Bikini corals recover from largest US atomic blast

They dove into the vast Bravo Crater left in 1954 by the most powerful American atom bomb ever exploded. At 15 megatonnes of TNT, it was 1,000 times more powerful than the bomb dropped on Hiroshima in World War II.

The Bravo bomb vapourised three islands, raised water temperatures to 55,000 degrees, shook islands 200 kilometers away and left a crater two kilometers wide and 73 meters deep.

"I didn't know what to expect - some kind of moonscape perhaps" - said Zoe Richards of the ARC Centre of Excellence for Coral Reef Studies and James Cook University. "But it was incredible, huge matrices of branching Porites coral up to eight meters high had established, creating thriving coral reef habitat.

"Throughout other parts of the lagoon it was awesome to see coral cover as high as 80 percent and large tree-like branching coral formations with trunks 30cm thick" - she added. "It was fascinating - I've never seen corals growing like trees outside of the Marshall Islands."

However, the team documented a high level of loss of coral species from the atoll. Compared with a famous study made before the atomic tests were carried out, the team established that 42 species were missing compared to the early 1950s. At least 28 of these species' losses appear to be genuine local extinctions - probably due to the 23 bombs that were exploded there from 1946 to 1958, or the resulting radioactivity, increased nutrient levels and smothering from fine sediments.

While corals, in general, have shown resilience, she says the coral biodiversity at Bikini Atoll has proven only partially resilient to the disturbances that have occurred there.

Maria Beger from the Commonwealth Research Facility for Applied Environmental Decision Analysis at The University of Queensland took a Geiger counter with her on the expedition. "The ambient gamma radiation the residential island of Bikini atoll was fairly low - pretty much like the background radiation in an Australian city. However, when I put the Geiger counter near a coconut - which accumulates radioactive material from the soil - it went berserk” - Beger said.

Extensive decontamination works have been carried out at Bikini atoll, making it safe to visit, but local produce is not safe to eat and it is unlikely the Bikinian people will return to live on Bikini Atoll in the near future.

For comparison, the team also dived on neighboring Rongelap Atoll, where no atomic tests were carried out directly - although the atoll was contaminated by radioactive ash from the Bravo Bomb and local inhabitants were also evacuated and, for the most part, have not returned.

The marine environment at this atoll was found to be in a pristine condition.

The team was of the opinion that Rongelap Atoll may be seeding Bikini's recovery, because it is the second largest atoll in the world with lots of coral reef diversity and biomass and lies upstream from Bikini.

Richards says that, due to the bombs, Bikini Atoll is a priceless laboratory showing how in the absence of ongoing stress, some corals have the capacity to recover from upheavals - a fact that may contain valuable lessons for the management of reefs in other parts of the world, including Australia.

Because of its incredible history and current undisturbed character, Bikini Atoll is now part of a larger project to have northern Marshall Island Atolls placed on the UNESCO World Heritage List for conservation.

"The healthy condition of the coral at Bikini atoll today is proof of their resilience and ability to bounce back from massive disturbances - that is, if the reef is left undisturbed and there are healthy nearby reefs to source the recovery" - said Richards.