Mangrove response to sea level rise: Palaeoecological insights from macrotidal systems in northern Australia
Accelerated sea-level rise threatens coastal wetlands; it is unclear whether sediment accretion beneath mangroves will be sufficient to keep pace. A conceptual framework, used to describe the response of reefs, can also be applied to mangroves, discriminating drowning or back-stepping with rapid rise from keep-up or catch-up under moderate rates. In macrotidal estuaries of northern Australia, different mangrove species grow across particular elevation ranges and accretion rates decrease with tidal elevation. Palaeoecological reconstructions, from drilling, dating and pollen analysis, record mangrove distribution over past millennia. Estuarine plains are underlain by a vertically continuous stratigraphy of muds, implying continuity of widespread 'big swamp' mangrove forests during decelerating stages of post-glacial sea-level rise c. 7000 years ago. In contrast, on higher-energy open coasts, mangroves back-stepped, but re-established as the shoreline prograded when the nearshore built to suitable elevation: a catch-up mode. These results demonstrate that mangrove response to sea-level rise has varied, determined by the availability of sediment and the oceanographic processes by which it is redistributed. How mangrove forests adjust in future will also vary as a function of local topography and sediment availability. Extensive plains flanking estuarine systems are particularly vulnerable to tidal creek extension and saline incursion under future higher sea levels.