Coastal ecosystems and environments

A psammosere is a plant succession that develops on sand — the sequence of vegetation from bare sand dunes near the beach through to a climax woodland inland.

A halosere is a plant succession that develops in salty (saline) conditions — for example the sequence from bare mudflats to a mature salt marsh.

On an embryo dune an early pioneer is sea couch grass (or lyme grass) — salt-tolerant grasses that trap the first sand.

On a mudflat at the start of a salt marsh a pioneer halophyte is glasswort (Salicornia) (or cord-grass, Spartina), which can survive daily tidal flooding by saltwater.

Coral reefs need:

• Warm water of roughly 23 to 29 °C (they are tropical organisms);
• Clear, shallow, sunlit water (generally less than about 25 m deep);
• Normal-salinity, clean saltwater away from large rivers.

The reason clear, shallow, sunlit water is essential is that coral polyps live in a symbiotic relationship with algae called zooxanthellae, which photosynthesise and supply the coral with food. Sunlight must reach the algae, so cloudy or deep water — or sediment from rivers — blocks the light and prevents the reef from growing.

The embryo dune is the youngest, harshest part of the succession, just above the strandline. The environment is dry, very salty, windy and mobile, with almost no soil — just sand with little humus, a high (alkaline) pH from shell fragments, and little moisture-holding ability. Only salt-tolerant pioneers such as sea couch and lyme grass can survive here.

The grey dune lies further inland and is much older and more stable. Generations of plants have added humus, so there is a genuine soil that is darker, more acidic (lower pH), more moisture-retentive and more sheltered from salt and wind. This supports a continuous, more diverse cover of grasses, mosses, herbs and lichens rather than bare sand.

The key distinction is that succession has modified the environment: the embryo dune is a hostile pioneer habitat, whereas the grey dune has a developed, less salty, humus-rich, slightly acidic soil supporting far more species.

The resource shows that as you move inland along the succession, the soil changes markedly. Humus content rises from about 1% on the embryo dune to about 8% on the grey dune — an eight-fold increase — while pH falls from about 8.5 (alkaline) to about 6.0 (slightly acidic). So the older dunes have far more organic matter and are much less alkaline.

This can be explained by the process of succession. On the young embryo dune there is almost no vegetation and no dead plant material, so humus is very low (~1%), and the sand is rich in shell fragments (calcium carbonate), which keeps the pH high and alkaline (~8.5).

As the dunes stabilise inland, more plants grow, die and decompose, steadily adding organic matter (humus) — this is why humus reaches about 8% on the grey dune. The accumulating organic acids from decomposition, together with rainwater leaching out the calcium carbonate, gradually lower the pH towards 6.0.

For the ecosystem this matters: the increasingly humus-rich, moist, less alkaline soil supports a greater diversity of species, which is why the grey dune has a continuous, varied plant cover while the embryo dune has only a few salt-tolerant pioneers. The two figures therefore capture how succession builds a soil and modifies the environment over time.

Mangroves provide a range of valuable ecosystem services:

• Coastal protection. Their dense prop-root network slows and absorbs wave energy and storm surge, reducing flooding and coastal erosion during tropical cyclones — a natural sea defence that repairs itself.
• Habitat and nursery. The sheltered, root-filled water is a breeding and nursery ground for fish, prawns and shellfish, supporting biodiversity and coastal fisheries that people depend on for food and income.
• Carbon storage ('blue carbon'). Mangroves and their waterlogged soils store large amounts of carbon, helping to regulate climate; they also trap sediment, building and stabilising the shoreline.

These services show why mangroves are so valuable: they combine physical protection, biodiversity and climate regulation in one ecosystem, which is why clearing them for aquaculture or development is so damaging.

A salt marsh is a halosere — a succession that develops in salty conditions, usually in the sheltered water of an estuary or behind a spit.

Stage 1 — mudflats. Fine silt and clay carried by rivers and tides settle out where the water is calm, forming bare mudflats that are exposed at low tide but flooded twice a day by saltwater.

Stage 2 — pioneer halophytes. Salt-tolerant pioneers such as glasswort (Salicornia) and cord-grass (Spartina) are the first to colonise, because they can survive daily submergence in salty water.

Stage 3 — the plants modify the environment (accretion). This is the key idea: the stems and roots of the pioneers slow the tidal flow and trap more sediment, so the mud surface builds up (accretes) and is flooded less and less often. Decaying plants also add humus and lower the salinity of the higher ground.

