In our last Reefscape story, we explored the northern Marshall Islands in search of answers about the long-term effects of nuclear fallout on coral reefs. We visited Rongelap and Ailinginae atolls, where Cesium-137 and other highly radioactive material from 1954’s Castle Bravo hydrogen bomb test on Bikini Atoll had drifted downwind, falling like snow on the people, lands, and reefs.
Sixty-four years later, in 2018, our coral reef findings at Rongelap and Ailinginae were double-edged: on the one hand, we discovered pristine shallow and deep-water reefs teeming with corals, fish, invertebrates and predators. There was seemingly no sign that these reefs had once been exposed to a nuclear winter. On the other hand, we found recent coral bleaching caused by an ocean heat wave in 2014 that generated vast stretches of undersea desert with relatively little life.
Our findings were a shock, challenging our understanding of how much a reef can take before it disappears, and confronting us with the possibility that climate change may be worse for coral reefs than nuclear fallout.
The voyage to Rongelap and Ailinginae also drew new questions about the repeated atom bomb tests, which occurred 100 kilometers (60 miles) west of Rongelap on the famous Bikini Atoll. It was the Castle Bravo hydrogen bomb that famously caused a massive downwind radiation plume that engulfed Rongelap. But eight years before it, in 1946, there were another two nuclear blasts, code-named Able and Baker, that purposely sank a fleet of warships in Bikini’s 594 square kilometer (230 square mile) lagoon. Today, these ships lie on a seafloor averaging 55 meters (180 feet) in depth, well out of range for recreational SCUBA diving.
The direct impact of multiple nuclear bomb tests on the ecosystems of Bikini Atoll may seem obvious: it destroyed vegetation on the atoll’s small islands and left mile-wide bomb craters strewn across the lagoon’s seafloor. Radiation levels were sky high. Surprisingly, however, some reefs around the atoll have rebounded, and they now harbor nearly as many coral species as those surveyed prior to the bomb-testing years.
But what about the wrecks? Could Bikini’s nuked ships have become the foundation for new reef life following the bomb tests? In November 2018, I returned to the Marshall Islands with my team of technical divers to survey the largest warships lying ghostly on the seafloor.
We planned to focus our search on hard corals, the ones that form calcareous structures and can easily scrape the skin off a snorkeler’s knee. Hard corals create the architecture of a reef, generating the three-dimensional habitat for all other organisms including fish and invertebrates, which, in turn, attract predators like sharks. Hard corals are to reefs what trees are to forests: habitat makers.
To make the 27-hour ocean passage from our entry point at Kwajalein Atoll north to Bikini Atoll, we chartered a trawler with an experienced crew and the necessary technical equipment to support deep diving operations. The long trip gave us time to prepare gear, cameras and survey protocols, and to get to know our hosts. One Marshallese crew member had been among the first to dive the nuked ships in the late 1980s. But no surveys have ever been undertaken by ecologists to ascertain whether the ships have been accumulating corals since their sinking 72 years ago. The voyage was a new exploration for both my team and the ship’s crew.
We arrived at Bikini with the morning light glistening off wind-chopped lagoon waters. In the distance, strewn around the atoll’s edge, small islands bristled with bright green palms against a blue sky. As we approached the warship graveyard, we spotted fuel oil seeping up from the bottom to form long shiny meandering ribbons, a sign of pending changes in our limited surface view of the tropical paradise.
Once moored on site, we worked through our final gear preparations. Deep dives require long decompression stops at 80, 70, 60, 50, … up to 10 feet (24 to 3 meters) of water depth on the way back up to the surface, which limits valuable survey time. Most team members carried three full-size tanks on their backs and hips, rigged for procedures to get a maximum survey time of just 30 minutes on each hull. Two of us used closed-circuit rebreathers with helium-oxygen-nitrogen mixtures to maximize bottom time and to reach the deepest parts of each wreck. Emergency tanks were staged at different depths below the dive ship, and a recompression chamber remained ready on deck.
Our first surveys focused on the U.S.S. Saratoga, a 277-meter (888-foot) aircraft carrier sunk in the Baker bomb test of July 25, 1946. The ship sits upright on the seafloor, creating an eerie sense that she’s still at war, with guns drawn outward along her main deck. Descending onto the Saratoga was a surreal experience as the flight deck slowly came into view in subtle tones of rusty brown against a dark blue backdrop. Unable to see across the Saratoga from any point of approach, the deck faded into a dark zone of the unknown.
