Seagrass meadows (the inshore pasture and nursery)

The seagrass meadow is the easiest of the coast’s great habitats to walk straight past. It hazes pale-green over the sand in the clear, sheltered shallows of the bays and passages, and most people take it for a patch of underwater weed. It is nothing of the kind. Seagrass is a true flowering plant, one of the small handful of land plants whose ancestors turned around and went back to the sea, and there it managed what almost nothing else with flowers has: it rooted in soft sediment below the tide and made a living underwater. On this coast the meadows run to low, unshowy genera, Zostera muelleri, Halophila and Halodule. They hold the sheltered sand flats where the water stays shallow and calm and clear. They do not look like much. They do more for the inshore sea than almost anything else growing in it.

It is the pasture first. Seagrass feeds the two great inshore grazers directly. The green turtle crops it, and the dugong ploughs slow furrows through the beds as it feeds, an underwater cow that cannot live where the meadow has gone. For the plant, the wonder is that it is grazed at all: very little in the sea eats living vascular leaves, and the meadow feeds a specialist economy of grazing that the bare sand around it cannot.

It is a nursery next. Most of the fish and prawns the region’s anglers and trawlers haul from the sea spend their vulnerable juvenile stage hidden among the blades, where the water runs rich with food and the cover grows thick enough to shut the bigger predators out. Over sand, the seagrass does what the mangrove root does in the tide: it grows the fishery. Demolish the maternity ward and the losses surface years later and kilometres away, as empty nets.

And as it feeds and shelters, it works the sediment. The bed traps and holds the fine mud that would otherwise hang in the water and cloud it. That matters to the plant, because clear water is the very thing it needs, and the bed manufactures the clarity it lives on. Left alone, the loop reinforces itself: the clearer the water, the more light reaches the blades, the more the meadow spreads, the more sediment it pins down, the clearer the water. Beneath all of it, in the airless mud among the roots, the plant buries carbon, as its cousins the mangrove and the saltmarsh do, in sediment where decay all but stops and the carbon stays put for centuries. The “blue carbon” store is real, and that much is well established; how much these particular meadows hold is not settled enough to put a number to. A genuine service, quietly rendered.

All of which turns on one fragile thing: light. The plant lives on the sunlight that reaches it through the water, so its greatest enemy is anything that dirties the water above it. Sediment run-off from cleared or built land, an algal bloom fed by nutrient pollution, a dredge, a propeller. Here the self-reinforcing loop runs the other way, and runs fast. Cloud the shallows and the meadow starves of light; a thinning meadow holds less sediment; looser sediment clouds the water further; and a bed that took years to build can shade itself out in a season. When it goes, it does not go alone. The dugong loses its food, the green turtle loses its grazing, the fishery loses its nursery, and the water loses the thing that kept it clear — all at once, and all from the same cause.

So the humble, overlooked seagrass earns the same protection as the reef and the mangrove it works alongside. It is the floor of the inshore world. A great deal of what swims over the sheltered shallows of this coast stands, without knowing it, on a flowering plant that lives on light.