Abstract
The effects of heavy metals on the structure of phytoplankton and zooplankton communities were determined in Sado River, Portugal, during the period of highest production (summer). Two tributaries, receiving wastes from pyrite mines, enter the river and gave rise, in the middle reaches, to dissolved metal concentrations up to 88 μg l −1 Cu, 2.6 μg l −1 Cd and 1800 μg l −1 Zn. At the most polluted sites phytoplankton responded mainly by a shift towards metal resistant species: dominant species such as Rhodomonas minuta, Synedra ulna, Crucigenia tetrapedia and Stephanodiscus hantzschii were replaced by Gomphonema parvulum, Scenedesmus armatus and Nitzschia frustulum, while the total algal standing crop was not significantly affected. In contrast, zooplankton displayed a strong reduction in the abundance of major groups (40–100%), species richness (59%) and diversity (58–75%), and only a few taxa, namely Acanthocyclops robustus, Arcella vulgaris, Philodina sp., seemed to tolerate high metal concentrations. At the lower river reaches, where metal contamination decreased substantially, a recovery in species composition and abundance was observed for both plankton communities.
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