Sub-littoral zone of southern Brittany
°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°Collection of the samples and analysis
Twenty six samples were collected by diving during the may-august period, in 2003, 2004 and 2005, around the "Ile aux moutons" (southern Finistère) and around the "Ile de Groix" (Morbihan), between 5 and 23 meters deep (middle water spring tide). Six samples were collected around this last island, during july-august 2006, seven ones during august-september 2007 and nine during july-september 2008.
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.
Brown area on a sandy bottom, due to the présence of a dense biofilm :
The biofilm is either homogenous or distributed in small patches :
After dilution with 0.5 volume of sea water, the samples were shaken deeply and after some differential sedimentations to separate the diatoms from the sediment, the frustules were treated and prepared according to Loir (2004a and 2004b ; see Bibliography).
The species present in each of the 2003-2005 samples were identified. For 10 samples, counting of about 1 000 valves has allowed to know the relative presence of every species.
To analyse the results, the 26 samples were gathered in the 6 following groups, according to their geographical origin and to the depth :
A : Groix, various sampling sites, depth : 5-7 meters (5 samples),
B : Ile aux moutons, various sampling sites, depth : 9-10 meters (3 samples),
C : Groix, one sampling site, depth : 9 meters (5 samples),
D : Groix, one sampling site, depth : 18 meters (4 samples),
E : Groix and Ile aux moutons, various sampling sites, depth : 20-23 meters (4 samples),
F : Groix and Ile aux moutons, various sampling sites with different granulometry of the sediment, depth : 6-14 meters (5 samples).
B : Ile aux moutons, various sampling sites, depth : 9-10 meters (3 samples),
C : Groix, one sampling site, depth : 9 meters (5 samples),
D : Groix, one sampling site, depth : 18 meters (4 samples),
E : Groix and Ile aux moutons, various sampling sites, depth : 20-23 meters (4 samples),
F : Groix and Ile aux moutons, various sampling sites with different granulometry of the sediment, depth : 6-14 meters (5 samples).
The samples collected in 2006, 2007 and 2008 were not analysed in detail, but only to list taxa which were not previously observed.
Obtaining of carrots of sand.
In 2007, during the period aout-september, 8 carrots of sand were obtained between -5 and -14 metres by means of the device illustrated below, realized with two 50 ml syringes.
Once the body of the device was pushed about 3 cm in the sand, the spoon in stainless steel is introduced below to allow to remove the syringe without to disturb the carrot, after which, the small piston is introduced into the syringe and pushed entirely in the tube.
Either immediately, or approximately 1.5 hours later (the device was then stored vertical to the darkness), the superior 20 mm of every carrot were split, by means of a wide blade, in 5 slices : 2 superiors with a thickness of 3 mm, then 1 of 4 mm, and 2 of 5 mm. Every slice was put in 10 ml of sea water. After deep shaking, then sedimentation of the sand, the supernatant was removed and let at rest for at least 24 hours. The obtained pellet was stored in 1ml of 4 % of formalin in water.
Sands were dried and and the grain size distribution was determined (classification of Larsonneur, on 1977, see bibliography). In five carrots, (among which 3 were taken in the site C; see above), the diameter of the grains was between 0,2 and 1 mm (fine/medium sand), in another one it was between 0,5 and 2 mm (medium/coarse sand) and in the both last ones, it was between 0,5 and 4 mm (coarse sand).
Charaterization of the diatom communities
A total of 512 taxa was observed in the 48 samples. Twenty eight genera were represented by only one species. The genera represented by the greatest number of taxa were :Amphora Ehrenberg : 73 taxa,
Diploneis Ehrenberg : 47,
Cocconeis and Amphicocconeis Ehrenberg : 45,
Navicula Bory : 39,
Nitzschia Hassall : 37.
Diploneis Ehrenberg : 47,
Cocconeis and Amphicocconeis Ehrenberg : 45,
Navicula Bory : 39,
Nitzschia Hassall : 37.
