Growing salt marshes

Atmosphere – salt marshes

Climate: Salt marshes as discussed in this building block are located in temperate zones. In tropical zones mangrove systems can be regarded as salt marshes.

Precipitation: In early development, rain will increase the germination rate of marsh plants. In older marshes the impact of rain becomes increasingly important. In time, marsh zones nearest to the marsh edge get inundated more frequently and sediment with the largest particles settles here. This results in a faster marsh accretion rate near the marsh edge. A limited flooding frequency, poor drainage and high precipitation could cause the high marsh to become more brackish. Long-term standing water could cause plant die-off and local erosion to occur. This could reduce the resilience of a marsh to cope with extreme events such as storms.

Storm events: A few times each year most marshes have to cope with storm events. Storms can have a large impact on the marsh accretion and erosion rates (Cahoon 2006). A marsh surface stabilized by local vegetation and with a high plant diversity will increase the resilience of a marsh to withstands these strong waves and prevent erosion (Ford et al. 2016).

Sea level rise: Marshes can increase in elevation and thereby keep pace with the increase in sea level, making them very valuable ecosystems for coastal defence. The critical threshold value for an increase in sea level seems to be 10 mm per year (Kirwan et al. 2010). Beyond this threshold, intertidal flats start to disappear and this may result in the pioneer zone to start eroding (Bakker et al. 2005). Stone dams could be necessary to protect the marsh edge and prevent erosion at this point. When estimating the ability of marshes to keep up with an enhanced sea-level rise, presence of soil subsidence should be taken in account as well.