Phytoplankton structure and dynamics in Lake Sanabria and Valparaiso reservoir (NW Spain)

The aim of this work is to compare the composition and seasonality of the p hytoplankton population in a natural oligotrophic lake (Lake Sanabria) and a mesotrophic reservoir (Valparaiso). Both ecosystems are located on the Te ra river course (NW Spain), which runs along an area of ancient metamorphic and plutonic rocks. Some physical and chemical parameters, chlorophyll a a nd phytoplankton biovolume were studied from monthly samples collected at d ifferent depths during the periods 1987-1989 (Lake Sanabria) and 1991-1992 (Valparaiso). Phytoplankton biovolume and chlorophyll a concentration were about five times higher in Valparaiso than in Lake Sanabria. Species compos ition (and main phytoplankton groups) were different. Valparaiso was highly dominated by diatoms and Lake Sanabria by cryptophytes and small chlorophy tes. In spite of the fact that both sires were nitrogen limited, heterocyst ous cyanophytes (Annbaena sp.) were detected only in Valparaiso. The relati onships between phytoplankton structure and trophic level, hydrological con ditions and nitrate content are discussed.