Stage 4 — zonation and later colonisers. Because the marsh is now higher, drier and less salty, less salt-tolerant species (sea aster, sea lavender, marsh grasses) can move in, creating clear zones from the low pioneer mud up to the higher, densely vegetated marsh.

Stage 5 — towards a climax. Eventually the surface rises above most high tides, creeks develop, and the community approaches a terrestrial/freshwater climax. The crucial point is that succession is driven by the plants themselves changing the conditions — trapping sediment and adding humus — so that later, less-tolerant species can replace the pioneers.

Coral reefs are among the most biodiverse — and most threatened — coastal ecosystems, facing both global (climate) and local (direct human) pressures. This answer assesses which matters more.

The case that global climate change is the greater threat. Two climate processes attack reefs at their core. Rising sea temperatures cause coral bleaching: when the water warms even 1 to 2 °C above normal for a sustained period, corals expel their symbiotic zooxanthellae, turn white and, if the heat persists, die. Mass bleaching events during strong El Niño years have already damaged reefs on a huge scale (for example large parts of Australia's Great Barrier Reef). Ocean acidification, as the sea absorbs more CO₂, makes it harder for corals to build their calcium-carbonate skeletons. These are global, unavoidable and worsening, and no local management can stop them — a powerful case that climate change is the dominant threat.

The case that local human pressures are also serious. However, reefs are also degraded by direct local pressures on crowded coasts: sediment and nutrient run-off from farming and coastal development smother and cloud the reef; sewage and pollution cause algae to overgrow the coral; overfishing and destructive dynamite/cyanide fishing remove key species; and tourism (anchors, trampling divers, boat damage) harms fragile reefs directly. These can devastate a reef locally even where the climate is stable.

Interaction and management. Crucially, the two combine: a reef weakened by pollution and overfishing is less able to recover from bleaching. This matters for management because, unlike climate change, local pressures CAN be reduced — through marine protected areas, fishing controls, and limits on run-off and tourism — which buys reefs resilience while global emissions are tackled.

Judgement. On balance, global climate change is the more fundamental long-term threat, because bleaching and acidification act everywhere and cannot be managed locally, so even a perfectly protected reef will suffer if warming continues. However, local pressures are the more immediately manageable threat, and reducing them is the best way to give reefs the resilience to survive warming. The most accurate view is therefore that climate change sets the ultimate threat, but local human pressure decides how well an individual reef copes with it.

Sand dunes, salt marshes, mangroves and coral reefs all act as natural ('soft') coastal defences, absorbing wave and storm-surge energy before it reaches the land. This answer assesses how important they are compared with the alternatives.

Ways they defend the coast. Each ecosystem reduces energy in its own way:

• Sand dunes form a flexible barrier that absorbs wave energy and stores sand, which is fed back to the beach after storms; marram grass binds the sand so the dune can rebuild itself.
• Salt marshes cause tidal water to lose energy over a wide, vegetated surface, reducing wave height and flood risk behind them, while accretion helps them keep pace with rising sea level.
• Mangroves use their dense prop roots to slow storm surge and waves and trap sediment — after major tropical cyclones and the 2004 Indian Ocean tsunami, coasts fronted by wide mangroves suffered fewer deaths and less damage than cleared coasts nearby.
• Coral reefs act as offshore breakwaters, forcing waves to break before they reach the shore and protecting beaches and lagoons behind them.

Why they are so important. Unlike concrete sea walls, these defences are self-repairing, cheap and sustainable: they rebuild after storms, grow with sea-level rise and deliver extra services at the same time — biodiversity, fish nurseries, carbon storage and tourism. This makes them highly cost-effective, which is why 'working with nature' is now central to Integrated Coastal Zone Management.

Limits to their importance. However, their protection is not unlimited. Very large events can overtop dunes or overwhelm mangroves; ecosystems take years or decades to establish; and on crowded coasts they are being cleared, trampled, polluted and squeezed by development, so the defence is removed just where people most need it. Reefs weakened by bleaching protect the coast less well. Where valuable property must be protected immediately, hard engineering may still be needed alongside them.

Judgement. On balance, coastal ecosystems are a very important — and increasingly favoured — form of natural defence, because they are sustainable, self-repairing, cost-effective and multi-functional in a way hard engineering can never be. However, their importance depends on their being conserved and on the size of the threat: they work best against everyday and moderate events and as part of an integrated approach, so the most accurate view is that they are a vital first line of defence, not a complete substitute for other management.