SEDIMENT-WATER EXCHANGES OF AMMONIUM AND PHOSPHATE IN INTERTIDAL AND SUBTIDAL AREAS OF A MESOTIDAL COASTAL LAGOON (RIA FORMOSA)

We measured the ammonium and phosphate concentrations in near-bottom a nd pore waters collected across one year, at several stations of the s ubtidal and intertidal areas of Ria Formosa. In addition, the concentr ations were determined in flood water collected during the first 20 mi n of inundation of the extensive intertidal areas. Ammonium and phosph ates were higher in the pore waters than in near-bottom waters, and th e predicted flux of NH4+ and PO43- by molecular diffusion varied seaso nally, and was higher in intertidal than in subtidal sediments. Transp ort induced by the short-term mixing of intertidal sediments were comp arable to the predicted diffusive transports between low tides. The NH 4+ flux was production-rate controlled, while PO43- flux was slowed do wn by chemical interactions with solids. NH4+ was released preferentia lly to PO43-; but the stoichiometry of decomposition/transport varied seasonally. Removal of NH4+ from the intertidal sediments was independ ent of season. As a yearly average, the amounts of NH4+ and PO43- flus hed out of the intertidal area of Ria Formosa were one-two orders of m agnitude higher than the quantities released from the subtidal area.