COMMUNITY STRUCTURE AND FROND SIZE DISTRIBUTION OF A DEEP-WATER STANDOF CYSTOSEIRA-SPINOSA (PHAEOPHYTA) IN THE NORTHWESTERN MEDITERRANEAN

Populations of Cystoseira spinosa, an endemic Mediterranean brown alga , are restricted to deep water at undisturbed sites. The species compo sition of an old stand in the Scandola Marine Reserve (Corsica) at a d epth of 26-29 m reflects the dominance of Cystoseira spinosa over othe r brown and red algae, with an average biomass of 450 g dw m(-2), and a mean density of 28.8 plants m(-2). The length of the main axis was c hosen as a convenient parameter related to size. The number of seconda ry axes increases exponentially with the length of the main axis, sugg esting the existence of environmental constraints (possibly hydrodynam ism), which may impose severe restrictions on the unlimited growth of the main axis. The size pattern of the Cystoseira spinosa plants from the population studied showed a log-normal distribution, with the high est frequency within size classes of 9-15 cm (main axis length), and a maximum size of 44 cm. Areas devoid of adult Cystoseira plants and la cking other erect algae had a significantly higher (8 %) number of rec ruits than undisturbed areas. We suggest that the log-normal size dist ribution found in the population studied is due to an inhibition of re cruitment by erect algae, and that disturbances that remove these alga e are necessary to increase Cystoseira recruitment.