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 Cystoseira mediterranea, and nutrient concentration in the ambient water 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 lower 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 biomass cycle. A remarkable seasonal and plant part variability characterized 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 composition of Cystoseira mediterranea has been pointed out. Ranges of tissue P and N were 0.04-0.34 and 1.51-3.11% ash free dw, respectively. Tissue nutrients exhibited the lowest values during summer and the highest values during winter, in concordance with dissolved nutrients in the water. Tissue P showed an almost two-fold greater seasonal variability 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 commonly higher than 16, and very often higher than 100. According to our results, P is the most likely nutrient limiting Cystoseira mediterranea growth in the oligotrophic Northwestern Mediterranean waters although seasonal N limitation has not been eliminated.