Island salt marshes in the Wadden Sea vulnerable, mainland remains stable

Researchers from Wageningen Marine Research have examined which salt marshes in the Dutch Wadden Sea are capable of keeping pace and which are likely to encounter problems as sea levels rise. They conclude that mainland salt marshes are relatively future-proof. Island salt marshes, particularly on Oost-Ameland, are, by contrast, vulnerable.

In the Netherlands, the total area of salt marshes has increased in most places over recent decades. Salt marshes are dynamic environments whose development is governed by sediment accretion and vegetation dynamics. Their future resilience depends on their capacity to accumulate enough sediment to keep up with sea-level rise (currently 2–3 mm per year). Mainland salt marshes grow through sediment accretion (11–17 mm per year) and are relatively future-proof, whereas island salt marshes (with growth rates of 1–5 mm per year) remain vulnerable.

In the response of the salt marsh vegetation to sea-level rise, the resilience of the vegetation is particularly striking: even under deteriorating conditions, it has so far managed to persist. This demonstrates the strength of salt marshes. However, fragmentation and poor drainage pose a risk, as these can lead to tipping points where salt marshes abruptly shift into bare tidal flats. This is a real risk for the salt marsh on Oost-Ameland, which is especially sensitive to sea-level rise due to land subsidence caused by gas extraction.

Management practices such as grazing influence both vegetation and sedimentation and therefore affect biodiversity and resilience. The researchers recommend limiting grazing in areas with low sediment accretion, poor drainage or land subsidence, and applying differentiated grazing management to promote variation and biodiversity. For island salt marshes, encouraging sediment supply is also recommended.