Abstract
The aim of this work is to compare the composition and seasonality of the phytoplankton population in a natural oligotrophic lake (Lake Sanabria) and a mesotrophic reservoir (Valparaiso). Both ecosystems are located on the Tera river course (NW Spain), which runs along an area of ancient metamorphic and plutonic rocks. Some physical and chemical parameters, chlorophyll a and phytoplankton biovolume were studied from monthly samples collected at different 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 composition (and main phytoplankton groups) were different. Valparaiso was highly dominated by diatoms and Lake Sanabria by cryptophytes and small chlorophytes. In spite of the fact that both sites were nitrogen limited, heterocystous cyanophytes (Anabaena sp.) were detected only in Valparaiso. The relationships between phytoplankton structure and trophic level, hydrological conditions and nitrate content are discussed.
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