Thirty four taxa were present in at least the half of the twenty six samples obtained in 2003-2005. Seven taxa were present in at least all the samples minus one of each of the 6 groups A to F. They are :
Cocconeis scutellum Ehrenberg (varieties scutellum, parva et minutissima) : 25/26 samples
Donkinia recta Grunow, var. intermedia Peragallo : 25/26
Plagiotropis lepidoptera (Gregory) Kuntze: 24/26
Trachyneis aspera (Ehrenberg) Cleve : 24/26
Grammatophora oceanica (Ehrenberg) Grunow : 22/26
Podosira stelligera (Bailey) Mann : 22/26
Incertae sedis 4 : 23/26 (see Incertae sedis)
The number of taxa present in one
sample varied between 34 and 140 and the total number of taxa present
in one group varied between 175 (group F) and 243 (group C). Donkinia recta Grunow, var. intermedia Peragallo : 25/26
Plagiotropis lepidoptera (Gregory) Kuntze: 24/26
Trachyneis aspera (Ehrenberg) Cleve : 24/26
Grammatophora oceanica (Ehrenberg) Grunow : 22/26
Podosira stelligera (Bailey) Mann : 22/26
Incertae sedis 4 : 23/26 (see Incertae sedis)
The three groups A, D and E correspond to samples collected at Groix (excepted one sample in group E), at 5-7 meters deep (A) and at 18-23 meters deep (D et E). For these groups, the number of taxa present in one sample was equal to 211, 229 and 184 respectively and the mean number of taxa observed in one sample was equal to 73, 101 and 74 respectively.
In each of the ten samples for which a counting of the valves was carried out, one of the following genera was dominant, representing between 16 and 47 % of the taxa : Amphora, Diploneis, Donkinia, Navicula and Nitzschia + Bacillaria. The 5 dominant taxa in a sample represented together between 24 and 52 % of all the present taxa. Usually, these dominant taxa were not the sames in the different samples. Nevertheless, the following taxa were among the 5 dominant ones in several samples : Nitzschia spathulata (7/10 samples), Nitzschia linkei ? (4/10), Bacillaria socialis, var massilensis (4/10), Navicula arenaria, var. arenaria (3/10), Donkinia recta, var. intermedia (3/10) and Incertae sedis 1 (2/10; see Incertae sedis).
After their distribution in the 6 groups of samples, Amphora costata, Toxarium undulatum and Rhoicosigma compactum would be species with a preference for reduced light, while Bacillaria socialis, var. massilensis, Navicula palpebralis, var. palpebralis and Rhabdonema adriaticum would have a preference for a mean light/depth and that Amphora costata would be a photophile species.
The species
Among the 488 observed taxa, 71 ones belonging to 16 genera were not identified. It was the same for 14 other taxa belonging to 13 genera which also were not identified. Pictures of some of these taxa are presented in :
About 15 observed taxa belong to the genera Actinocyclus, Coscinodiscus, Roperia and Thalassiosira, which are common in the plankton. Most often, they were rarely observed in the samples, with the exception of Actinocyclus octonarius which represented 9 and 10 % in 2 samples (C and D groups) and of Thalassiosira punctigera which was more seldom, but was present in 10 samples. As already observed (Hendey, 1964 ; see Bibliography), Actinoptychus senarius, Paralia sulcata and Podosira stelligera were often observed (in 11, 19 and 22 samples respectively) but they represented rarely more than 1 to 2 % of the valves.
About fourty araphid taxa were observed. They were more or less present. The genus Grammatophora was the more represented, G. oceanica and G. serpentina being present in 22 and 18 samples respectively and the first one representing 9 % of the valves in one sample. Psammodiscus nitidus, Fragilaria investiens and Hyalosynedra laevigata, were present in 18, 9 and 10 samples respectively. They represented no more than 6, 14 and 7 % of the valves respectively, each in a different sample. The genus Licmophora, represented by 10 taxa, represented 9 and 5 % of the valves in 2 samples.
The 2 monoraphid genera, Achnanthes and Cocconeis were represented by 13 and 45 taxa respectively. Usually, the Achnanthes taxa were not often present (1 to 7 samples / 26), with the exception of Achnanthes sp. cf. brockmannii (16 / 26 samples ; see Achnanthes brockmannii). Twenty Cocconeis taxa were present in at least 7 samples, twelve others were well represented in 1 to 6 samples , while for thirty others, only one or two valves were observed (Loir M., 2007 ; Bibliographie).
