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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).
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).
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 %) | 64 (21 %) |
| 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 %) |
The species.
Pictures of some of the collected species are presented in :Thirty nine taxa belonging to 15 genera were not identified. Twelve are
presented (hoping that at least some ones could be named ; contact :
maurice.loir(at)orange.fr) in :
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), 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
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), 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 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 | |
| Genres | 62 | 96 | 87 |
| Taxons | 295 | 405 | 560 |
| Mastogloia | 64 (21 %) | 28 (7 %) | 60 (11 %) |
| 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 %) |