The seasonal variation and the vertical distribution of the phytoplanktonic population of the lagoon of Cullera, an elongated coastal lagoon with estuarine circulation of water, has been studied in three sampling stations: mouth, centre and source. Seasonal variation is determined by a marinefreshwat,er interaction. In winter, the sea influence is important, a marine water wedge of anoxic water arrives a t the sampling station located a t the source and marine and brackish water species dominate the phytoplankton. Also marine species of zooplankton and fish enter the system, which may then be considered as exploited by the sea. Inspring the ninrine wedge retreats from the source but remains in the centre and mouth, salinity diminishes, vertical mixing persists and phytoplankton is dominated by Cyclotella species. From late spring t o autumn the freshwater influence prevails and a sharp stratification of the water is produced in the stations a t the mouth and the centre, by means of a steep halocline coincident with an oxycline. The phytoplankton in this period follows a typical succession like those described in freshwater eutrophic lakes.
Vertical distribution of phytoplankton is determined by the presence of the oxycline, originated by the marine water wedge, whose depth varies seasonally but which is always present in the mouth and centre of the lagoon; only few species of algae can be found below its level.