperature, topography, stratification and biological factors. In the summer, it was found that there was positive correlation between DO concentration and temperature, which was contrary in the autumn and winter. The trough obstructed the horizontal transport of DO and was filled by the TWC water. Strong halocline and thermocline appeared in spring and summer. Strong stratification obstructed the vertical transport of DO and the vertical transport of DO flux, which was obstructed by stratification, was calculated and reached 4.70 g m-2 day-1. Hypoxia was seasonal and occurred in summer. Large nutrient loading led to enhanced phytoplankton production, and more organic matter was decomposed, which could result in oxygen depletion. These special physical conditions and biochemical factors might result in the occurrence of hypoxia in the Changjiang Estuary.
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