BIOGEOCHEMICAL PROCESSES CONTROLLING NUTRIENTS AT THE TURBIDITY MAXIMUM AND THE PLUME WATER FRONTS IN THE CHANGJIANG ESTUARY

In order to study geochemical and biochemical processes in estuaries. particularly in the turbidity maximum and at the plume water fronts, t wo cruises were carried out in the summer and winter of 1988 in the Ch angjiang Estuary region. The study permitted to identify two major sou rces of nutrients: firstly the Changjiang River carried abundant nutri ents with 90-110 mumol/l of Si(OH)4, 70-95 mumol/l of NO3- and 0.5-0.8 mumol/l of PO43-. The annual average nutrient fluxes of the studied y ear were estimated as about 2.5 X 10(12), 1.0 X 10(12) and 1.9 X 10(10 ) g/yr for Si, N and P, respectively. However, NO2- and NH4+ did not p rincipally originate from the freshwater discharge. Their distribution was more affected by geochemical and biogeochemical processes. Second ly nutrient release in the turbidity maximum and from sediments outsid e the plume water fronts was observed. In the regions where vertical c onvection was strengthened due to the complex hydrographic features of the studied region, nutrients released from sediments dispersed upwar d to the surface waters providing a basis for an increase of primary p roductivity. In addition to seawater dilution, biological activities w ere another important factor for nutrient consumption, particularly ou tside the plume water fronts where biological consumption led to a not iceable removal of nutrients from surface waters.