SEASONAL-VARIATION IN TISSUE NITROGEN AND PHOSPHORUS OF CYSTOSEIRA-MEDITERRANEA SAUVAGEAU (FUCALES, PHAEOPHYCEAE) IN THE NORTHWESTERN MEDITERRANEAN-SEA

Biomass, C, N and P content of the different parts of the thallus of C ystoseira mediterranea, and nutrient concentration in the ambient wate r have been monitored twice a month during a period of 16 months at a locality on the Northwestern Mediterranean Coast. Minimum biomass was measured in the autumn (200-700 g dry weight m-2) while biomass maxima occurred in late spring, that in 1986 (1800 g dw m-2) being much lowe r than in 1987 (3300 g dw m-2). Persistent periods of low sea level in spring accounted for the observed inter-annual variability in the bio mass cycle. A remarkable seasonal and plant part variability character ized tissue nutrients of Cystoseira mediterranea, though an almost 50% of the variance remained unexplained. The importance of the frequency in the sampling strategy to get reliable information on tissue compos ition of Cystoseira mediterranea has been pointed out. Ranges of tissu e P and N were 0.04-0.34 and 1.51-3.11% ash free dw, respectively. Tis sue nutrients exhibited the lowest values during summer and the highes t values during winter, in concordance with dissolved nutrients in the water. Tissue P showed an almost two-fold greater seasonal variabilit y than tissue N (27-44% vs 6-22%, coefficient of variation). Nutrient storage in perennial parts of the plant, holdfasts and stipes, was not detected. The atomic ratio N : P in tissues was always higher than 16 , and frequently higher than 30, showing a high seasonal variability. Similarly, the DIN : SRP atomic ratio in the ambient water was commonl y higher than 16, and very often higher than 100. According to our res ults, P is the most likely nutrient limiting Cystoseira mediterranea g rowth in the oligotrophic Northwestern Mediterranean waters although s easonal N limitation has not been eliminated.