Advanced imaging technique could boost Great Barrier Reef recovery

Covering less than 0.1 per cent of the ocean surface, coral reefs support 25 per cent of all marine life. But according to Unesco, 21 of 29 World Heritage reefs have been damaged through heat stress and bleaching.

In warmer or polluted waters, corals expel algae, which provide sustenance through photosynthesis; the corals then turn white and can die if the water temperature remains high.

Following the latest bleaching, the world’s largest coral reef ecosystem, parts of the Great Barrier Reef off the coast of Queensland, Australia, is estimated to have lost two-thirds of its corals, mainly in the northern section.

The newly developed method is being used to monitor the recruitment success of newly settled corals through an artificial process whereby microscopic larvae are raised in floating coral nursery pools and released en masse on reefs damaged by the impacts of climate change to kickstart their recovery.

“Coral recruitment occurs at a scale invisible to the human eye and, up until now, required the use of artificial plates attached to the reef to later be examined under microscopes to quantify newly settled corals,” said Dr Marine Gouezo, doctoral researcher at Southern Cross University and lead author on the study.

“With this new method, observations of the recruitment of organisms 0.5mm in size, such as two-month-old corals, can be documented and tracked through time in their natural habitat: the reef.”

The study demonstrates the application of underwater macrophotogrammetry: a combination of macrophotography – the art of taking close-up photographs at very high resolution to see tiny organisms typically smaller than 1mm in size – and photogrammetry – a technique that ‘stitches’ these photos together to recreate three-dimensional models of small portions of the reef.

Unlike traditional field observations that rely on artificial devices, this approach allows scientists to directly monitor larval settlement and recruitment on a reef, making it valuable for assessing the success of restoration interventions.

“While reef-scale photogrammetry is commonly used in ecological surveys underwater … it lacks the resolution to spot baby corals,” Dr Gouezo added.

“This macrophotogrammetry technique can complement reef scale photogrammetry by magnifying small portions of the reefs to monitor the reef at submillimetre scale, which is the scale at which recruitment occurs for corals.”

For large-scale projects such as coral larval restoration on the Great Barrier Reef, the macrophotogrammetry technique is cost-effective and avoids the need for equipment installation on the reef or access to laboratory microscopes. It provides a permanent three-dimensional record of small reef areas.

Dr Christopher Doropoulos, senior research scientist and co-author, said: “This macrophotogrammetry technique enables us to examine the earliest stages of coral recruitment in field settings on the reef for the first time, rather than using proxies such as settlement tiles.

“It will deepen our ecological understandings of what limits and what promotes successful larval settlement and early survival for optimising the application and scale of on reef larval-based coral restoration operations.”