Ecogeomorphological assessment of a mangrove wetland in the Pacific Islands

Coastal mangroves are essential areas for the habitat of coastal and marine ecosystems, which are under constant anthropogenic and climatic pressure. Sediment and biological accretion can attenuate the effects of sea-level rise by rising the ground level. The balance between the soil accretion and the sea-level rise under climate change conditions will determine the response of the mangrove and its adaptation/survival.

This contribution presents the ecogeomorphological assessment of the Dreketi river wetland (northern coast of Vanua Levu - Republic of Fiji). This mangrove wetland belongs to the central area of the Great Sea Reef, which was declared as Ramsar site in 2018. The Dreketi river catchment is the main contributor of water and sediments to the wetland. The amount of water and sediments generated in the catchment were determined using a hydro-sedimentological, physically based watershed scale model (SWAT). Then, the response of the Dreketi mangrove wetland to sea-level rise and climate change was analysed using an eco-geomorphological (EGM) model.

The hydro-sedimentological model proved to be suitable to represent the sediment concentration in the Dreketi river catchment with a good performance against sediment concentrations obtained using remote sensing products. The EGM was able to represent the spatial distribution of suitable areas for mangrove habitats, given the current conditions. Under sea-level rise events, after 100 years, a significant amount of the suitable area could disappear for a sea-level rise. The model showed the profound effect that sea-level rise and sediment accretion have on the wetland's future evolution, highlighting the importance of the sediment input from the contributor catchment.