Sublittoral zone of Martinique 

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Collection of the samples and analysis.

Twelve samples  were obtained in March-April 2008and fifteen in March-April 2009, at depths between -3 and -9 meters, by snorkelling, as follows (from North towards South) :
- Anse Turin : 1 sample (medium sand),
- Anse à l'Ane : 1 sample (medium sand),
- Anse Noire : 2 samples (fine and medium sand),
- Anse Dufour : 9 samples (fine, medium and coarse sand),
- Grande Anse : 3 samples (fine sand),
- Anse d'Arlet : 5 samples (fine sand),
- Petite Anse : 2 samples (fine sand),
- Anse figuier : 3 samples (fine sand).

Cartemartinique 

The surface of the sediment was scraped on a thickness of 3-4 mm using 50 ml Falcon tubes. Sampling was done in areas where a yellowish-braun film was present. After deep shaking of the samples, the diatoms were separated from the sediment by 2 or 3 differential sedimentations, then fixed with acidic Lugol solution. Later, they were stored in the presence of 2% formaldehyde. The frustules were treated and prepared according to Loir (2004a and 2004b ; see bibliography).


Composition of the collected diatom populations 

In the 12 samples collected in 2008, 466 taxa were listed, représenting 83 genera, 35 of which were représented by only one taxus.
In the samples, the number of taxa varies between 54 et 163.

In 2009, 312 taxa were listed.
In one sample obtained at the Anse d'Arlet at  2,5 meters deep, 240 taxa were observed, 54 of which belonging to the genus Mastogloia. At the opposite, the population collected at the Anse Dufour at  9 meters deep was composed, at about 90 %, by the 2 species Diploneis smithii et Diploneis fusca.

Thhe most represented genera are the followings :
2008 2009 Total
Genres 83 87
Taxons 466 312 588
Amphora 75 (16%) 39 (12.5%) 79 (13.4%)
Mastogloia 50 (10.7%) 61 (19.5%) 89 (15.1%)
Nitzschia 53 (11.3%) 33 (10.6%) 62 (10.5%)
Navicula 44 (9.4%) 23 (7.4%) 51 (8.7%)
Cocconeis 26 (5.6%) 19 (6.1%) 34 (5.8%)
Diploneis 23 (5%) 15 (4.8%) 28 (4.8%)



The species

Pictures of some of the collected species are presented in  :
Genera Actinocyclus to Diploneis

Twenty eight taxa belong to the following centric genera  :
 Actinocyclus (1 taxon), Actinoptychus (1 t.), Anaulus (1 t.), Asterolampra (1t), Asteromphalus (1 t.), Auliscus (1 t.), Biddulphia (2 t.), Coscinodiscus (1 t.), Cyclotella (2 t.), Cymatosira (1 t.), Dimerogramma (2 t.), Dimerogrammopsis (1 t.), Eunotogramma (1 t.), Glyphodesmis (1 t.), Isthmia (2 t.), Odontella (1 t.), Paralia (1 t.), Plagiogramma (4 t.), Stictodiscus (1 t.), Thalassiosira (3 t.) and Triceratium (2 t.).

Forty taxa belong to the following araphid genera :
Ardissonia (2 taxons), Climacosphenia (1 t.), Delphineis (2 t.), Diplomenora (1 t.), Fragilaria (3 t.), Grammatophora (6 t.), Hyalosira (2 t.), Hyalosynedra (1 t.), Licmophora (10 t.), Omphalopsis (1t.), Opephora (1 t.), Perissonoe (1 t.), Podocystis (1 t.), Psammodiscus (1 t.), Rhabdonema  (1 t.), Synedra (3 t.) and Toxarium (3 t.).

The 3 monoraphid genera Achnanthes, Cocconeis and Planothidium are represented respectively by  4, 26 et 4 taxa. Most of these taxa are present in low numbers.

Forty five biraphid genera are represented by 417 taxa.
The genera Mastogloia and Amphora were dominant by the number of taxa. Most of the Amphora taxa were present in low numbers. According to the sample, the representation of this genus varies between 10 and 25 %. Similarly, that of the genus Mastogloia (see the list of taxa) varies between 3 and 18 % and that of the genus Nitzschia between 6 and 22 %.
In one sample collected at the Anse Dufour, it has been observed that some Amphora ocellata (between 2 and 24 cells ; see pictures) were included in mucilaginous bubbles. Some of these bubbles have a short tubular foot. This suggests that they were fixed to sand grains. 


Conclusions

The data that we have obtained point out that the diatom communities, present during march-april on the sandy sediments along the western coast of Martinique, are characterized by a high generic and specific diversity and by the dominance and the specific diversity of the genera Mastogloia and Amphora.

Taking into account the species collected, it appears that the diatom communities that we have collected at Martinique (in march-april) and at Basse-Terre (in january and march) differ noticeably from those that we have collected at Marie-Galante (in march and may).
If the period of collect can explain partly the difference, likely, the mineralogical nature of the sediments (coral origin at Marie-Galante and abrasion of volcanic stones at Basse-Terre and at Martinique) plays an important role in the composition of the two diatom communities.

The dominant genera in lesser Antilles :
                        
  Marie-Galante Guadeloupe Martinique
Genres 62 96 87
Taxons 298 411 588
Mastogloia 66 (22.1 %) 35 (8.5 %) 89 (15.1%)
Nitzschia 31 (10.4 %) 45 (10.9 %) 62 (10.5 %)
Amphora 27 (9.2 %) 68 (16.5 %) 79 (13.4%)
Cocconeis 25 (8.4 %) 33 (8 %) 34 (5.8 %)
Navicula 14 (4.7 %) 29 (7 %) 51 (8.7 %)
Diploneis 13 (4.4 %) 15 (3.6 %) 28 (4.8 %)


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