Open to the South, the Villefranche-sur-Mer bay is surrounded by mountains (North) and rather high hills (the smallest is Semaphore's, on the East side: 150m over the sea level). The Villefranche Bay is a very sheltered site. Prevailing winds come from East, corresponding to depression witch stay on the Ligurian Sea. The area isn't really under the influence of the Mistral, but it is subjected to a more or less strong northern wind during the winter.
This bay is a place historically known as being favourable to the plankton study. One of its distinctive feature is to possess a sill who separates a not very deep basin in the North (13 m) from a pronounced slope witch dive into the Villefranche's canyon. The latter is the submarine extension of one of the Paillon's affluents (Nice river). It is dug by turbidity currents due on the one hand to the continental slope formation, and in the other hand to the Ligurian current, permanent, of witch the general direction is parallel to the coast, going towards the south-east.
Proximity of important depths (>500m) clear off the bay creates a phenomenon allowing capture of the sub-superficial plankton, and it's made use of by the scientists : the liguro-provençal current, with a wind from east, "pushes" the surface water inside the bay, running along its east then west coast. Now plankton, making his vertical migration during the night, ends up on the morning inside the bay, and he remains stuck there, because of the high depths of the sill. Hence an impressive richness and biodiversity of the site, especially during winter, when plankton is concentrated in a thin layer, and so is catched in its entirety in the "trap" (ex : it's possible to study organisms who live usually at 600m down thanks to takings made on 20m depth...).
Daily observation of macroplankton has started in 1898 and carried on until 1917 spurred on by the russian biologist A. de Korotneff. In 1957, hydrologic measures start at the "B point", located at the bay opening (coordinates: 43°41'10''N and 7°19'00''E), at six standard depths (0, 10, 20, 30, 50 and 75m) accepted as representative of the upper water layer. This, in order to provide observation of hydrologic conditions, at the same time of plankton systematic observations witch have been started at that time.