Limnetica 40

Surface sediment samples from the Negro River, northern Patagonia, were examined to identify major environmental variables that influence the diatom distribution. Assemblages were highly diverse and heterogeneous and its community structure responds to a salinity gradient. Cluster analysis identified two major diatom zones: the former grouped sites with marine influence and the latter freshwater ones. Partial RDAs analysis indicates that salinity was the main factor affecting diatom communities. Riverine sites assemblages were dominated by tychoplanktonic fragilarioids, while estuarine sites were characterized by marine-brackish taxa. The occurrence of marine taxa in the freshwater estuary shows the maximum intrusion of marine biogenic particles at a more landward position. Results showed that the highest nitrate concentration is found at the lower valley of the river. Ecological preferences of dominant and less represented taxa also suggest the existence of anthropogenic processes that would be affecting the water quality of the river.