Sea country

The Sunshine Coast does not end where the sand slips under the water. The soils and slopes we spend so many of these pages reading do not stop at the beach; they pass into something we cannot walk on, and so have learned to overlook. We read the sea as a blank blue edge to the map. It is country in its own right, grazed and hunted and migrated through and depended upon. The name insists on what European charts left out: the sea is a place, its habitats linked one to the next. The science bears it out. The water off this coast is a mosaic, as legible, once you learn to read it, as the gradient ashore, and it runs in the same order.

The seabed near the shore continues the land’s gradient, only flooded. Off the surf beach, where rivers and tides drop their fine sediment in the sheltered bays and passages, the bottom is soft sand and mud. Wherever the water stays shallow and clear enough for light to reach it, seagrass grows: pale-green meadows over sand. Further out the bottom turns hard around the rocky headlands and offshore reefs, and a different community of seaweeds, sponges, corals and reef fish claims the stone. Between and around them lie the open sand flats, the channels, the surf zone. Each is its own world, sorted by depth, light, wave energy and the nature of the bottom, as the land’s ecosystems are sorted by soil and water and fire.

As on land, one habitat quietly carries more than its share. The seagrass meadow is pasture, nursery and filter to the inshore sea at once. It is a flowering plant, remember, not a weed: it carpets the shallows, feeds the animals that graze it, shelters a whole generation of young fish and prawns among its blades, holds the sediment steady and locks carbon into the mud beneath. It is the underwater wetland, humble, easily wrecked by anything that clouds the water and shades it out, and worth far more than it looks. It also feeds the great inshore grazers: the dugong that ploughs furrows through the beds and the green turtle that crops them. Lose the seagrass and you have not lost one plant. You have pulled the floor out from under the whole inshore food web, and starved the animals that cannot live without it.

Out past the seagrass and the open sand, where the bottom turns hard around the headlands and offshore reefs, the coast does something rarer: warm-water and cool-water life share the same stone, a two-oceans reef built by the current that runs down this coast. That overlap gets a page of its own, warnings and all. Here it is enough to mark the reef as the outermost band of the underwater gradient, the hard-bottom world beyond the meadow. Because so much of what lives there sits close to the edge of what it can bear, it is the part of the sea country that will feel a warming ocean first.

So read the sea as country. The pale-green shallows over sand are seagrass, the pasture and nursery of the inshore sea. The boulder reef off the headland, coral and cool-water sponge on the same stone, is the warm current’s handiwork. And the animals refuse to recognise the line we draw at the tide. The turtle that grazes the meadow must haul out on a dark beach to nest; the dugong staked on the seagrass is undone the moment the water clouds. Damage the water and you starve the grazer; light up the beach and you confuse the nester. Land and sea are one country. The gradient runs unbroken from the range to the reef.