Tidal exchange of water between a coastal marsh and the Río de la Plata estuary: the effect on the main physical and chemical variables

Resumen

At Punta Blanca, a Schoenoplectus californicus marsh of about 1,190 m long and 175 m wide is separated from the Río de la Plata estuary by a sand bar interrupted at one extreme by a channel through which tidal exchange of water occurs. Water movement in and out the marsh was estimated by installing a water stage height meter and following stage height variations through complete tidal cycles. Incoming river water was sampled twice and outcoming marsh water three times along six tidal cycles, and main physical and chemical variables were determined. The marked decrease of the suspended matter concentration in the outcoming water points out the large retention of sediments by the marsh. Oxygen concentration and water pH decreased in the outcoming water in four over six samplings. Higher concentrations of soluble reactive phosphorus in the outcoming tidal water suggest P release related with riverine suspended matter transformation upon sedimentation, in response to acidification and reduction within the marsh environment. Nitrate impoverishment in the outcoming water suggests denitrification losses. Since ammonium concentration in the outcoming water was larger than in the incoming water, a net flux from the sediments is suggested. Nevertheless, as a result of nitrate depletion and ammonium release, a net loss of inorganic nitrogen took place.