Coastal Coral Reef at Cape Tribulation

When the tide goes out near the famous tourist location of Cape Tribulation in tropical Queensland, reef flats and coral platforms are exposed. However these reefs at first sight appear to be mostly dead and some explanation is needed. The short answer is that these reefs flourished more than 5000 years ago when sea level was about one metre higher than it is today. After sea level feel to its present height, the top of the reef was about 80 cm above the level required for coral growth and coral cover was lost from the top of the reef. The mystery is why the reef platforms are still there at all after 5000 years of battering by rough seas from trade winds and cyclones.

Some areas have a wide flat platform

Fissures and porous reef absorb waves and no waves swashed over the top

Emmagen Creek is the best place to observe these old reefs, however almost all sections of coastline near Cape Tribulation have well developed reefs. Corals are still present in the fissures of the reef and on the reef slope. In fact these reefs hold the record for coral diversity on the Great Barrier Reef nd have even more coral species than the best of offshore reefs. In the order of 175 species of coral have been recorded. However most of what is visible is a yellow brown surface riddled with holes and coated with algae and sediment. It is opposite of an attractive coral reef.

In some areas there are shallow enclosed lagoons

The coral matrix is about 7 metres thick and is so porous that swells can flow into the reef. Twenty five metres back from the edge, I could hear the reef beneath my feet breathing, it sound like a sleeping baby. The reef platform is quite strong, although occasional thin sections can break underfoot causing me to lose some skin. Deep fissures surge with swells and even in the middle of the broad reef, I could not see the bottom through the clear waters. I am keen to investigate this underwater world, however it will likely have hazards including jellyfish which often concentrate in fissures and even moray eels and crocodiles which could be inconvenient. In one of the fissures, I observed large fish being attended by a cleaner fish.

A large fish at a clearer station in the middle of the reef platform

Live corals fill the deep fissures

When the corals could no longer grow on the reef platform, encrusting coralline algae took their place. The top 80 cm of the reef platform consists of a coralline algae matrix with embedded chunks of coral and terrestrial rock. I wonder if aborigines carried the rocks onto the reef as it is hard to imagine a natural process doing so. In any case, I consider the reef to be a living reef that is maintained by algae rather than coral.

Pink coralline algae is easily overgrown by brown algae

Some patches of reef edge have hand-sized stones in the reef matrix

Coralline algae are easily grown over by other types of algae and can only thrive when something is grazing the other algae. At Emmagen Reef, the grazing appears to be performed by air-breathing slugs. These slugs (Onchidium sp.) usually cruise the ground in mangrove swamps and to find them as the dominant herbivore on a coral platform is most unusual.

Onychidium slugs grazing on the algae

There are some specialised fish associated with this reef. Peppered moray eels hunt the crabs which scurry along the margins of fissures. Dart fish inhabit small pools. Larger pools have a subset of reef fish including angel fish and damsels. In wide fissures, there are concessional coral colonies and wave washed edges are covered with dense sargassum. Water clarity is limited.

Peppered moray eel under a coral ledge at the toe of the beach

The most interesting thing and the only thing I failed to record were two small fish that bounded over an exposed two metre wide sand bank on the tips of their tails. The fish were vertical and bouncing away like pogo sticks in a motion that was very different from a mudskipper. When they reached a pool, they swam away underwater. This leaves me wondering if they were rock skippers, which are an amphibious blennie known from islands in the Pacific Ocean.

Ships sail close to the coastline here