Long-term impacts of embankments on coastal marsh vegetation and carbon sequestration

Coastal marshes, encompassing saltmarshes and freshwater marshes, are important environments for carbon sequestration and coastal protection; however, they are at risk due to climate change, sea level rise and human activity. Freshwater wetlands sit inland of saltmarshes and are often characterised by high biodiversity and productivity. Increasingly, conservation efforts are focussed on freshwater habitats, sometimes even at the expense of saltmarshes, through the construction of defences and embankments. However, these sea defences have unintended consequences on both coastal freshwater and saltmarshes. In this study we investigate the implications of an embankment on a freshwater marsh and saltmarsh from Gibraltar Point, UK using a palaeoecological approach. A multiproxy approach was used on archives from the freshwater marsh and saltmarsh to reconstruct the marsh geomorphological characteristics before and after the embankment was built. In the Freshwater marsh our results show the development of a transitional marsh over the past 200 years with ecological and morphological changes mainly dictated by storm surges with some influence from the seawall installation (ca. 1880 CE). The Old Saltmarsh shows the succession of a mature saltmarsh including rising elevation and vegetation development with negligible impact from the embankment construction. Whilst sea defences contribute to potential future coastal squeeze, by truncating areas of valuable freshwater marsh and cutting them off from their external sediment supply, they create risk to future inundation by sea-level rise. Natural development into freshwater marshes habitats have a greater potential for more biodiverse and multifunctional ecosystems and improved carbon storage capacity whilst enhancing coastal protection. • Embankments show no impact on the accreting saltmarsh habitats. • Storm surges have a greater impact on freshwater marsh and saltmarsh habitats. • Transitional habitats have more biodiversity than embanked and natural saltmarshes. • Embankments may increase risk of coastal flooding due to loss of sediment supply.