Sedimentary and elemental dynamics as a function of the elevation profile in a semi-arid mangrove toposequence

The effect of elevation and seasonal variations on the elemental status of sediments was investigated along a semi-arid mangrove profile in the Heart of Voh, New Caledonia. As other mangroves in the world, this mangrove site experienced an increase of tidal range that led to the recent colonisation of salt flats at the highest elevations landward by Avicennia marina (Forsk.) Vierh subsp. australasica (Walp.) J. Everett. This young Avicennia stand was compared with an old Avicennia stand at lower elevations on its edge. Down the elevation profile, the sediment properties of a short Rhizophora stylosa Griff stand were compared with a taller Rhizophora stand seaward that experienced longer immersion periods. Our results show that centimetre-scale variation in elevation significantly affects all sediment properties along the semi-arid profile and induces a strong seasonal variation in reduction potential and pH at high elevations over the year. We suggest that during the dry season, the oxygenated sediments enhanced the oxidation of organic matter (OM), which led to the dissolution of Fe-S compounds and the subsequent acidification of the sediments. This in turn induced a loss of metal content (Fe, Cu Ni, Mn) compared to the sediments at lower elevation. Moreover, our results show that the accumulation of OM during the colonisation phase by Avicennia coincided with higher water content and higher total and exchangeable concentrations of N, P, Mg and K within the surface sediments than in the old Avicennia stand. The tall Rhizophora stand at the borders of the channel was characterized by an increase in elevation, denser sediments and a depletion of elements in several horizons of the depth profile compared to the short Rhizophora landward. These results provide a better understanding of i) the impact of elevation differences on major and minor elements in a semi-arid mangrove ecosystem, and ii) the changes of sediment properties mediated by a pioneer species within the early phase of succession in hypersaline mangroves.