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

The estuary of the Krka river on the eastern Adriatic coast of the Mediterranean sea was studied. A high stratification reduced the river/seawater interface to a well-defined, visible region. Water samples at 10 and 25 cm intervals around the interface were collected by divers. Temperature, salinity, total suspended solids, particulate organic carbon, chlorophyll-a, phaeophytin, surfactant activity and surface-active organic matter were measured. Measurements of the last-mentioned indicated the presence of a well-defined film, formed at the freshwater/seawater interface by accumulation and condensation of plankton-derived organic matter under the influence of salinity and shear gradients. It was composed of dissolved and insoluble, liquid, surface active material, contained a potential food source for heterotrophic organisms and accumulated pollutants. It might be characteristic of other salt-wedge estuaries elsewhere in the world.