Numerous papers have been published on the animal and algal communities of fresh-water and marine biotopes, and their relation to the physical and chemical factors of their environment. Brackish-water biotopes have been studied much less intensely, presumably due to their limited distribution and to the complexity of their bioco- enoses, composed of fresh-water, brackish-water and marine organisms. The reduced size of the species present forms a supplementary difficulty, which is especially pronounced in the algal communities, a major part of which are composed of nanno-phytoplankters. The...