Two oceans on one rock (the tropical–temperate overlap)

Some of the reefs off this coast have no business existing the way they do, and the culprit is a current. The East Australian Current, the same warm southward flow a cartoon turtle made briefly famous, runs down the Queensland coast carrying tropical water, and in that water drift the larvae of tropical fish, corals and invertebrates spawned far north on the Great Barrier Reef. The current sweeps them south and sets them down here, at or near the edge of where warm-water animals can hang on, and a good many of them settle and do exactly that.

At the same moment, cool-water species from the temperate south are reaching their own limit, their northern one, not far away. So this stretch of coast becomes a meeting place, a biogeographic overlap where two faunas that belong to different seas end up living on the same rock. Ashore, what the ground is made of does the sorting. Out past the seagrass and the open sand, around the headlands and offshore pinnacles, the bottom simply turns hard, and the sorting passes to temperature.

Duck under off a headland reef (the shoals off Point Cartwright, or the boulders ringing Mudjimba Island) and the first surprise is two communities colliding on one rock. On a single boulder, tropical hard coral grows within a fin’s-length of the leathery brown seaweeds and sponges of the cool south. Bright tropical reef fish, butterflyfish moving in pairs, clouds of small damselfish over the coral, a parrotfish crunching at the rock, mingle with the stouter, sober wrasses of temperate water. A frilled carpet shark, a wobbegong, may lie in a gutter looking exactly like a frill of weedy rock until it resolves, unnervingly, into a shark with a fringed beard.

Two things to carry down with the mask, because the temptation is to oversell it. First, “temperate” here means sponge-and-seaweed, not the swaying kelp forest of cooler New South Wales water; kelp is marginal this far north, and what stands in for it on these rocks is a low, tough turf of algae and sponges. Second, these are overlap reefs, not a Great Barrier Reef in miniature. The corals are present and real, but they are guests near the southern end of their range, and they do not build the massive limestone framework of the tropics. What you are looking at is a mongrel margin, and the mongrelling is why it is worth the swim.

That edge is also the risk. A community living right at the boundary of its members’ tolerances feels every change in the water keenly, and a degree either way decides who can stay. The warming and marine heatwaves now reaching down the current are already leaning on that boundary, nudging tropical species further south, year on year, into water that used to be too cool for them, while the cool-water residents have no cooler sea to retreat to. The gradual southward creep of the warm-water crowd even has a name, tropicalisation, and it is being read most clearly at these transition reefs.

So the two-oceans reef is one of the glories of the coast, a subtropical improbability hidden a few metres beneath a surface most of us only ever see from above. And it earns its keep twice over. Because it is stitched together from species living at the far edge of what they can bear, it registers change before almost anywhere else, and the slow reshuffling of who lives on the rock is the plainest local reading we have of how fast the sea is warming.