The biraphid species were dominant. It may be supposed that their motility allows them to be not definitively recovered by sand when the sediment moves.
Some of the taxa that we have observed would be known -as much as I know at the present time- only from the Mediterranean sea and/or from warm waters. These are (between brackets, the number of samples in which each species was observed) :
Amphora biggiba Grunow, var. interrupta Grunow (11)
A. bioculata Cleve (7)
A. cuneata Cleve (12)
A. egregia Ehrenberg, var. ininterrupta Peragallo (3)
A. formosa Cleve (1)
C. nummularia (Greville) Peragallo (1)
C. fluminensis (Grunow) Peragallo (5)
Cymatoneis circumvallata Cleve (2)
C. sulcata Gregory (5)
Dimerogrammopsis furcigerum (Grunow) Ricard (2)
Diploneis beyrichiana A. Schmidt (5)
D. bombus Ehrenberg, var. kützingii (Grunow) Hustedt (2)
D. weissflogii (A. Schmidt) Cleve (5)
Fallacia ny (Cleve) D.G. Mann in Round et al. (2)
F. reichardtii (Grunow in Van Heurck) Witkowsky et al. (3)
F. versicolor (Grunow) D.G. Mann in Round et al. (2)
Lyrella polysticta A. Schmidt, var. circumsecta Grunow (2)
Mastogloia erythraea Grunow, var. anocellata Peragallo (1)
M. subaffirmata Hustedt (1)
Navicula archibaldiana Foged (1)
N. salva A. Schmidt (8)
Thalassiophysa hyalina (Greville) Paddock & Sims (9)
Toxonidea balearica Cleve (1).
A. bioculata Cleve (7)
A. cuneata Cleve (12)
A. egregia Ehrenberg, var. ininterrupta Peragallo (3)
A. formosa Cleve (1)
C. nummularia (Greville) Peragallo (1)
C. fluminensis (Grunow) Peragallo (5)
Cymatoneis circumvallata Cleve (2)
C. sulcata Gregory (5)
Dimerogrammopsis furcigerum (Grunow) Ricard (2)
Diploneis beyrichiana A. Schmidt (5)
D. bombus Ehrenberg, var. kützingii (Grunow) Hustedt (2)
D. weissflogii (A. Schmidt) Cleve (5)
Fallacia ny (Cleve) D.G. Mann in Round et al. (2)
F. reichardtii (Grunow in Van Heurck) Witkowsky et al. (3)
F. versicolor (Grunow) D.G. Mann in Round et al. (2)
Lyrella polysticta A. Schmidt, var. circumsecta Grunow (2)
Mastogloia erythraea Grunow, var. anocellata Peragallo (1)
M. subaffirmata Hustedt (1)
Navicula archibaldiana Foged (1)
N. salva A. Schmidt (8)
Thalassiophysa hyalina (Greville) Paddock & Sims (9)
Toxonidea balearica Cleve (1).
Cocconeis schmidtii Heiden in Heiden & Kolbe (5/26) and Cocconeis scutellum Ehrenberg, var. minutissima Grunow (23/26) are known from arctic seas, Amphora holsaticoides Nagumo & Kobayasi (5/26) from the baltic sea and from the japanese coasts.
Lunella ghalebii Witkowsky (1/26), Navicula fortis Gregory (2/26), Navicula longa Hustedt (6/26), Nitzschia macilenta Gregory (16/26), Pinnularia claviculus (Gregory) Rabenhorst (1/26), Planothidium lilljeborgei (Grunow) Witkowsky (3/26), Progonia musca Gregory (12/26) et Toxonidea insignis Donkin (6/26) are considered as being relatively rare.
Three samples collected in July 2005, at the east of Ile de Groix, at depths between 8 and 10 meters, contained numerous resting spores, which likely, after their morphology, corresponded to Chaetoceros or/and Bacteriastrum species.
