Phosphorus recycling in the seagrass Zostera marina L.  <B>Phosphorus recycling in the seagrass Zostera marina L.</B><P><B>Morten Foldager Pedersen & Jens Borum</B><I><P> Freshwater-Biological Laboratory, University of Copenhagen, Helsingørsgade 51,<BR> DK-3400 Hillerød, Denmark</I><B><P> Abstract</B><BR> The role of internal and external phosphorus recycling to support high seagrass productivity was evaluated by an <I>in</I> <I>situ</I> study of eelgrass (<I>Zostera</I> <I>marina</I> L.). A phosphorus budget including estimates of requirements, external uptake, recycling and losses was constructed from field measurements of biomass, plant turnover and phosphorus concentrations in individual plant parts.<BR> The phosphorus requirements during rapid growth in spring and summer exceeded uptake from external sources, and the deficit was met by internal recycling of phosphorus reabsorbed from old tissues before they were sloughed. Nutrients internally recycled covered 57% of annual requirements, thus reducing the demand for nutrient uptake from the surrounding media. Old roots and rhizomes of eelgrass decompose within the sediments in close contact with living roots, thereby closing nutrient cycles further. Nutrients recycled externally accounted for <I>c.</I> 16% of the annual phosphorus requirements, thus increasing the amount of potentially recycleable phosphorus to <I>c.</I> 70% of the phosphorus incorporated into newly produced tissues.<I><P> Keywords</I>: <I>Zostera marina</I>, phosphorus, recycling.<P>