SEASONAL FLUCTUATION OF THE PHYTOPLANKTONIC POPULATIONS IN CULLERA LAGOON (VALENCIA, SPAIN)

Abstract

The seasonal fluctuations and the vertical distribution of the phytoplanktonic populations of the Estany of Cullera has been studied. The Estany of Cullera is a stratified coastal lagoon due to a marine water wedge underneath the flowing fresh water. The higher phytoplanktonic densities were found in winter and spring, in conditions of vertical mixing; being 86x10⁹ ind/m² the maximum density detected. In these seasons the diatoms were dominant; the most abundant species were Stephanodiscus subsalsus, Cyclotella meneghiniana & Cyclotella atomus. I n these periods m arine species can also be found such as Skeletonema costatum. Diatom dorninance is switched by the dorninance of green algae in summer, being the density then arround 8x10⁹ ind/m². In autumm, a remarkable minimum is produced due to an important water flux, reducing the phytoplanktonic density to 0.5x10⁹ ind/m². Most species are not found below the level of the chemocline where there ¡S a growth of photosynthetic bacteria. However, the existence of a few species of euglenoids, cyanobacteia and small flagellates has been detected in these botton water layers; they must have interesting adaptations to be able to live in the anoxic conditions of these waters.