Etude biometrique d'une population de laminaria digitata lamouroux de l'etage infralittoral profond 1. - résultats relatifs aux dimensions des thalles

During the year 1964-1965, J. COSSON monitored the growth of a Laminaria digitata population living off the coast of Luc-sur-Mer (Calvados), on Quihot Rock. This population, which emerges during spring tides' low tides, is located at the upper limit of the infralittoral level, that is, in very peculiar conditions for a species that usually lives below this limit. J. COSSON himself, incidentally, pointed out that for this reason, his results could not be generalised. The difference between his own observations and the small number of measurements done by undersea divers on colonies living in the neighbourhood of Quihot Rock, but deeper, leads us to suppose a different development of the species on the surface and down below. It was in order to verify this that, on a Laminaria digitata population located below the level of the lowest seas (so-called "deep" or "sea" population), we repeated research identical to COSSON's on Quihot Rock population (so-called "shore" population). You can imagine that the development of such a programme posed several problems that we would not have been able to resolve without the help of the Algology Laboratory of the Caen Faculty of Science headed by Professor GAYRAL and the support of the Luc-sur-Mer laboratory. We used a team of undersea divers who had accepted to attend a weekly training in order to be able to stay under water for two to three hours in summer as well as in winter. Whereas, in summer, the work was in fact relatively easy, in winter, on the contrary, in water whose temperature did not rise above 6°C, it was extremely hard.