Concept · Ch 14

Seagrass meadows (the inshore pasture and nursery)

Seagrass is a flowering plant, not a weed, carpeting the clear shallows — the pasture that feeds the dugong and green turtle, the nursery that hides young fish and prawns, and a quiet store of carbon in the mud beneath.

First, meet: The gradient rule (substrate writes the country)

The seagrass meadow is the easiest of the coast’s great habitats to walk straight past. It is a pale-green haze over the sand in the clear, sheltered shallows, and most people take it for a patch of underwater weed. It is nothing of the kind. Seagrass is a true flowering plant that went back to the sea, rooted in the sediment, and it does more for the inshore coast than almost anything else growing there.

It is the pasture first. The dugong grazes it like an underwater cow, ploughing slow furrows through the beds, and the green turtle crops it too; both animals are staked, more or less entirely, on the meadow staying alive. It is a nursery next: a whole generation of young fish and prawns hides among the blades before it is big enough to face the open water, which is another way of saying the fishery is grown here. And as it works, the meadow traps the fine sediment that would otherwise cloud the water, and buries carbon in the mud beneath it.

All of which turns on one fragile thing: light. The plant lives on the sunlight that reaches it through clear water, so the fastest way to kill a meadow is to dirty the water above it — with run-off, with a nutrient bloom, with a dredge. Cloud the shallows and the seagrass shades out, and when it goes it does not go alone. The dugong loses its food, the turtle loses its grazing, and the fishery loses its nursery, all at once. The humblest-looking habitat on the coast is holding up a great deal of what swims over it.

In depth

Seagrasses are true flowering plants — not seaweeds — that returned to the sea and now grow rooted in soft sediment wherever the water stays shallow and clear enough for light to reach the bottom. On this coast the meadows are chiefly low-growing genera such as Halophila and Halodule, holding the sheltered sand flats of the bays and passages. One habitat doing a disproportionate share of the inshore work, the seagrass meadow is pasture, nursery, filter and carbon store at once:

Pasture. It is the direct food of the two great inshore grazers — the dugong, which ploughs furrows through the beds as it crops them, and the green turtle. A nationally significant, strongly seasonal dugong population grazes the wider bay and the Pumicestone Passage (Lanyon's 1995 aerial surveys ranged from several hundred to about a thousand across the year); the dugong's whole existence is staked on the meadow, so foul or cloud the water until the seagrass dies and the animal simply has nothing to eat.
Nursery. Its blades shelter a generation of juvenile fish and prawns, feeding the inshore fishery exactly as the mangrove root does (McPhee 2017).
Filter and anchor. The bed traps and holds fine sediment, keeping the water clear — a self-reinforcing service, since clear water is the very thing the light- hungry meadow needs.
Blue carbon. Like mangrove and saltmarsh, seagrass buries carbon in the airless mud beneath it. The advantage is well established in kind, but kept qualitative here: specific per-hectare sequestration figures for these particular meadows are not settled enough to quote.

The vulnerability follows from the biology: because the plant lives on light, its single greatest enemy is anything that clouds the water — sediment run-off, algal blooms fed by nutrient pollution, dredging, boat damage. Shade it out and the whole inshore web that leans on it — dugong, turtle, and the juvenile fish of the fishery — loses its floor at once. That is why the seagrass, humble and easily overlooked, earns the same protection as the reef it feeds alongside.

Primary sources & further reading — the doorway

McPhee, D. (2017). Environmental History and Ecology of Moreton Bay. CSIRO Publishing. — Seagrass ecology of the bay — nursery role, grazing and the threats that cloud the water.
Davie, P. (1998). Wild Guide to Moreton Bay. Queensland Museum. — Regional seagrass and inshore natural history (Wild Guide to Moreton Bay).

See it in the country

Bribie Island & the Pumicestone Passage The dugong The Noosa oyster reef Point Cartwright & the Mooloolaba rocky reef Sea country Wolf Rock (Double Island Point)