Distribution of diatoms in the sand
In
the 8 carotts obtained in 2007, "empty" frustules (no living cell) were present in
the 15 to 20 first mm of sand.It was always in the 3 first mm of sand that living diatoms were the most numerous and the most diverse ; their concentration decreased then more or less fast. In 2 carrots of fine/medium sand, alive diatoms were observed only in the superior 6 mm whereas in the 3 others, they were present up to 10 mm deep. In 2 of 3 carrots of medium/coarse or coarse sand, alive diatoms were observed up to 15-20 mm deep.
In the majority of carrots, it has not appeared a differential distribution of the diatom species according to the depth in the sediment. The species observed under the first 3 mm of sand were often diverse small-sized species present in the 3 first mm.
However, in the carrot of medium/coarse sand, alive diatoms present between 6 and 20 mm were essentially active colonies of 1 to 3 tens of Bacillaria paxillifer cells. Between 3 and 6 mm this species also dominated, whereas it represented a lesser proportion of the diatomic population present in the 3 first mm (approximately 8 % of the totality of the alive species).
On an american coast, it was observed (Round, 1979, see bibliography) that the diatoms present on the surface of the sand, were mobile species whereas, a little more deeply (between 4 and 7 mm), lived little mobile diatoms enfeoffed in grains of sand (strictly epipsammic species). Other works (reported by Riaux-Gobin, 1997, see bibliography) indicate that small and mobile species occupy the surface of the sediment (mud), whereas bigger and more little mobile species, but also, adapted to the anoxia and to the weak illumination (capacity in the heterotrophy), are present some mm more deeply.
These studies concerned sediments of the littoral zone. Our first observations concerning sub-littoral sands do not suggest such micro distributions. The observed presence of Bacillaria paxillifer in the depth of the sediment suggests that this euryhaline species could have a capacity in the heterotrophy, although we cannot exclude that the mobility of its colonies allows it to make relatively brief raids in the depth of the sediment.
The study of the micro distribution of diatoms in sandy sediments is made complex by the fact that several factors interfere. Let us quote among others the composition of the diatomic population which undergoes temporal variations, the granulometry, the compaction and the mineralogical composition of the sand, as well as its content in organic matter.
Besides, the fact that alive diatoms were observed between 15 and 20 mm deep in carrots of medium/coarse or coarse sand, the porosity of which facilitates the circulation of the water, asks the question of a possible artefactual displacement of the diatoms after obtaining of carrots.
Conclusion
The diatom communities living on the sandy sediments present in the sub-littoral zone of southern Brittany are characterized by the quasi-permanent presence during the may-august period of 7 taxa and they appear to be particularly rich in various species (about 500 taxa). This is due to the fact that nutrient substances such as phosphates and nitrates are there in greater concentration than in the overlying sea water, as organic materials (feces, dead invertebrates,...) and fragmented algae present on the sand quickly disintegrate and mineral components are freed by bacteria.
In southern Brittany, the inferior limit of the sub-littoral zone is around 15-20 meters deep. In the case of the stations 18 and 23 meters deep where we collected samples, no laminarians were present on the rocky surfaces in the vicinity of the considered sandy places. The limit for these macroalgae was some meters above. So, the 18-23 meters deep stations were in the upper circalittoral zone, as defined in biological terms (Hiscock & Mitchell, 1980).
The critical depth for the growth of laminarians is at about 1% of surface illumination. Hendey (1964) has proposed that the depth at which diatoms live, normally does not exceed 10 meters. Our results point out that, at least on sand, the diatom communities are as rich in species at a depth of 18-23 meters (groups D and E, 229 and 184 taxa respectively) as at a depth of 5-7 meters (group A, 211 taxa) and that the 7 above-mentioned taxa are present in the whole height of the sub-littoral zone. We know now that benthic diatoms possess, among various physiological adaptations to the environment, a capacity to heterotrophy which allows them to survive in a reduced light or in the darkness under some millimeters of sand (Admiraal & Peletier, 1979, Riaux-Gobin, 1997 ; Bibliographie).
About twenty species known from warm seas would be present ine the sub-littoral zone along the coasts of Brittany. This would need to be confirmed.
Among the species that I have not named, some ones are likely new species for the science (cf. non-identified taxa). For instance, it is the case for six Incertae sedis taxa (see Incertae sedis) and for two taxa belonging to the genus Achnanthes .