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 University of Ghent considered a detailed survey sufficiently attractive to set up because of the high species diversity, the gaps in the knowledge of brackish-water nannophytoplankton at the species and population levels, and the convenient proximity of brackish-water biotopes. Moreover, the brackish-water biotopes proved to be unique, and consist of a series of landlocked creeks, con nected to the sea by means of an underground salt water supply. VII Contents Chapter 1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Chapter 2 Study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. 1. Topographical situation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. 2. Historical aspects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. 3. Edaphic factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. 4. Hydrology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 5 2. 4. 1. Salinity fluctuations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. 4. 2. Origin of the brownish colour of certain creeks. . . . . . . . . . . . . . . . . . . . . . . . 6 2. 4. 3. Hydrography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2. 5. Biotopes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2. 6. Climatic features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Chapter 3 Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3. 1. Sampling procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3. 2. Physical properties, water chemistry, chlorophyll a and phaeophytin a . . . . . . . . . . . . . 11 3. 3. Phytoplankton samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3. 4. Periphyton and benthos samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .