Salt marsh accretion

How can a marsh keep its head above a sea that will not stop rising?

Sea level has risen by metres since the last ice sheets went, and it is rising now. A salt marsh is a flat plain of grass sitting within a few tens of centimetres of high water — the most obviously doomed landform imaginable. Raise the sea by a metre and it should simply be gone.

Yet marshes of the same age as that rise are still there, still at the same height relative to the tide. Something has been keeping pace. The tempting explanation is that the sediment happens to have arrived at the right rate, but coincidence maintained over millennia is not an explanation. The question is whether a marsh can be responsive — whether rising water somehow provokes the very process that lifts the marsh clear of it.

Begin with where new marsh surface comes from. Two sources, and it matters that they are different. Mineral sediment settles out of the water column when a flooding tide slows over the vegetated flat. Organic material accumulates from below, as roots and rhizomes grow and their dead remains decay only partially in waterlogged, oxygen-poor soil. In a sediment-rich estuary the mineral half dominates; in a clearwater setting such as parts of the Gulf coast, marsh soil is largely peat the plants made themselves.

Now the crucial move. Ask what happens to each source when the marsh sits slightly lower relative to the tide — which is exactly what a rising sea produces. Flooding lasts longer and floodwater is deeper, so more sediment-laden water passes over each square metre and more particles have time to settle. Deposition rises. And the marsh grasses, which are tolerant of salt and inundation, tend to grow more vigorously in the wetter conditions up to a point, putting more carbon into roots. Organic accumulation rises too.

So the drowning itself increases the rate of building. That is a negative feedback — a system that answers a disturbance by opposing it — and it is why a marsh is not a passive heap of mud but something closer to a self-levelling surface. Push it down, it builds faster; let it build too high, it floods rarely, deposition falls off and growth of the pioneer grasses declines, and the surface stalls. There is an equilibrium elevation in the upper part of the tidal frame, and marshes tend to sit at it.

Now interrogate the feedback rather than admiring it, because a negative feedback only stabilises within a range. Where does it fail?

Two places. First, the plant response is not monotonic. Vigour rises with inundation only to an optimum and then collapses: too deep, too long, too often, and the grasses drown, root production stops, and the marsh loses its organic contribution and its ability to slow water and trap particles at the same moment. Past that threshold the feedback reverses sign — lower elevation now means less building, which means lower elevation. The system runs away, and marsh converts to open mudflat abruptly rather than gradually.

Second, the mineral half depends on a supply the marsh does not control. If the estuary carries little suspended sediment, deeper flooding delivers more water but not more mud, and the deposition response is weak. Dams that trap sediment upstream, and channelisation that fires it past the marsh into deep water, both starve the feedback of its raw material. This is why marshes in sediment-rich, large-tidal-range systems are far more robust than those in microtidal, sediment-poor ones — the same rate of sea-level rise is survivable in one and lethal in the other.

There is also a subtraction we have ignored. What accumulates at the surface is not what the marsh gains in elevation, because the column beneath is compacting and decomposing. A marker horizon records accretion; a rod driven to depth records the net, which can be much less. Confusing the two has produced real over-optimism.

Two clarifications, and one honest limit on what is known.

The first is that vertical building is not a marsh's only response. Given room, a marsh migrates landward: as the seaward edge erodes, the upland margin becomes salty and floods, and new marsh establishes there. Over the Holocene this transgression preserved far more marsh than accretion alone would have. It is precisely this route that hard shorelines close off — a marsh with a seawall behind it can only build upward, and the phenomenon is called coastal squeeze.

The second is that "keeping pace" is a comparison of two rates and both are local. What matters is relative sea-level rise: the global signal plus regional ocean dynamics plus vertical land motion. Coasts still rebounding from the ice sheets face less; deltas subsiding under their own sediment, or over depleted groundwater and hydrocarbon reservoirs, face much more, which is why Louisiana loses marsh at rates the global average would not predict.

The limit worth stating plainly: the threshold rate at which the feedback breaks is genuinely uncertain. Published estimates vary widely with tidal range and suspended sediment concentration, and models that include the biological feedback yield markedly more optimistic thresholds than those that do not. That disagreement is unresolved, and any single number quoted as the survivable rate should be treated with suspicion.

• How surface elevation tables and marker horizons are used together to separate accretion from net elevation gain.
• Whether sediment can be added deliberately, and what thin-layer placement has achieved so far.
• Mangroves as the same feedback in a warmer climate, and what happens where they are displacing salt marsh.

Every term the collection defines is gathered in the glossary.