Distribution of diatoms in the sand


Obtaining of carrots of sand.

In 2007, during the period aout-september, 8 carrots of sand were obtained between -5 and -14 metres  (Groix island) 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). 

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.

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