RATES OF CHANGES IN ORGANIC-MATTER AND NUTRIENT STOCKS DURING SEAGRASS CYMODOCEA-NODOSA COLONIZATION AND STAND DEVELOPMENT

The rates of changes in stocks of organic matter and major nutrients ( N and P) during colonization and development of seagrass Cymodocea nod osa biomass in a semi-closed estuary, Alfacs Bay, Spain, were studied. The non-vegetated sediments of Alfacs Bay contain large reserves of N and P, but most of these nutrients are not readily available for plan t growth as they are tied up in organic matter or mineral fractions wi thin the sediment. Seagrass biomass per unit area increased linearly d uring the first 5 yr after colonization although the very low tissue c oncentrations of P in leaves of C. nodosa suggested P Limitation. Deve lopment of plant biomass was accompanied by a net accumulation of N at a rate of 3.6 g N m(-2) yr(-1). Half of this increase was caused by t he build up of plant-bound N in living and dead biomass, while the oth er half was accounted for by accumulation in other N fractions. P accu mulated as plant-bound P, but this fraction constituted less than 1% o f the total P pool. Accumulation of plant-bound P was of the same magn itude as depletion of other P fractions, leaving the pool of total P u naffected by plant development. These results suggest that colonizatio n and development C. nodosa in Alfacs Bay facilitate capture and accum ulation of external N within the stands, while P may be mobilized from mineral-bound P reserves within the sediments.