Salt marshes

Salt marshes consist of communities of rooted halophytic plants of terrestrial origin, dominated by grasses, herbs and dwarf shrubs, and animals. They share many characteristics with mangroves but replace them geographically in higher latitudes. Only at the extremes of mangrove distribution do they co-occur, in the Gulf of Mexico, Japan and southern Australia and northern New Zealand. Salt marshes are estimated to cover 3.5 x 105 km2.

Initial colonisation does not occur unless the sediment is stable enough to permit the growth of roots and sand is unfavourable because it dries too easily. Therefore salt marshes tend to develop in sheltered areas of mud and silty sand which are flat, slow draining and on which mats of diatoms, bacteria and protists flourish. These mats initially serve to stabilise the sediment, a process which is accelerated as the salt marsh plants grow. With time therefore the level of the shore rises, the time of tidal submergence decreases and the physiography becomes characterised by dendritic networks of creeks ramifying through densely vegetated areas through which the sea floods and ebbs. The two genera which are most prominent as pioneer saltmarsh plants are Salicornia, or samphire, and Spartina, or cord grass. Species of Puccinella, Scirpus and Juncus are also common.

Salt marshes are highly productive aquatic systems producing around 2500 gC/m2/yr. Occurring in highly seasonal latitudes salt marshes rapidly take up and accumulate nutrients during the growing season. In the autumn, when plants die or become dormant, the uptake of nutrients is greatly diminished and dead organic matter may be transported out of the marsh. The physical features of salt marshes mean that pollutants are not easily or rapidly flushed out. Other threats include infilling, especially around the North Sea and increased erosion through channel dredging. Causes of salt marsh decline are not always local. Populations of lesser snow geese have increased by more than an order of magnitude as a result of increased feeding on of agricultural crops on their wintering grounds in the southern United States. This species now exceeds the carrying capacity of its summer breeding grounds in Canadian Arctic salt marshes striping the vegetation.