Coastal Wetland Elevation Dynamics, Sedimentation, and Accommodation Space Across Timescales

The capacity of coastal wetlands to maintain their position within a tidal frame is a key indicator of resilience to climate change. A range of techniques can be used to assess this capacity, but few studies have focussed on describing wetland elevation dynamics across timescales. In this study, annual-scale wetland elevation dynamics within intertidal coastal wetlands located at different tidal positions in south-eastern Australia were quantified using both shallow and deep rod surface elevation tables and marker horizon (rSET-MH) techniques. This was supplemented by analyses of sediment accumulation rates across the decadal-centurial timescale using 210 Pb dating techniques. The rSET-MH technique indicated slight variation in surface elevation change between sub-sites and processes contributing to surface elevation gain was a product of processes occurring over the full substrate volume. This included sediment (both mineral and organic) accretion on the surface and belowground substrate expansion in tidal positions where accommodation space and inundation frequency were higher (i.e. in the mangrove). 210 Pb data provided the means to consider sedimentation and wetland elevation trends over decadal timescales over which relative sea-level rise has been operating. Sedimentation responded to localised accommodation space processes, exceeding sea-level trends lower in the tidal frame, but corresponded to rates of sea-level rise where accommodation space was increasingly limited (i.e. higher in the tidal frame). We demonstrate that anticipated sea-level rise will create new accommodation space for wetland vegetation and that where sea-level rise is not matched by an equivalent increase in surface elevation, coastal wetlands will either die, retreat landwards, or transition to lower tidal positions that support mangroves.