SUCCESSIONAL DYNAMICS OF THE PHYTOPLANKTON IN THE LOWER PART OF THE RIVER EBRO

The temporal and spatial dynamics of phytoplankton have been studied in four sites located along the last 60 km of the river Ebro, over a period of 1 year. Diatoms and green algae were the most abundant groups; blue-green algae were frequent only in autumn. Asterionella formosa dominated the winter phytoplankton assemblages. In autumn, spring and early summer centric diatoms were dominant: Aulacoseira granulata (Ehr.) Simonsen in autunm; Cyclotella sp. p1., Skeletonema potamos (Weber) Hasle and StephanodisciLs sp. p1. in spring. A great abundance of green algae was observed during the summer, mainly in the lower sites. In the sites closer to the mouth, the spring maximum of centric diatoms extended to the summer. Mainly in the downstream sites, a remarkable growth of Acrinocyclus normanii f. subsalsa (Juhl.-Daunf.) Hustedt and Stephanodiscus hantzschii f tenuis (Hust.) Hak. & Stoerm. was added to green algae in the late summer. As has been investigated through a principal component analysis, the phytoplankton temporal succession and longitudinal differences between the sites may be affected by the variations in flow and the increase of water conductivity downstream; both factors seem to act together. The river is rather homogeneous with respect to the phytoplankton assemblages during the winter and spring months, and from late spring to the following autumn, differences greatly increase both in time and downstream. © 1990 Oxford University Press.