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Corresponding authorAs a result of the deliberate introduction of Undaria pinnatifida (Harvey) Suringar into the Rance Estuary for aquaculture purposes, this kelp has now established itself naturally in the coastal area around St Malo. A population of U. pinnatifida in the Dinard area was subjected to a monthly monitoring programme, conducted in the lower eulittoral zone during low water of Spring tides. Tagging of thalli and punching of holes on the blades were methods used to estimate growth rate. Five parameters were measured: total length, stipe length, midrib width, sporophyll number and sporophyll width. During the 20 months of this study, 4 series of thalli (two per year) developed successively, in spring and autumn. Thalli appeared throughout the year, although very few were present during a period of three weeks, from the end of August to mid-September. Maximal daily growth rates occurred in MarchϪApril (21 mm/day) for spring thalli appearing from January to March, and in November (18 mm/day) for those appearing in October. The highest growth rate was observed both in the spring rise and autumnal fall of temperature and illumination. Thalli that appeared in autumn and winter produced the widest sporophylls. Thalli that appeared in spring and early summer gave rise to sporophylls less wide but more numerous. The maximal life-span noted in the field was 222 days (about 7.5 months); but most had a life-span of about 6 months. Maximal size observed was 1.90 m, with 1.470 g wet weight and 190 g dry weight.

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Zonation of algae and sessile invertebrates in the Gl6nan Archipelago. Biologic definition of bathymetric zones. The GI6nan Archipelago was investigated, from the surface to a depth of 60 m, by SCUBA diving. Four distinct communities ("peuptements") have been determined that fit into the system of zonation defined by the Genoa Symposium (1957). The results are compared with observations made in the English Channel (CA~IOCH 1968) and in the Atlantic Ocean (GLEMAI~C 1971). Since illumination depends upon the angle of the rocky face, photophilic species exhibit a maximum range when the rock surfaces are horizontal. The sublittoral zone is dominated by laminarians. They form two distinct communities with the animal and vegetal species they overshadow. The upper community is characterized by Laminaria digitata (LINNI~) L~ouRovx (horizontal rocky face from 0 to 6 m; vertical ro&y face from + 0.5 to --3 m). The lower cummunity, characterized by L. hyperborea (Gv~,mER) FOSLIE has a greater vertical extension (horizontal: --6 m to --26 m; vertical: --3 to --30 m). The circalittoraI zone is colonized by several animal species growing in a ramified manner: the upper community is characterized by Axinellidae (--30 m), the lower by DendrophyIlia eornigera (LAMAgCX) (--55 tO --60 m). Upright algae disappear somewhere between--40 and --47 m.

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