We studied variations through 1 year in concentrations of several macronutrients (C, N, P, Na, K, Ca, Mg, S) and micronutrients (Fe, Mn, Cu, Zn) and water content in tissues of the brown seaweeds Ascophyllum nodosum , Fucus vesiculosus and Fucus spiralis at two sites in a coastal embayment in northwest Spain. The annual mean concentrations of the nutrients in A. nodosum were very similar at both sampling locations. There were no significant differences between sites in the concentrations of N, P, Na, K, Ca, S, Fe, Mn or Cu, the C:N or N:P ratios or in the water content, whereas the chemical composition of this species was notably different at the site where it coexisted with F. vesiculosus (the only parameters that were not significantly different were concentrations of Ca, Cu and the N:P ratio). The C:N:P ratios and the clear seasonal variability in N in all species, with minimum values in summer, suggest limitation by this element. Along with N, K was the only other element that varied throughout the seasons in all of the species, and this was most notable in F. vesiculosus . Minimum concentrations of K were also observed in summer, which might indicate limitation by this element. Seasonal variations in C were only observed in the Fucus species; they followed a reverse trend from those of N and K, as the highest levels occurred in summer. The highest concentrations of P and Zn in F. vesiculosus were also observed in spring. No obvious seasonal trends were observed in the remaining minerals.