The tidal estuary

For two centuries we answered a mangrove swamp the same way: drain it, fill it, build something tidier on top. It was one of the great misreadings of the Australian coast. The tidal flats where the rivers meet the sea are muddy, sulphurous and hard to love — the mangroves, the saltmarsh behind them, the seagrass in the shallows. They are also the most productive ground on the whole gradient, and once you know what you are looking at, the least deserving of contempt.

To read the estuary you have to stop looking for solid ground and start thinking in tides. Twice a day the salt water floods in and drains away, and a handful of unrelated trees have each cracked that impossible pairing of salt and airless mud (see how a mangrove beats salt and mud). Having cracked it, they sit in bands by tidal height, seagrass to mangrove to saltmarsh (see estuarine zonation). How the machine gets assembled matters less than what it does once it is running. It makes food out of rubbish.

A paddock runs on sunlight caught by green pasture. The estuary runs on litter. A mangrove forest sheds an enormous fall of leaves, and dark sesarmid crabs drag them backwards into their burrows, shred them and pass them out as fragments that microbes finish off. That flood of rotting, microbe-rich detritus is the base of the food web, the reason the mud is so fertile, and the source of the fat prawns and fish everyone downstream is keen to catch. Out on the drained flats, pale-blue soldier crabs march forwards in their thousands, sieving the surface film into little rolled pellets of cleaned sand; in the deeper creeks the mud crab lies dug into a bank; and among the roots hides the next generation of almost everything.

The great majority of the fish and prawns pulled from the region’s waters spend their vulnerable juvenile stage hidden in the estuary, among the roots and the seagrass, where the food is rich and the cover thick enough to keep the bigger predators out (McPhee 2017). Drain an estuary and you demolish a maternity ward whose losses turn up years later and kilometres away as empty nets. The mangroves and the fishery are the same system; you cannot keep one and bulldoze the other, though we spent a long time trying.

The mangroves get the attention, but they are only the middle of three worlds. Landward, on flats the spring tides reach only now and then, lies the saltmarsh, a low grey-green carpet of salt-storing succulents that looks like barren nothing and is in fact a high-tide dinner table, where fish push up to gorge on the flooding tide and the crabs and birds take over as it drains. Seaward, in the clear shallows, lie the seagrass meadows, the underwater pasture that feeds the dugong grazing quietly and unseen offshore, a nationally significant, strongly seasonal presence in the wider bay and the Pumicestone Passage (Lanyon 2003). All three do one more thing. Because decay all but stops in airless mud, every buried leaf and root locks its carbon away as blue carbon, gram for gram outpacing even rainforest.

Read the three bands together, because they are one machine, run by the moon. The tide lays down the fine mud the rivers strip off the land; the mangroves pin it in place and rain their leaves onto it; the crabs shred the leaves to detritus; and the detritus, worked over by microbes, feeds the worms and prawns and juvenile fish, which in turn feed the mud crabs, the herons, the shorebirds down from the Arctic, and the boats. The saltmarsh baking above and the seagrass in the clear shallows run the same economy at their own heights on the tide. None of it is pretty in the postcard sense, and all of it does more for the coast’s fish, floods, climate and birds than the manicured foreshore we so often put in its place. The engine room of the whole coast was hiding, all along, in the place we held our noses and looked away from.