Why Does Mud Between Land and Sea Matter So Much? A Habitat That Absorbs Storms

A salt marsh develops where fine sediment accumulates in sheltered water and where the tidal range allows a surface to be exposed for part of each cycle. Sediment settles in slack water, raising the surface until it emerges at low tide, at which point salt-tolerant plants colonise it. Those plants slow the water flowing over them, which causes more sediment to settle, which raises the surface further, which allows less tolerant species to establish, and the process feeds on itself. The result is a surface built by the interaction of sediment and vegetation rather than by either alone, cut by a network of creeks that drain it, and rising in step with sea level as long as sediment is available.

Marshes show sharp bands of vegetation and the reason is submergence:

• •The lowest zone is flooded on nearly every tide and carries only the most tolerant pioneer species
• •The middle marsh is flooded regularly and carries a distinct community
• •The upper marsh is flooded only on the highest tides and is the most diverse
• •The transition to fully terrestrial vegetation is abrupt where grazing or a wall stops it
• •Small differences in height produce large differences in flooding frequency
• •The zonation is therefore a direct map of elevation, which makes marshes ideal for studying it

The services these habitats supply are substantial and quantified. They absorb wave energy, with measurements showing wave height reduced by a large fraction across a modest width of vegetated marsh, which reduces the height and cost of any wall behind and which is why restoring marsh in front of defences is now standard practice in several countries. They store carbon at rates per unit area exceeding most forests, accumulating it in waterlogged sediment where decay is slow, which has brought them into climate policy under the heading of coastal carbon. They support fisheries, since a great many commercially important fish species use them as nurseries. They filter nutrients and sediment from runoff. And they support enormous numbers of wintering and migrating birds.

Living in soil saturated with seawater poses two problems and the solutions are visible in the plants. Salt draws water out of tissue, so a plant must maintain an internal concentration higher than its surroundings simply to take water up, which several species do by accumulating salts and compatible solutes in their own cells. Excess salt must then be dealt with, and the strategies differ, with some species excreting it through glands on the leaf surface, where it can be seen and tasted, some concentrating it in old leaves that are then shed, and some diluting it by holding water in thick fleshy tissue. Waterlogged soil is also short of oxygen, which roots need, so several species run air spaces down through the stem into the roots.

Losses have been enormous and the causes are identifiable. Reclamation for agriculture and development removed a very large share historically, since a marsh is flat land requiring only a wall to convert. Coastal squeeze is the current mechanism, where sea level rises while a fixed wall prevents the marsh migrating inland, so it is compressed between the two and eventually drowned, and this is the dominant cause of ongoing loss in developed coastlines. Sediment supply has been reduced by dams and by river management. Pollution and nutrient enrichment weaken the plants that hold the sediment. Restoration by deliberately breaching walls is being done at scale in several countries and works, and it requires giving up the land behind.

Sediment settles, plants colonise, the plants slow the water so more settles, and the surface builds itself and rises with sea level while sediment lasts. Vegetation bands map elevation directly, since small height differences change flooding frequency sharply. Marshes absorb a large fraction of wave energy across a modest width and store carbon faster per unit area than most forests.