Ana I. Negro

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 phytoplankton population in a natural oligotrophic lake (Lake Sanabria) and a mesotrophic reservoir (Valparaíso). 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 (Valparaíso). Phytoplankton biovolume and chlorophyll a concentration were about five times higher in Valparaíso than in Lake Sanabria. Species composition (and main phytoplankton groups) were different. Valparaíso 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 Valparaíso. The relationships between phytoplankton structure and trophic level, hydrological conditions and nitrate content are discussed.

Diatom and desmid relationships with the environment in mountain lakes and mires of NW Spain

The mountain ranges in NW Spain have a large number of little known wetlands. We report the results of a study carried out on a group of 77 small lakes and mires in the Sierra Segundera and Cordillera Cantábrica. The main physical and chemical variables, and phytoplankton communities from littoral samples were studied. Cantabrian wetlands showed greater variability in all environmental variables measured as well as higher values in those related to mineralisation than the Segundera ones. Many of these ecosystems were oligotrophic and showed a high species richness. Desmids and diatoms were the two most abundant groups, both in the species number and in biovolume. Desmids were the most numerous group in taxa in Sierra Segundera, whereas diatoms were in Cordillera Cantábrica. Differences in species composition of algae communities between both mountain ecosystems were studied. Canonical Correspondence Analysis (CCA) was carried out on diatom and desmid flora composition. This analysis showed that alkalinity was the most important parameter in diatom distribution and pH the most important one in that of desmids.