Demographic study of a Cystoseira humilis Kutzing (Fucales : Cystoseiraceae) population in the western Mediterranean

A demographic study of Cystoseira humilis Kutzing present in a tidepool sit uated on the rocky shore south of Livorno (Ligurian Sea) was carried out. I n order to assess density, growth, reproduction, mortality and biomass of t he population, the plants, present in three permanent marked squares which had been randomly chosen, were tagged and monitored over three years. The results suggest that the C. humilis population displayed periods charac terized by high mortality and low recruitment alternating with periods char acterized by low mortality and high recruitment. During the first year of t he study the greatest abundance and growth of plants was recorded in spring , while a population decline was observed in the autumn-winter period. New plants appeared between October and April. Density variations were linked t o seasonal fluctuations in recruitment and mortality. The seasonal trend of biomass was related to changes in density and plant height. In the second year there was a marked decline in the entire population of C. humilis due partly to absence of recruitment and partly to the high mortality rate duri ng the winter-spring period affecting adult and juvenile plants. This high mortality coincided with an appreciable decrease both in frond length and s tem number of the surviving plants, resulting in a marked decrease in bioma ss. In the third year the resulting availability of substratum facilitated recruitment. The development of reproductive structures showed a seasonal t rend throughout the three years of the study. The greatest frequency of rep roductive individuals occurred in July, while in January only sterile plant s were found.