In estuaries, salty swamps and most of the brackish waters generally, the salinity can vary more or less quickly, as a result of the tide, of the pluviometry and of periods of sunshine. The species which live in these waters thus have to be euryhaline, that is that they have to support, daily or not, variations of salinity, sometimes of big amplitude. These species, more or less euryhaline, have three origins. Some ones are generally met in sea (¨euryhaline marine species¨); others are present usually in freshwaters (¨euryhaline freshwater species¨); some ones (¨brackish species¨) are considered as limited to waters the salinity of which is situated between 0,5 and 30 %o (or in very mineralized continental waters).

Benthic diatoms are either motile or fixed to various substrates  (minéral, végétal or animal). In this last case, some species adher tightly to the support (Genera Cocconeis, Achnanthes ...). Other species are fixed by a mucous peduncle either branched or not to plants (epiphytic species) or, for some species on invertebrates (epizotic species). It is the case for diatoms belonging to the genera Falcula, Pseudohimanthidium et Protoraphis.
Benthic diatoms form on various substrates and on plants organic films which are grazed on by various invertebrates (sea urchins, gastropods...). Also, - but this case is trivial- they can be catched by big ciliated Protozoa. When benthic diatoms are suspended in the water by the waves or by the currents, they are catched by invertebrates which filtrate the water for feeding (barnacles, bivalves ...).

In addition, in samples collected in estuaries, stenohaline marine and freshwater diatom species are also found. However, the fact of finding, after preparation of the samples, frustules or valves of these stenohaline species does not allow to know if they were alive or not in situ. They could have been brought by tidal streams or by brooks. 


The species illustrated below are characteristic of brackish littoral zones.

Marine diatoms 1 : genera Achnanthes to Berkeleya

Marine diatoms 2 : genera Biremis to Diploneis

Marine diatoms 4 : genera  Navicula and Nitzschia

Marine diatoms 5 : genera Oestrupia to Trachysphenia

Marine diatoms 6 : the genus Haslea

Euryhaline marine diatoms 1 : genera Achnanthes to Diploneis

Euryhaline marine diatoms 2 : genera Entomoneis to Nitzschia

Euryhaline marine diatoms 3 : genera Opephora to Tryblionella

Brackish marine diatoms 1 : genera Amphora to Navicula

Brackish marine diatoms 2 : genus Nitzschia