Coral near Nikumaroro Island (Photo: Randi Rotjan)

Overall, coral cover was almost halfway back to where it was before the bleaching, which is a phenomenal speed of recovery in six short years. This was an incredible affirmation of the expectation we have in the science and conservation communities that ecosystems that are not suffering from a range of different threats have a much greater ability to recover from any one. In this case, the lack of local human impacts has made the Phoenix Islands reefs able to recover faster from a global change impact than most of the reefs that I study anywhere else in the world.

(Photo: Jim Stringer)

This alone is a great result that needs to be better known to help support people and places that are trying to limit damage to ecosystems and reefs. But the story was very nuanced, and some parts difficult to absorb still. It's like watching a loved one recovering from a major illness--even when its clear they are recovering and out of the main danger zone, their continued suffering is heartbreaking. Looking at the reefs in Phoenix we could see reefs still struggling to get firmly on the road to recovery, held back by local factors that differed from site to site.

One factor confirmed a long-standing suspicion that has been growing in my mind--whereas reefs on sheltered reef slopes near lagoon entrances can be the best developed and most spectacular and diverse when mature, when they are knocked back nearly to zero it can take a lot longer for them to recover. It may be that lagoon water, which tends to be warmer, has higher nutrient levels and a higher sediment load, is less supportive of settlement, growth and survival of small corals because it fertilizes algal and microbial growth. In a mature community this is not critical as adult corals have passed their most sensitive stages, but when recovery depends on successful colonization and growth of young corals, the lagoon effect may be critical.

Litter seen during the expedition (Photo: Randi Rotjan)

Another more insidious factor seems to be pollution by iron … remote islands are a magnet for shipwrecks. Ships are drawn to them for shelter and the resources they provide, and the litter of shipwrecks around them can be mind-boggling. Having spent more time in the Central Pacific in the last 4-5 years with more experienced scientists such as Jim Maragos, who strongly advocates removing shipwrecks immediately from remote reefs to prevent them from poisoning the small vulnerable reef communities. We saw clearly this effect--large wrecks on Kanton and Nikumaroro had a clear effect on reefs downstream, with almost no live corals and near-100% cover of the black algal turf. On Orona, at a site that back in 2000 I thought was impacted by eutrophication, turned out to also be degraded by a shipwreck that was mostly broken up and not visible from the surface. What the islands do for us is help distinguish the action of different factors from one another--climate change, lagoon waters, shipwrecks--in many ways this is clearer than along a major populated coastline where pollution, eutrophication, fishing and climate change impacts are all mixed in with one another. It'll take some months to really understand what our results are saying.

But for now, with the dives behind me, at least there's the satisfaction of having returned after so many years, re-visiting these magical islands in the middle of the largest ocean on the planet. A real privilege, and now the mission to get the story out about these islands rising again from the ashes of the worst coral bleaching event I've ever seen, and hoping we can make this happen for other reefs around the world.

-David Obura