ABSTRACT: The rates of changes in stocks of organic matter and major nutrients (N and P) during colonization and development of seagrass Cymodocea nodosa biomass in a semi-closed estuary, Alfacs Bay, Spain, were studied. The non-vegetated sediments of AlfacsBay contain large reserves of N and P, but most of these nutrients are not readily available for plant growth as they are tied up in organic matter or mineral fractions within the sediment. Seagrass biomass per unit area increased linearly during thefirst 5 yr after colonization although the very low tissue concentrations of P in leaves of C. nodosa suggested P limitation. Development of plant biomass was accompanied by a net accumulation of N at a rate of 3.6 g N m^-2yr^-1. Half of this increase was caused by the build up of plant-bound N in living and dead biomass, while the other half was accounted for by accumulation in other N fractions. P accumulated as plant-bound P, but this fraction constituted lessthan 1% of the total P pool. Accumulation of plant-bound P was of the same magnitude as depletion of other P fractions, leaving the pool of total P unaffected by plant development. These results suggest that colonization and development C.nodosa in Alfacs Bay facilitate capture and accumulation of external N within the stands, while P may be mobilized from mineral-bound P reserves within the sediments.