Bio-morphodynamics of coastal wetlands with mangrove vegetation

Abstract

Mangroves, occupying the transition between land and sea, provide valuable ecosystem services for hundreds of millions of people across the tropics and subtropics. What would happen in a world without mangroves? Large amounts of carbon dioxide would be released into the atmosphere, contributing to global warming and accelerating global climate change, affecting humans, animals, plants and almost all other organisms on the planet. Dismal scenarios would be seen in almost every (sub)tropical coastal city, for example, a barren coast with unprotected shorelines periodically inundated by polluted water, driving away coastal inhabitants from the shore. Although this extreme situation is predicted not to occur within this century, global loss of mangrove coverage is currently occurring under the pressure from sea-level rise and human interventions. While the appeal to the conservation of mangrove forests has become globally recognized, limited knowledge on the complex interactions between ecosystems and coastal morphology hampers coastal zone management and the safeguarding of mangroves. External forcings, such as sea-level rise, tidal currents, wind waves, river flows, sediment supply, and human activity vary with mangrove systems influencing profile changes and vegetation dynamics. It is often thought that mangroves can keep up with sea-level rise and withstand natural and human-induced pressures, but to what extent is this true and which non-linear relations with the external forcings and internal dynamics are most important? Sustainable coastal management needs to be based on a better understanding of the underlying mechanisms controlling the development of mangrove ecosystems, especially in the face of predicted sea-level rise and intensifying human interventions.Mangroves are often characterized by numerous pneumatophores, which are their special aerial root systems growing upward out of the mud and water so that mangroves can stand in seawater without asphyxiation. These root systems add flow resistance when water flushes through mangrove-covered areas. As a result, currents slow down, which promotes sedimentation and increasing bed surface elevation. Locally, flow between vegetated flats, such as in tidal channels, becomes more concentrated due to convergence of water flow, leading to a larger flow velocity and sediment erosion and thus lowering channel bed elevation.These hydro-morphodynamic interactions in turn affect vegetation biological processes, such as seedling establishment, mangrove growth and mortality, which then further change the impacts of vegetation on the above physical processes. Such non-linear interactions between vegetation dynamics, water movement, sediment transport and morphological changes are called bio-morphodynamic feedbacks. Sea-level rise and increasing human interventions have increasingly affected mangrove ecosystems and caused more uncertainty about future system evolution. Rising sea levels disturb hydro-morphodynamic processes through modulating water levels, which directly an...