Sublittoral zone of Basse-Terre (west coast)

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Collection of the samples and analysis.

Three samples (A-C) were obtained in January 2006 and five (D-H) in march 2007, at depths between -4 and -6 meters, by snorkelling, as follows :
- Petite Anse (north of Marigot), 3 samples : 1 sample (A) on fine sand, 1 sample  (B) on coarse sand and 1 sample (D) on medium sand,
- Anse Leroux (south of Deshaies) 2 samples : 1 sample (C) on coarse sand and 1 sample (E) on medium sand,
- Petite Anse (North of Pointe Noire), 1 sample (F) on medium sand,
- Anse de Malendure, 2 samples (G and H) on fine sand.
Cartebasseterre 
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 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 3 samples obtained in january 2006 (A-C), 223 taxa were listed, corresponding 68 genera.
In the 5 samples obtained in March 2007 (D-H), 252 taxa were listed corresponding to 74 genera. 192 of these taxa and 28 of these genera have not been  observed in 2006.
So, the 8 samples have allowed to establish a list of 411 taxa and of 96 genera  present on the sublittoral sandy sediments along the western coast of Basse-Terre.

The  most represented genera in each of the two series of samples and in the whole are the followings :
January 2006 March 2007 Total
Genera 68 74 96
Taxa 223 252 411
Amphora 44 (19.7 %) 37 (14.7 %) 68 (16.5 %)
Nitzschia 24 (10.8 %) 28 (11.1 %) 45 (10.9 %)
Mastogloia
 18 (8.1 %)
 21 (8.3 %)
 35 (8.5 %)
Cocconeis
 11 (4.9 %)
 29 (11.5 %)
 33 (8 %)
Navicula 16 (7.2 %) 16 (6.3 %) 29 (7 %)
Diploneis 14 (6.2 %) 3 (1.2 %) 15 (3.6 %)


Twenty seven genera are represented by only one species.

Two observations characterize the four samples A,B,C and D :
- One different species was dominant in each sample. At Petite Anse, Lyrella clavata represented around 98% of the frustules collected on fine sand (sample A), Amphora coffeaeformis represented around 95% of those collected on coarse sand (sample B) and Amphora elegans represented 92 % of those collected on medium sand (sample D) . Diploneis vacillans represented around 90% of the frustules from the Anse Leroux (sample C).
- Most of the other species were small species. In addition, the size of the frustules was frequently near the inferior limit indicated by the diagnoses .

In the four other samples, most of the species were also of small or medium size and, at least for the genus Cocconeis, the frustules had measurements near or below the inferior limit given by the diagnoses.

The species
Pictures of some of the collected species are presented in  :

Several taxa not identified are presented in :


Twenty five taxa belong to the 18 centric genera Actinocyclus (2 taxa), Actinoptychus (1 t.), Anaulus (1 t.), Biddulphia (1 t.), Cyclotella (2 t.), Cymatosira (1 t.), Dimerogramma (2 t.), Dimerogrammopsis (1 t.), Eupodiscus (1 t.), Odontella (1 t.), Paralia (1 t.), Plagiogramma (2 t.), Pleurosira (1 t.), Podosira (1 t.), Terpsinoe (2 t.), Thalassiosira (2 t.), Triceratium (2 t.) et Trigonium (1 t.).

Thirty four taxa belong to the 16 araphid genera Ardissonia (3 taxa), Asterionella (1 t.), Climacosphenia (1 t.), Delphineis (4 t.), Diplomenora (1 t.), Fragilaria (3 t.), Grammatophora (3 t.), Hyalosira (3 t.), Hyalosynedra (1 t.), Licmophora (7 t.), Neosynedra (1 t.), Opephora (1 t.), Podocystis (1 t.), Psammodiscus (1 t.), Rhabdonema  (2 t.) et Toxarium (2 t.).

The 3 monoraphid genera  Achnanthes, Cocconeis andPlanothidium were represented respectively by 9, 33 and 4 taxa. Most of these taxa were present in low number.

Fifty nine biraphid genera were represented by 309 taxa.
The genus Amphora was dominant by the number of taxa, most of which being present in low number. Twenty four taxa among the 37 which were observed in 2007 have not been observed in 2006.
The percentage of all the frustules represented by the genera Nitzschia, Mastogloia and Navicula in the samples of 2006 on one hand and in the samples of 2007 on the other, changes slightly (see the table). However, for each of these genera, the taxa observed in the two series of samples differ conspicuously : for the genus Nitzschia, 20 among the 28 taxa observed in 2007 were not observed in 2006, for the genus Mastogloia (see the list of taxa), it was 17 among 21 and for the genus Navicula it was 14 among 16. 

Conclusions
The data obtained with the eight samples collected in the first meters of the sublittoral zone of Basse-Terre point out that the diatom communities, present during january and march on the sandy sediments along the western coast of this island, are characterized by a high generic and specific diversity and by the dominance and the specific diversity of the genus Amphora. However, in spite of this diversity, four samples contained populations which were, if not monospecific, at least dominated by one species. 
 
The analysis of the populations collected, on one hand in january and on the other in march (at one year interval), shows differences in the collected species. Seventy six per cent of the species listed in march 2007 were not found in january 2006. We suggest that, although these differences could be due to the low number of samples, they likely reflect variations throughout the year (seasonal variations ?).

Taking into account the species collected, it appears that the diatom communities that we have obtained at Basse-Terre (in january and march) and at Martinique (in march-april) differ noticeably from those that we have obtained 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 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 %)