Benthic diatoms from the littoral zone 

of Southern Brittany (brackish waters)

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Introduction

Estuaries, as well as rias, correspond to arms of the sea penetrating in a river valley until the superior limit where the tide leads to a rise of the water level. These mouths are places of accumulation of sediments of river origin, which feed intertidal mud flats (slikke)  appearing as almost flat, grey and gleaming areas.
The slikke is also present in the submersible salty swamps where the schorre represents then the part dressed in a vegetable coverage, covered only during the high spring water tides.
In estuaries, the tide presents a dynamic component which corresponds to the penetration of the wave of tide and a salt component corresponding to the rise of the sea water. This last one induces a gradient of the salinity. In the salty swamps, often gone through by one or several brooks, the effect is the same although it is less marked. Animals and plants present in estuaries and salty swamps thus have to be euryhaline, that is that they have to support variations of salinity of big amplitude. Indeed, in brackish waters, the species of marine origin tend to loss essential body salts whereas those of dulcicole origin tend to dehydrate. Besides the variations of salinity, the species living in these intertidal areas undergo violent variations of the illumination and the temperature.

In the South of Finistere, the triangle Bénodet-Quimper-Fouesnant presents one ria -that of the river Odet- with three long digitations, a salty swamp -the Mer Blanche- gone through by a little consequent brook and presenting a schorre rich in puddles reached by the sea at the time of the high water spring tides, and a micro-estuary -the Anse de Penfoulic- gone through by waters from a ground marsh. It seemed to me interesting to try to establish an inventory of the epipelic diatoms (living on the mud) presents on these areas.  



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Porz guen mud flat on the left bank of Odet

Material and methods

A  -  From february 2000 to May 2004, samples were collected, all the year round, on the 4 following stations :

- ¨Porz Guen¨ : mud flat on the left bank of the river Odet, in 3 km upstream from the ¨Pointe de Combrit¨ (free sea) (7 samples),
- ¨Anse Saint Cadou¨ : mud flat 12 km upstream from the ¨Pointe de Combrit¨ and in 2 km of the main course of the river Odet ; this digitation of   the valley is gone through by a brook (11 samples),
- ¨Mer Blanche¨ : slikke and puddles on the schorre, 1 to 2,5 km from the entry of the swamp (10 samples),
- ¨Anse de Penfoulic¨ : mud flat in 1 km of the entry of this small estuary (10 samples).

The 3-4 first mm of the sediment were taken in a dozen points (50 ml Falcon tubes). In some cases, the mud deposited on stones or dead wood was also collected. The samples were then diluted and shaken several times. Diatoms were separated from the sediment by 2 or 3 differential sedimentations  and stored in the presence of 2 % formalin. After the frustules were prepared according to Loir (2004), the taxa present in every sample were identified.


B-  During june 2016, 3 samples were collected in the Anse de Penfoulic (micro-estuary) in a ten of points. During december 2016, 2 samples were collected at the same place. The samples were treated as indicated above.

Results

A - Only the samples collected from 2000 to 2004 were quantitavely analysed :

Total specific diversity
:


The 38 samples collected on the 4 stations have allowed to list a total of 221 species.
 
Le number of species found on each station was :
Pors Guen : 77
Penfoulic : 61
Mer Blanche : 69
St Cadou : 113

For each of the 4 stations, the percentage of species common to all the 3 others was of :
Porz Guen : 36 %
Penfoulic : 60 %
Mer Blanche : 53 %
St Cadou : 44 %

Composition of the diatom assemblage

We have not realized a quantitative study allowing to identify the dominant species. Only was considered the number of species for the genera represented by at least 4 species in one of the 4 stations:

Genera Porz guen Penfoulic Mer Blanche Saint Cadou 4 stations
Amphora 5 5 4 2 12
Caloneis 0 0 7 2 7
Cocconeis 5 4 1 4 7
Diploneis 0 2 8 9 14
Gyrosigma 6 2 3 7 12
Lyrella 4 3 4 3 8
Navicula 7 5 9 10 19
Nitzschia 7 3 7 19 29
Pinnularia 0 4 2 7 9
Stauroneis 1 2 0 4 5
Surirella 2 3 0 8 11
Synedra 4 2 2 2 7


The species present in the 4 stations are inevitably more or less euryhaline. Some ones are usually met in sea (¨marine species ¨), whereas the others are present usually in fresh water (¨freshwater species¨, see Diatoms of the fresh waters) ; Some ones (¨brackish species¨) are usually found in brackish waters (or in very mineralized continental waters). These three types of species divided up as follows in the 4 stations (in percentages):

¨ marine species¨ ¨brackish species¨ ¨freshwater species¨
Porz Guen 85 5 10
Penfoulic 78 7 15
Mer Blanche 70 8 22
St Cadou 46 4 50


B -  In the 5 samples collected in june and december 2016 120 taxa were present. About forty ones were not found previously. It is the case for 4 Mastogloia taxa.


The taxa observed in the samples A and B are listed:


About 140 taxa present in the samples A and B are illustrated in :

 

Diatoms from the littoral zone

Some non-identified taxa  are illustrated in :

Littoral zone, non-identified taxa

Some taxa were photographed in vivo


Conclusion 

Since Carter (1932), several studies dealed with the epipelic diatom flora of the littoral zones of Western Europa (Round 1960, Hendey 1964, Malissen 1973, Riaux et Germain 1980 ; see Bibliography). In the studied brackish zones (estuaries, mud flats, salt marshes), the number of epipelic species varies around 50 to hundred. The specific variety of the diatom flora of the 4 stations that we have considered is of the same order. Most of the species we have listed are characteristic of brackish littoral zones.
We have not realized maesurements of salinity. Nevertheless, the differences observed by the proportions of marine diatoms and freshwater diatoms in the 4 stations are coherent with 1) the situation of the stations with regard to the free sea and 2) with the relative importance of the contributions of fresh water. Notably, it is likely that St Cadou mud flat, where 50 % of the species are freshwater species, undergoes a salinity much weaker than that of Porz Guen where 85 % of the species are marine species. As for both swamps of Mer Blanche and of Penfoulic, they constitute intermediate situations. Let us mention that in a ditch fed by a source, situated in the Anse St Cadou, 26 (93 %) of the 28 diatom species listed were freshwater species and 2 were brackish water species.

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