The structure of the phytoplankton communities on a stretch of the Jacuí River and some chemical and physical variables of its aquatic environment were studied during July 1986. The sampling zone was divided into three sections: the area under direct influence of the industrial wastes (mineral coal residues), and the areas upstream and downstream, trying to get a preliminary environment evaluation. According to the local organic and industrial pollution, changes on the structure of the communities and abiotic factors were expected, but they were not confirmed since the specific diversity of the communities, as well as some chemical and physical variables analysed did not show significant differences among the sections studied, probably due to the great discharge of the river and, consequently, high dilution power. Considering the cells density, Bacillariophyta, Chlorophyta and Cyanophyta were the representative groups. One hundred and thirty taxa were identified, six of them being new records for the State.
phytoplankton communities structure; chemical and physical variables; coal wastes influence