A film of organic matter at the fresh-water/sea-water interface of an estuary

Abstract

In estuaries one finds three types of interface: air/water, freshwater/sea-water and water/solid. Physical, chemical and biological processes are relatively well understood at the air/water1,2 and solid/water interfaces3, which are common to other aquatic systems; processes and, in particular, organic structures at the freshwater/sea-water interface are less well known. Here we report measurements of surface-active organic material along the vertical profile of a stratified estuary that point to the existence of a well-defined film, and subsequent visual observations by divers which confirm this discovery. The film is formed at the freshwater/sea-water interface by accumulation and condensation of plankton-derived organic matter under the influence of salinity and shear gradients. It is composed of dissolved and insoluble, liquid, surface-active material. The film contributes to the stability of the fresh-water/sea-water interface, contains a potential food source for heterotrophic organisms and accumulates pollutants. Because we have found the film in an estuary with a low content of organic matter, we expect that it occurs in most salt-wedge estuaries of the world.