Benthic diatoms from the saline-marshes 

of Guérande


The saline-marshes in the Guérande peninsula:

The Guérande peninsula is located in the southeast of Brittany, northward of the estuary of Loire. For fifteen centuries, people have fitted out this zone of swamps situated at the level of the ocean to produce the sea salt by natural evaporation of the sea water. At the present time, the surface occupied by the saline-marshes is around 2000 hectares.
From the open sea to the rectangular pools
("oeillets") were the salt crystallizes, the salinity increases progressively until 27%.

The samples :

Nine samples of the surface of the sediment and of the water which covers it (only some cm) were collected on october 2nd 2008, in a "bondre", in a tidal reservoir, in a neglected tidal reservoir, in several "fares" and in an "oeillet". The salinity was not measured at the different points of collect.

After dilution with 0.5 volume of sea water, the samples ware shaken deeply and the diatoms were separated from the sediment by 2 or 3 differential sedimentations. The frustules were treated and prepared according to Loir (2004a et 2004b ; Bibliography).

Résults :

None diatom was present in the sample collected in the "oeillet". In the other samples a total of 87 taxa was found (see list of taxa). In each sample, between 15 and 48 taxa were observed. 
According to the samples, between 10% (neglected tidal reservoir) and 20% (tidal reservoir) of the taxa were stenohaline marine species. Between 40% (tidal reservoir) and 61% ("fare") were euryhaline marine species able to support, according to the species, a salinity under 30-33 
or an higher salinity. Between 22 and 26% were brackish-water species. Between 0% ("fare") and 13% (tidal reservoir) were euryhaline freshwater species. Two stenohaline freshwater species were found in the tidal reservoir.

In the whole of the samples, the genera Nitzschia (14 taxa) and Navicula (13 taxa) were dominant. The genera Amphora, Cocconeis and Gyrosigma were represented respectively by
5, 4 and 5 taxa.

In the sample collected in one "fare", many Nitzschia sigma and Nitzschia rigida cells diplay fine expansions, often in the shape of small sticks with a length around some µm. These sticks could be either bacteria or fungi. 


Many marine species and brackish-water species present in the saline-marshes of Guérande are presented elsewhere in this site (see Sublittoral zone of Brittany and Eulittoral zone of Brittany). Some other taxa are presented in :

Diatoms from the saline-marshes of Guérande 1
Diatoms from the saline-marshes of Guérande 2

Conclusions :

Because of the small number of samples and because they have been collected only once, in autumn, our data are obviously partial and temporary. In samples obtained in the saline-marshes of Guérande between march and june 1980, Y. Rincé et J.M. Robert (see Bibliography) have found 55 species. Only 17 are present among the 87 species that we have listed. In such environments where the salinity and the temperature vary strongly, the planktonic and benthic populations undergo pronounced qualitative and quantitative variations.

During the Summer, the salinity in the tidal reservoirs is around 4-4.5% and in the "fares" around 5.5-20%. Likely, at the time of our sampling it was clearly lower because of previous rains. This could explain the presence of euryhaline freshwater species.

Three euryhaline species present in the saline-marshes during the Spring and in our samples are remarkable (Rincé & Robert 1980). Gyrosigma balticum and Nitzschia sigma var. sigmatella are able to support variations of salinity from 0 to 6.1% and 0 to 9.6%, or even 16.8%, respectively (holeuryhaline species), while for Navicula ramosissima the supportable variation of salinity is from 2 to 9.6% (polyhaline species).

return to communities