Why and how are they threatened?
Salt marshes and tidal flats are under pressure from a range of threats (Melville et al. , 2016). It has been estimated that 16% of tidal flats were lost globally between 1984 and 2016 (Murray et al. , 2019), while salt marshes are being lost at a rate of 0.3% per year (Campbell et al. , 2022).
Some of the main threats include:
Coastal development (including land reclamation): The expansion of human populations on coastlines has put increasing pressure on developing infrastructure and housing in these areas (Charlier et al. , 2005; Lai et al. , 2015; Murray et al. , 2019). Reduced sediment supply: Activities upstream, such as river damming, limit the amount of sediment reaching estuaries, therefore the rate at which sediment is replenished is reduced relative to the rate it is eroded (Syvitski et al. , 2005; Dethier et al. , 2022). Similarly, the removal of sand from rivers is a major global environmental issue (Rentier & Cammeraat 2022) that reduces sediments reaching the coasts, including in the Yangtze and Yellow Rivers (Yang et al. , 2006; Yi et al. , 2022). Sea level rise: Coastal areas are vulnerable to sea level rise, which leads to the erosion of shorelines and increased risk of flooding (Fujii, 2012; Passeri et al. , 2015). The combination of rising sea levels with coastal development, which prevents the landward movement of salt marshes and tidal flats, means there is simply less space available for these habitats. The resulting loss of intertidal habitats is called coastal squeeze (Pontee, 2013). Sinking river deltas: Due to sediment compaction and reduced sediment supply, it is estimated that 85% of the world’s deltas have experienced subsidence (Syvitski et al. , 2009). This impacts salt marshes and tidal flats by increased wave exposure, altered tidal inundation characteristics and increased erosion. Habitat degradation: Intertidal habitats can become degraded from human activities such as bottom trawling, dredging and digging, which impact the benthic fauna (Dieter & McConnaughey, 2003). Many pollutants including heavy metals, pesticides, plastics and excess nutrients end up in estuaries from agriculture, aquaculture and domestic waste (Islam & Tanaka, 2004; Bessa et al. , 2018). Invasive species: Invasive species spread through a variety of routes infiltrating coastlines and outcompeting native species (Reise et al. , 2023). The highly invasive Smooth Cordgrass Spartina alterniflora is a considerable threat to tidal flats and salt marshes on many shorelines including the Chinese (Zuo et al. , 2012; Stokstad, 2023) and Korean (Kim et al. , 2015, 2023) coasts. The deliberate introduction of native species for commercial purposes, for example molluscs, can be a threat to other native species by taking over ecological roles (Peng et al. , 2021).
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