Estuarine zonation (the tide sorts the bands)

Walk out onto a tidal flat at low water. The Maroochy will do, where a boardwalk carries you over the mud without your sinking to the knees, and the muddle of grey-green resolves, if you know how to look, into stripes. A meadow in the clear shallows, a dark forest of trees standing in the tide, a low succulent carpet baking above them. It looks like a random patchwork and is nothing of the kind. It is a gradient: the great slope of the coast all over again, the reef-to-range sequence compressed into a few hundred metres and driven, this time, by the tide instead of the rock.

One variable does nearly all the sorting: how often the salt water reaches. Ecologists call it hydroperiod, the hours a day and the days a month that a patch of ground spends drowned. Two things run with it, the saltiness of the water and how little air the waterlogged mud beneath it holds. Set those against a plant’s tolerances and the estuary falls into three bands as tidily as books on a shelf.

Lowest of all lie the seagrass meadows, true flowering plants returned to the sea rather than seaweed, needing water clear enough to let the light down. Much of it sits below the low-tide mark, but do not read that as a rule: the dominant local species, Zostera muelleri, runs well up into the intertidal and is regularly exposed on a low tide. In the intertidal band, flooded and drained twice a day, stand the mangroves, the only trees that can take the beating of salt and airless mud. And on the higher flats, which the spring tides climb to only now and then, sits the saltmarsh: low, salt-tolerant succulents and grasses, the samphires and glassworts, storing fresh water in fleshy stems and baking in the sun between soakings.

Here is the engine. Each plant lives at the one height where it can just tolerate the salt and the soaking, and cannot afford a step in either direction. Push it seaward, into a band with more hours underwater, and it drowns. Let it drift landward, onto ground the tide reaches less often, and its neighbours do what the salt could not: freed from the stress that had been holding them back, the plants of the drier band grow faster and shade it out. So each band holds no more than the specialists that can stand a little more salt and inundation than the band above it, sorted by their tolerance of hardship as the dune plants are sorted by their tolerance of poverty.

The sorting does not stop at three. Look inside the mangrove band and it, too, splits by the same rule. The shrubby river mangrove holds the upper, fresher reaches and the landward fringe; the caustic milky mangrove keeps to the driest margin the tide barely wets; the grey mangrove, Avicennia marina, takes the broad middle where the tide runs hardest. Zones within zones, every one of them a reading of how long the water stays.

And because the tide sets the pattern, nothing about it holds still. The boundaries are edges, not painted lines. They shift with the sediment the river brings down, the freshwater draining off the land, and the shape of the flat. Over longer time they migrate. As the sea rises, each band tries to move landward with the tide that defines it, and on a floodplain hemmed in by roads and cane that is where the trouble starts.

Read the three bands together, then, and you no longer have a mudflat. You have a tide-gauge you can walk along, or better, paddle, each stripe of green telling you, near enough to the hour, how long its ground spends under the sea.