The western lagoon marshes of the Ria Formosa (Southern Portugal): Sediment-vegetation dynamics, long-term to short-term changes and perspective

The study concerns the lagoon marshes of the “cul-de-sac” of Ancão (CDSA), situated in the western part of the Ria Formosa. The aim of this article is to propose a quantification of the sedimentary volume in lagoon marshes on the long-, medium- and short-term, to precise the respective part of the natural and human factors driving the sedimentary changes, and to put in perspective the results with the predictions on the rise of sea level. The environmental analysis is based on a set of botanical, morpho-sedimentary and chronological (14C, 137Cs, 210Pb, 226Ra) indicators, on the use of surface marks to measure the vertical growth of saltmarshes, and on the photo-interpretation. Results show that the present available volume of fine-grained sediments represents ∼1,549,215 m3. The muddy deposits are concentrated both in the upstream (30%) and downstream (63%) part of the CDSA. The pluri-secular accretion rates (0.4 mm/a between 1681 BC and AD 2001) obtained in the Holocene Ludo ria are relatively low, compared with the pluri-10-year accretion rates (8–9 mm/a between AD 1941 and AD 2000) calculated from the excess 210Pb and 137Cs activity concentrations. On the short-term (AD 2000–2002), the measures of accretion rates confirm field observations, namely a critical situation where saltmarshes are in dominant erosion. Changes in sedimentary secular trend, spatial heterogeneity of present accretionary deficit and preservation potential of the CDSA marshes are discussed.