BBC Science Focus
VIRTUAL E-REEF BUILDERS
Not so long ago, the main way that marine biologists studied coral reefs was to scuba dive for an hour or so at a time and note down what they saw on waterproof slates. Now, during a single dive, they can take photographs that can be stitched together into an intricate, three-dimensional view of the reef. “It’s underwater virtual reality,” says Prof Stuart Sandin, a marine biologist at Scripps Institution of Oceanography, UC San Diego. “It makes you feel like you’re immersed.”
Using a system with two cameras rigged at different angles, a diver swims up and down a reef as if they are ‘mowing the lawn’. Around 3,000 images taken from standard 10 x 10m plot are then analysed by computer using a technique known as ‘structure from motion’. The result for a 10 x 10m plot is a three-dimensional digital model of the reef, made up of a billion coloured dots.
The technique, which took Sandin years to develop by collaborating with teams of computer scientists and engineers, is now being rolled out around the world. So far 30 hectares of reef have been mapped, equivalent to dozens of city blocks, at a resolution of one millimetre. Besides producing stunning underwater vistas, all sorts of valuable information can be extracted from these e-reefs.
At Boston University, undergraduate student researcher Coretta Granberry meticulously traces on a digital tablet the outlines of individual corals, so she can calculate their areas and make comparisons over time. “You get a very detailed, intimate image of the reef and how everything is connected,” she says.
The corals she studies are growing thousands of miles away, in the Phoenix Islands in the middle of the Pacific. Her professor, Dr Randi Rotjan, leads expeditions every three to five years to these extremely remote reefs. “Out there, the closest people to you are on the International Space Station,” says Rotjan. These isolated, protected islands are helping to show how reefs respond to rising sea temperatures. “If you leave reefs alone locally, what are they going to look like when global change is the only stressor?” says Rotjan.
Armed with images taken from the same plots in 2012 and 2015, Granberry and her colleagues will track how the individual corals in the Phoenix Islands change, to see if they shrink, grow or get overgrown by something else. As time capsules, e-reefs will allow scientists in the future to wind back the clock and answer new questions that nobody can anticipate. “You’re in essence exploring in four dimensions,” says Sandin.
E-reefs are also a powerful tool for showing what the reefs are like right now. “You watch everyone’s eyes glow, from the most seasoned scientists, to a politician, to a community leader, to a child,” says Sandin. This is especially important in the Phoenix Islands Protected Area, where local people of Kiribati who drove the conservation efforts live too far away to visit the reefs across the enormous archipelago. “E-reefs become the mechanism to show people in country what they’re protecting, and why,” Rotjan says.