With each descent onto the carrier, we quickly split up into pairs, with two groups working their way down the port and starboard sides of the flight deck while two of us worked the deeper reaches of the hull. We took photos and videos up and down the hull, deck and on the guns — anything that might serve as a coral anchor. Later, we congregated at the ship’s superstructure and bridge extending 12 meters (40 feet) above the flight deck. When time was up, we started our three-hour ascent, stopping in 10-foot increments to decompress. At the 30-foot mark, we lined up on a decompression safety bar suspended below the stern of our trawler. Sharing notes on underwater slates, we guessed at how many types of corals would be tabulated once we got on board to analyze our camera footage. The guesses seemed implausible.
In biology, we organize living things on a tree of life, also known as a phylogenetic tree. Closely related organisms called species are placed near to one another, like leaves on a branch. More distantly related organisms are separated by branches, and so on until you get to the main “trunks,” such as the plant and animal kingdoms, on the world’s phylogenetic tree of life.
We think there are more than 800 hard coral species throughout the world’s shallow tropical oceans. Many of these species are closely related and so difficult to tell apart that they often require genetic testing. One notch up the phylogenetic tree, however, where groups of species are organized at the genus level, the differences become more obvious to the trained eye. Any two coral genera will have become distinct long ago, evolutionarily driven apart by cumulative environmental and genetic forces.
This is interesting because if a habitat hosts just two coral species within a genus and nothing else, it suggests that other genera cannot access or cannot adapt to the habitat, thereby lowering the biodiversity rating. On the other hand, if a habitat hosts multiple genera of hard corals, it is considered more biodiverse, which can also produce a more resilient system. The relationship between biodiversity and resiliency is akin to the difference between investing in a Wall Street stock or a mutual fund: the former carries a greater risk of loss, whereas the latter is more resilient to a loss in any particular contributing stock.
Back aboard ship, we carefully tabulated the photographic results from the Saratoga surveys. To our amazement, we discovered 28 different coral genera along the flight deck, hull and superstructure. Some we found on the carrier’s huge gun barrels and turrets — life on former instruments of destruction. To put our findings into perspective, hard coral species of the world are arranged in 122 genera on the phylogenetic tree, which means we had found 23 percent of them on one nuked aircraft carrier.
In the following days of diving, we surveyed seven more warships, including the battleship Arkansas, two destroyers, two submarines, and the Japanese Imperial Navy’s battleship Nagato. Admiral Yamamoto used Nagato as his flagship to orchestrate the 1941 attack on Pearl Harbor. We found ourselves literally submerged in a history of violence and heroism: each ship and submarine we surveyed finally met its end in the 1946 nuclear tests as big as the Hiroshima and Nagasaki bombings that had ended World War II a year earlier.
Collectively, our surveys ultimately revealed two surprise findings. First, the total number of hard coral genera tallied was 33, or 27 percent of the world’s stock. Second, we discovered a positive relationship between ship length and the number of coral genera, with the Saratoga taking the prize for maximum coral diversity.
A famous pattern in ecology is known as the species-to-area curve, which tracks the increasing number of species per area of natural habitat. It works on genera too. Observed increases in the number of genera per area eventually diminishes or asymptotes, revealing the biological carrying capacity of an ecosystem. In our surveys, we never reached that asymptote, suggesting that even larger ships could harbor additional distinct corals.
Our findings are important in conservation and marine management because the hulls of these ships rest well below surface waters exposed to the modern havoc of the hot-water events that go hand in hand with climate change. Reef bleaching typically occurs during ocean heat waves that reach a depth of about 10 to 15 meters (30 to 50 feet). Sitting well below this 21st-century danger zone, Bikini’s massive warships have literally become arks of coral biodiversity.
More recently, we carried out surveys on the sunken Japanese fleet of Truk (Chuuk) Lagoon in Micronesia, tallying results from 24 more ships that echoed our findings at Bikini Atoll. We found that Truk’s shipwrecks are home to some 35 percent of the world’s hard coral genera. Bikini and Truk lagoons also share an important feature that bears on our thinking about artificial reefs: both sites are protected by law. Protection of artificial reefs facilitates settlement and habitation by corals, fish, and perhaps a widening network of branches drawn from our planet’s tree-of-life.
This article originally appeared on Mongabay.
Greg Asner is director of the Center for Global Discovery and Conservation Science at Arizona State University. He thanks Indies Trader, Oceans 360, Hollis Rebreathers, Huish Outdoors, and Backscatter Underwater Photo & Video for their support of the Coral Haven voyage to Bikini Atoll. The Reefscape Project of the asnerlab.org is supported by the Leonardo DiCaprio Foundation and is journalistically hosted by Mongabay.