Rocky substrate communities of the infralittoral fringe of the Boulonnais coast,NW France: a quantitative survey

Infralittoral fringe communities were investigated at 26 stations, in all four seasons (summer 1982, spring, autumn, winter 1983), in different habitats. The investigated habitats can be divided into undersurfaces of boulders, cavelike habitats, crevices between rocks, horizontal and vertical surfaces on rocks. The stations were distributed over four locations, all in the district Pas de Calais, arranged in a sequence of increasing exposure to wave action: Boulogne, Audresselles, Ambleteuse and Cap Gris Nez. Data on quantitative distribution of the biota were collected using quadrat sampling techniques; the data were processed by using cluster analysis. Several communities could be distinguished as separate units, viz. those of undersurfaces of boulders, caves, medium-exposed/exposed vertical/horizontal surfaces. A comparison with previous, preliminary quantitative investigations carried out in 1980 revealed that communities from shaded habitats are apparently stable enough to yield completely reproduceable results. When all habitats are compared, 78% of the samples of 1980 were clustered together with corresponding samples of 1982/1983.