When CSIRO marine ecologist Dr Christopher Doropoulos stepped out of his office and onto the boat during his seven-year study of the reefs off the Micronesian island of Palau, the first thing to hit his senses was the evidence of flourishing ocean life.
“We’d see fish jumping out of the clear water as our boat zigzagged through the maze of the reef. It was full of life. The corals were so diverse – in size, structure and colour," shared Doropoulos.
But elsewhere on the reef, the scene was not quite so idyllic.
In December 2012 a super typhoon occurred in the oceans around Palau that could have spelt long-term devastation.
“It was equivalent to a category-five cyclone in Australia, which hadn’t occurred in Palau for roughly 70 years. It removed all the living coral cover from the eastern reefs,” said Doropoulos.
“The contrast between that reef and other intact reefs was so striking. A lot of the reef’s complexity was gone. As well as the structure and the colour."
Over the next seven years, Doropoulos, together with research co-lead Dr George Roff and team members from The University of Queensland and Palau International Coral Reef Center, were surprised to see the reef recover at a rate never seen before.
Rapid reef recovery
“The reef’s recovery was initially slow. We recorded no new coral recruitment in the first four years. But then it began to recover in such a rapid way that we didn’t understand what was going on,” said Doropoulos.
“Reefs are traditionally understood to recover following major disturbances after coral spawning events, when millions of coral larvae find their way onto those impacted reefs from nearby intact reefs."
Synchronous coral spawning events offer awe and inspiration to many a coral scientist. They are a key mechanism by which coral reef recovery occurs.
The process includes mass synchronous spawning of eggs and sperm into the water column. Coral larvae is dispersed for up to hundreds of kilometres via ocean currents. Swimming coral larvae settle onto the reef. Growth of microscopic coral recruits happen until they are big enough to survive long-term.
It didn’t take the team long to surmise that the rapid recovery they were recording wasn't from the delivery of new coral larvae following the disturbance event.
“That’s not what was happening here,” said Doropoulos. “We had to reassess what was happening – it took a lot of discussions that challenged our thinking about what we thought we knew.”
In searching the reefs for the source of this surprising recovery, the team turned their attentions to the new recruits that appeared from unseen nooks and crannies in the coral framework. Like dormant seeds, these hidden coral recruits had stayed dormant until the super-typhoon removed the adult corals. It was then they were ready to spring into action.
Secrets in the coral seed banks
“We’ve known about dormancy in seed banks in plants for a long time. This is when plants release a mass of seeds that can sit in the soil for decades, waiting for the right conditions to germinate. It happens in seagrass too. But this is the first time we've seen or conceptualised it in coral,” said Dr Doropoulos.
The germination of seed banks in forests are triggered by environmental cues such as bushfire. For corals, those environmental cues include variations in light and water flow, and relaxation of competition from species such as algae and adult corals.
“This is a now-known mechanism by which reefs recover from disturbances,” he added.
Is reef recovery more complex than we thought?
Doropoulos and his colleagues’ did a two-year study in Northwest Western Australia. They set out to diagnose why degraded reefs in the UNESCO Ningaloo Marine Park and neighbouring Exmouth Gulf were not recovering from a mass bleaching event and two cyclones that occurred 2011 and 2012.
“We need to understand the dynamics of a system before we actively try to restore reefs,” said Doropoulos . “We need to ask firstly, ‘what’s the natural potential for reefs to recover?’ and secondly, ‘what are the factors limiting that recovery?’”
Dr Doropoulos’ findings in Western Australia challenged many of his assumptions. This included his expectation that coral cultured from clear-water conditions would die in marginal reefs with turbid, warmer water.
“That’s not what we found at all,” he said. “Instead, we found we can produce coral in benign conditions that do really well in more marginal, turbid reefs.”
Other major factors affecting coral recovery included competition with algae and smothering of new coral recruits by sediment.
All in the one study
The research is among the first to combine such a variety of elements in one study. It assessed limitations to coral recovery across environmental gradients including laboratory experiments, culturing of larvae and deployment to reefs spanning 150 kilometres, field monitoring, remote sensing, and larval dispersal modelling.
The team collected and cultured coral larvae onto tiles made of a mix of carbonate sand and cement, and measuring 10 x 10 cms. These were settled in different environmental settings within the reef, then mapped and monitored over an 18-month period.
“We saw coral in this area grow from 3mm to 150mm in the space of four years in marginal conditions. This was pretty exciting,” said Doropoulos.
"These findings will help inform conservation and management strategies for agencies during the establishment of a new marine park in the Exmouth Gulf."