2015 Expedition: Black reefs: an under-recognised threat to remote Pacific Island reefs

Aquarium researchers and staff are on expedition to the remote Phoenix Islands Protected Area (PIPA) to study the natural history of the islands, surrounding reefs and connecting open water ecosystems. Research on the 2015 expedition will directly inform the management and maintenance of this world-renowned MPA. Today's post comes from the expedition's co-leaders Randi Rotjan and Sangeeta Mangubhai.

We have just spent two days diving McKean Island, one of the smaller islands in the Phoenix group. Around 1km in diameter, we surveyed these reefs in 2000 and 2009. I remember this reef had one of the lowest natural coral cover of all the islands, around 20% in 2000 when the reefs were near-pristine and undisturbed, with an abundance of fish and shark life around the island. The island itself also supports an important seabird colony. There are remnants of the old guano mining settlement on the island.

It has been a shock to dive and find that the reefs around McKean largely devoid of corals especially from the shallows to 15m depth. What was a flourishing reef is now covered in dark fine filamentous-like algae. The island has become what scientists are starting to refer to as ‘black reefs’. Under the water are bits of metal and anchor chain from ships that have wrecked themselves on McKean’s reefs. Our theory is that these wrecks are triggering and/or causing these black reefs to form. On the eastern side is a large wreck from some time in the last 10 years, perched high on the reef flat pushed up by waves and storm action.

The phenomenon of black reefs is still a new and we are in the early days of studying and understanding how and why they occur. We know there are wrecks have been sitting on reefs for decades and reefs have remained largely healthy around them. In fact corals are known to grow on sunken ships and can become a dive tourism attraction. But this is not the case in the Phoenix Islands.

So as scientists we are asking questions like: What is triggering these black reefs? Is this related or triggered by increase in sea surface temperatures like what the Phoenix Islands experienced in 2002/3, 2010 and now 2015? What is the process that is happening? And most importantly is it in any way reversible?

As a Pacific Islander, I am worried about abandoned shipwrecks, this emerging threat to remote Pacific Island reefs. Our countries are made up of many small islands, and our reefs are vital for our lives.

Right now, I am trying to hold onto a glimmer of hope that we did see live coral in some of the deeper waters and on one windward corner. If we can remove the shipwrecks and the scrap metal, perhaps can the reefs come back to life, though we cannot predict how long the recover process would take. These are the questions that plague me, as we head off to our last and final atoll, Nikumaroro.