Benthic diatoms from the Seine estuary

Le Havre harbour and Rouen harbour



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. 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.  
Animals and plants present in estuaries 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 freshwater origin tend to dehydrate. Besides the variations of salinity, the species living in these intertidal areas undergo violent variations of the illumination and of the temperature. In addition, they have also to support various pollutions brought by the river.

In the Seine estuary, because of the high range of the tides (about 7 meters), the dynamic component is felt until 150 km from the English Channel (Barrage of Poses, upstream from Rouen). The salinity (> 0,05 %) would be felt only until 40 km from the English Channel (Vieux-Port, between Le Havre and Rouen).
Le Havre harbour and the one of Rouen accommodate ships coming from various regions of the world, which can bring into the estuary exogenous animal and vegetable species.

Collect and analysis of the samples :

Samples were collected, either by diving or not, by Dr. Gérard Breton, at the following dates and stations :

-- A :  30 jan. 2010, Le Havre harbour, dock La Barre, at the surface level; collect of filaments on Ulva lactuca and on Bryozoans,
-- B : 13 mar. 2010, Le Havre harbour, quay La Moselle, 3 meters deep,
-- C : 14 oct. 2010, Le Havre harbour, river dock, collect of filaments present on the bottom, 0.5 meters deep,
-- D : 14 oct. 2010, Le Havre harbour, river dock, collect of sediment under 5 cm of water,
-- E : 16 oct. 2010, Rouen harbour, dock Saint Gervais, collect of sediment, about 2 meters below the lowest water level.

These samples were stored in 95% alcohol then treated according to
Loir (2004a et 2004b ; see Bibliography).

Le Havre harbour

Composition of the collected diatom populations :

The analysis of samples A-D has allowed to list a total of 115 taxa, belonging to 35 genera. Sixteen genera were represented by only one taxus  (see list of taxa).

The most represented genera are :

Genus Nitzschia : 22 taxa,
Genus Amphora : 15 taxa,
Genus Navicula : 13 taxa,
Genus Cocconeis  : 9 taxa,
Genus Achnanthes : 7 taxa.

Number of taxa and dominant taxa in the 4 samples  :

A : 63 taxa. Achnanthes brevipes, Bacillaria paxillifer, Melosira moniliformis var. moniliformis et Navicula agnita (?).
B : 29 taxa. Haslea crucigera.
C : 20 taxa. Achnanthes brevipes, Ardissonia crystallina.
D : 31 taxa. Achnanthes brevipes.

The marine diatoms considered as being stenohaline species represent 50% of all the species. This pourcentage varies between the stations: it is equal to 40% in B and to 63% in C. The euryhaline marine diatoms represent 38% of all the species (27% in C and 50% in B). Ten per cent of the species are brackish-water species.

Exogenous species : only one species (two valves were observed), Amphora kolbei, would not be present on the coasts of the Northeast Atlantic Ocean and of the North Sea. It is  present in the Méditerranean Sea  and in the Pacific Ocean.

Pictures of some of the collected species are presented in :
Diatoms from Le Havre Harbour

The genus Achnanthes Bory de St-Vincent :

Achnanthes brevipes C.A. Agardh var. brevipes and Achnanthes brevipes var. intermedia (Kützing) Cleve are the dominant taxa in the samples C and D. In addition to these taxa, Achnanthes parvula Kützing, Achnanthes longipes C.A. Agardh, Achnanthes javanica Grunow, Achnanthes sancti-pauli (Heiden) Kobayasi & Swatari and one non-identified species, are present in the sample D.
Three tens of frustules and valves observed in the sample D present all the characters of
Achnanthes sancti-pauli (syn. Cocconeis sublittoralis Hendey)) excepted for the number of striae : on the sternum valve it varies from 5-6 to 11 / 10 µm. (diagnoses : 6-8,5)  and on the raphe-sternum valve  from 11 to 12-13 / 10 µm. (diagnoses : 5-8,5).

Rouen harbour

Composition of the collected diatom population :

In the sample collected in October we have found a total of 53 taxa, belonging to 22 genera. Nine genera are represented by only one taxus (see list of taxa).

The most represented genera are :

Genus Nitzschia : 8 taxa,
Genus Navicula : 6 taxa,
Genus Caloneis : 4 taxa.

The species Navicula gracilis  is slightly dominant.

The stenohaline freshwater species represent 50% of all the species and the euryhaline freshwater species, 25%. Species found in brackish water and marine species constitute one quarter of the totality.

Pictures of some of the collected species are presented in :
Diatoms from Rouen harbour


The analysis of the 5 samples allows to establish a first preliminary inventory of the benthic diatoms present in 2 stations, far one from the other, in the Seine estuary.

We do not know how the salinity varies as a function of the tide, in the 2 stations. In the Le Havre harbour, the salinity varies throughout the year around values which most often are around 3,0- 2,5%. These values allow the prolifération of marine diatoms either stenohaline or
euryhaline, even of some species found in brackish water.

It is admitted that the variation of the salinity due to the tide becomes nil far downstream from Rouen. Nevertheless, the stenohaline freshwater diatoms constitute only an half of the population, the other half being constituted by euryhaline freshwater diatoms, brackish-water diatoms and euryhaline marine diatoms. So, it would be interesting to know the mean value of the salinity and its variations throughout the time.

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