Intertidal wetland vegetation dynamics under rising sea levels.

Intertidal wetlands have historically been in decline and are increasingly at risk due to climate change, particularly sea level rise (SLR). Different intertidal wetland communities can adapt to SLR via lateral upslope retreat to higher ground, capture and accumulation of allochthonous sediment, and/or organic accretion. In this paper, a case study is presented to assess the impact of the overall sediment accretion rate (i.e. allochthonous and organic accumulation) versus possible SLR rates on wetland species composition. Initially, an eco-hydraulic calculation method is developed to estimate existing spatial and temporal tidal inundation statistics of saltmarsh species at a Ramsar listed wetland on the south-east coast of New South Wales, Australia. SLR and accretion scenarios were then tested using high resolution hydrodynamic models to predict future saltmarsh species composition based on the eco-hydraulic calculation method. Saltmarsh species composition and extents were found to persist if sea levels continue to rise at present-day rates, as observed rates of SLR are similar. However, if the SLR rate accelerates beyond the accretion ability of the wetland, a significant shift in species composition and an increase in open water coverage was predicted. These results indicate that the current rate of sediment capture by wetland species, and the subsequent rate of elevation change, will need to increase significantly to adapt with projected future rates of SLR.