We live on a rotating planet, differentially heated by the Sun, and mostly covered in seawater. The largest body of water is the Pacific Ocean, and on its southwestern edge is the Great Barrier Reef.
This is arguably the largest coral reef system to have ever existed on planet Earth. It is but a thin veneer of limestone that has grown on top of at least five previous extensive reef systems; each destroyed by past dramatic falls in sea level.
Ribbon Reef No. 10 is the longest of the most northern outer barriers at the edge of Australia's continental shelf, with an inside (westerly-facing) edge that drops to 40 metres and an outside (easterly-facing) edge that drops vertically to 2,000 metres. Both edges are covered in a great diversity of colourful corals. This last week I got to dive both edges! It was so much fun sinking below the waves with the reef sharks and trevally, meeting manta rays and cuttlefish.
At the northern tip of this ribbon is an opening that was once where the Starke River entered the sea; that was more than 16,000 years ago when sea levels were up to 120 metres lower than they are today.
That river canyon is now underwater with strong currents, that wash in nutrient-rich upwelling from the Pacific Ocean twice a day. The water rushes in, and then out. So I was pleased that there was a lookout on the top deck of our boat the entire time I was diving, with a tender handy, should I come-up in a current that I couldn’t kick against and needed to be picked up.
In fact, I always surfaced at the stairs to the boat, thanks to the great navigation skills of my underwater buddy.
Beyond the southern end of this coral reef system is the much smaller Ribbon Reef No. 11, and a dive site known as Goggle Gardens. The corals here are at 15 metres and were totally bleached white from March to October 2016.
What I learnt from a very experienced diver, who documented this event through photography, is that white and bleached coral is not necessarily dead coral.
The zooxanthellae — unicellular algae that give coral its colours and normally feeds it with energy from the sun via photosynthesis — were expelled, as the corals were stressed by the exceptionally warm waters during the summer of 2015 – 2016. But the corals at this dive site didnotdie.
Coral polyps also have tentacles, and these tentacles were used to feed on small animals and plankton and also to clean away bad algae that would otherwise settle and smoother it.
So, corals are not necessarily totally dependent on zooxanthellae, they can be omnivorous.
In fact, bleached coral can take-back zooxanthellae, become colourful again, and reshoot after months of being stark white and bleached.
I hope to show how this happened at this dive site in my next mini-documentary.