M. Falcao et C. Vale, SEDIMENT-WATER EXCHANGES OF AMMONIUM AND PHOSPHATE IN INTERTIDAL AND SUBTIDAL AREAS OF A MESOTIDAL COASTAL LAGOON (RIA FORMOSA), Hydrobiologia, 374, 1998, pp. 193-201
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.