Oceans and lakes are characterized by large residence times and can thus be considered as being in an equilibrium or quasi-equilibrium state. In contrast, estuaries, which constitute the interface between the fresh water and marine ecosystems, are biogeochemically and physically very dynamic. Strong gradients in physical and chemical variables such as temperature, pH, salinity, dissolved oxygen, nutrients, amount and composition of particulates result from the mixing of fresh water and saline end members. These gradients provoke increased biogeochemical processes, generally resulting in a...