Marine benthic diatoms from Marie-Galante

(sublittoral zone)

°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°

Collection of the samples and analysis.

Four samples were collected during may 2005 and six during march 2007, by schnorkelling at depths between 3 and 5 meters. Four were collected at "Anse canot" (NW of the island), 2 at "Petite Anse" (SE of the island), 1 at "Grande Anse" (SE of the island) and 3 at "Saint Louis" beach (W of the island).
CarteMarGal
Using 50 ml Falcon tubes, the surface of the sand was scraped on a thickness of 3-4 mm. Samplings were done in the 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.

A  total of 137 taxa was observed in the 4 samples obtained in 2005, representing 41 genera. A  total of 189 taxa was observed in the samples obtained in 2007, representing 56 genera ; 154 of these taxa and 21 of these genera were not observed in 2005. So, a total of 295 taxa and of 62 genera present on the sandy sediments of the sublittoral zone of Marie-Galante was listed. 

The  genera the most represented in each of the two series of samples and in the whole are the followings :

May 2005 March 2007 Total
Genera 41 56 62
Taxa 137 189 295
Mastogloia * 39 (28.4 %) 38 (20 %) 68 (23 %)
Nitzschia 13 (9.3 %) 22 (11.6 %) 31 (10.6 %)
Diploneis 8 (5.8 %) 4 (2.1 %) 13 (4.3 %)
Navicula 8 (5.8 %) 6 (3.2 %) 14 (4.8 %)
Amphora 7 (5 %) 22 (11.6 %) 27 (9.2 %)
Cocconeis 7 (5 %) 22 (11.6 %) 25 (8.4 %)


Twenty six genera were represented by only one species

The species.

Pictures of some of the collected species are presented in :

 Genus Mastogloia (1)
Genus Mastogloia (2)
 Genera Actinocyclus to Climacosphenia
Genera Cocconeis  to Navicula
Genera Nitzschia to Tryblionella


Thirty nine taxa belonging to 15 genera were not identified. Some are presented in :

Non-identified species of the genera Mastogloia
Other genera, non-identified species


Thirty taxa belong to the 17 araphid genera Ardissonia (2 taxa), Climacospheniaon (1 t.), Delphineis (4 t.), Diplomenora (1 t.),  Fragilaria ( 1 t.), Grammatophora (4 t.), Hyalosira (1 t.), Hyalosynedra (1 t.), Licmophora (5 t.), Licmosphenia (1 t.), Opephora (1 t.), Plagiogramma (1 t.), Podocystis (1 t.), Psammodiscus (1 t.), Rhabdonema (1 t.), Raphoneis (2 t.) and Toxarium (2 t.). Often, they were little present.

The 2 monoraphid genera Achnanthes (3 taxa) and Cocconeis (23 taxa) represented also a low percentage of the frustules.

The biraphid genera were dominant and among them, particularly, the genus Mastogloia. Being represented by 69 taxa (see the list, Loir & Novarino 2013 and figures), it constituted 84 and 67 % of the valves present in 2 samples from "Anse Canot", 53 % of those in a sample from "St. Louis " beach (72 % if Actinocyclus gallicus, which represented 26 % of  the valves, was not taken into account) and 40 % of the valves in a sample from "Petite Anse".

The genera Nitzschia, Amphora, Navicula, Diploneis, Pleurosigma and Lyrella were more or less present in the various samples. The five last genera presented a lower specific diversity than in the sublittoral zone of South Brittany

Conclusions

The data obtained with the ten samples collected in the first meters of the sublittoral zone of Marie-Galante point out that the diatom communities, present during march and may on the sandy sediments along the SE, W and NW coasts of this island, are characterized by a high generic (62 genera) and specific (293 taxa) diversity and by the dominance and the specific diversity of the genus Mastogloia.
 
The analysis of the populations collected, on one hand in may and on the other in march (at a two years interval), on the same areas, shows differences in the species. Eighty one per cent of the species listed in march 2007 were not found in May 2005. For the genera Amphora, Cocconeis and Nitzschia especially, the number of taxa was higher in march than in may. We suggest that, although these differences could be due to the low number of samples, they likely reflect variations throughout the year.

Taking into account the species collected, it appears that the diatom communities that we have obtained at Marie-Galante (in march and may), at Basse-Terre (in january and march) and at Martinique (march-april) differ noticeably. 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 predominant genera in lesser Antilles :
                       
  Marie-Galante Guadeloupe Martinique
Genera 62 98 87
Taxa ** 295 540 560
Mastogloia 68 (23 %) 57 (10.6 %) 60 (11 %)
Nitzschia 31 (10.4 %) 71 (13.1 %) 62 (10.5 %)
Amphora 27 (9.2 %) 78 (14.5 %) 79 (13.4 %)
Cocconeis 25 (8,4 %) 49 (9.1 %) 34 (5.8 %)
Navicula 14 (4.7 %) 39 (7.2 %) 51 (8.7 %)
Diploneis 13 (4.4 %) 25 (4.6 %) 28 (4.8 %)

** See the liste of taxa


Return to top of the page
Return to communities of diatoms
Return to tittle page