Deposition gradients across mangrove fringes

Abstract

Observations in a mangrove in the Whangapoua Harbour, New Zealand, have shown that deposition rates are greatest in the fringing zone between the tidal flats and the mangrove forest, where the vegetation is dominated by a cover of pneumatophores (i.e. pencil roots). Current speeds and suspended sediment concentrations dropped substantially across this zone. Near-bed turbulence within the fringe was substantially lower where the pneumatophore canopy was denser, facilitating the enhanced deposition in this zone. However, the near-bed conditions were not the primary control on the instantaneous sediment concentrations at this site. The total deposition across the different zones was the combined result of the reduced near-bed turbulence inside the vegetation and the larger-scale dynamics over the spatially variable vegetation cover, along with other confounding factors such as changing sediment inputs.