The hydrodynamic properties of estuaries are extremely variable and depend on the relative ratio of tidal mixing of freshwater and seawater. This ratio influences essential properties of the estuarine system such as the degree of vertical stratification of the water column, the residence time of freshwater, and the existence and intensity of a turbidity maximum. The estuaries, which are the obligated pathways for the transfer of freshwater to the ocean, act often as a filter for many constituents, whose efficiency increases with the residence time of freshwater in the system. This is especially the case for suspended particulate matter, which is often removed to a large extent from the water column by flocculation and sedimentation, generally occurring in the area of the turbidity maximum. Particulate organic matter and trace metals are therefore largely trapped in the estuarine sediments and never reach the sea. The turbidity maximum may, furthermore, generate anoxic conditions, which affect the behaviour of numerous components such as nutrients and redox-sensitive metals like iron and manganese. Finally, the intense biological activity, characteristic of the estuarine environment, is responsible for emissions of various biogases (CO2, CH4, N2O) to the atmosphere. Due to the complexity of the hydrodynamic properties, the existence of strong gradients of chemical and biological parameters as well as the numerous processes occurring in estuaries, the evaluation of elemental fluxes requires the development of elaborated coupled hydrodynamic-biogeochemical models. Large errors could be generated by the use of simplified hypothesis (such as in the AZE method). In addition, estuaries are usually far from steady-state conditions and the estimation of fluxes requires intensive measurements at the boundaries of the systems.
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