Barker Inlet

Drive north out of Adelaide along the coast and you will pass, without much noticing, one of the most productive nurseries in the whole gulf. Barker Inlet is not scenic in any postcard sense: a broad, shallow, mud-bottomed tidal reach, threaded with channels, fringed grey-green with mangrove and rust-red with samphire depending on the season. It looks, from the causeway, like the leftover ground nobody wanted. In 1973 the state government decided otherwise, and declared it South Australia’s first aquatic reserve — not to protect a view, but to protect a nursery.

The logic was straightforward and practical. Western king prawns, King George and yellowfin whiting, and blue swimmer crabs — the backbone of the gulf’s commercial and recreational catch — spend their first weeks and months in exactly this kind of sheltered, shallow, food-thick water, tucked out of the surf and out of most predators’ reach (Bryars 2003). Take away the inlet and you do not lose a patch of mud; you cut the supply line to fisheries the length of Gulf St Vincent. Reading Barker Inlet for what it is, not what it looks like, means reading it as a maternity ward.

The inlet does its work with the same two-handed trick as the rest of the samphire coast. Along the seaward channels, grey mangrove — South Australia’s only mangrove species, and among the most southerly stands on Earth — holds a tropical way of life at its cold limit, excluding salt at the root and breathing through mud via a fringe of pneumatophores. Landward, samphire glassworts take the opposite approach, drinking the brine and storing it in swollen, jointed flesh. Both are halophytes, built to solve the one problem that matters here: too much salt for an ordinary plant to survive. Between them, mangrove root and samphire mat build a sediment that is airless, waterlogged, and unusually good at locking carbon away. Australian tidal marsh like this holds roughly 165 tonnes of organic carbon in every hectare of its top metre, and keeps accumulating it year on year, which makes an unglamorous mudflat one of the coast’s most effective carbon stores (Macreadie 2017).

None of it survives on looks. Coastal saltmarsh nationally is now listed as a threatened, Vulnerable ecological community, drained and reclaimed for two centuries before anyone thought to count what it was worth (Saltmarsh 2013). Barker Inlet’s own water answers to the strange breathing of Gulf St Vincent beyond it — an inverse estuary where evaporation, not river flow, drives the circulation (Lennon 1987). Look at the inlet properly, and the plain grey mudflat turns out to be doing three jobs at once: raising the gulf’s seafood, banking its carbon, and running on physics found in only a handful of places on the planet.