Contenido y dinámica del fósforo en el sedimento de praderas de fanerógamas marinas.

Resum

Dynamics and content of phosphorus in the sediment of seagrass meadows. The sediment phosphorus concentrations of different seagrass meadows was determined, with special emphasis on the spatial variation of PRS in pore water. Additionally, a preliminary attempt to evaluate phosphorus exchange at the sediment-water interface and the role of the plants in thesc processes was made. Pore water dissolved phosphorus ranged between 1 and 5 ug-at P r \ and total sediment phosphorus concentrations ranged between 150 and 250 ug P g dry weight r1. In sediment below Posidonia oceanica (L.) Delile meadow a clear increase of PRS in pore water is shown in a depth gradient, related to a decrease of Posidonia cover. In addition, comparing these data with a nearfound bare sediment, two differences arose: total phosphorus content in the bare sediment is higher, but below Posidonia the labile fraction is múch more important; both facts appear to reflect the influence of the rizhosphere upon sediment dynamics. The rnineralogical composition of the sediment and preliminary results of laboratory experiments suggest that carbonate fluorapatite can play a major role in the equilibrium between particulate and dissolved phosphorus forms in the Posidonia system.That seagrasses exert an important influence upon sediment phosphorus dynamics was also evident in other seagrass meadows (dominated by Cymodocea nodosa (Ucria) Ascherson, Ruppia cirrhosa Petagna Grande, Zostera no/tii Homemann), where plant activity had a clear influence upon the vertical distribution of pore water PRS. The labíle phosphorus fraction of this sediments was, however, small compared to that in the Posidonia system. This difference may be related to differences in the mineralogical composition of the sediments, with the presence of montmorillonite. Whatever the relative importanee of total phosphorus compared to labile phosphorus, this last fraetíon appear sufficient to meet the phosphorus requirements of the plants, and phosphorus JimÍlation of plant growth appears unlikely.