There is a reason why we normally associate corals with clear, shallow waters. They are one half of a symbiotic relationship with zooxanthellae, a microscopic algae, which produces oxygen and other products through photosynthesis. In other words, corals need light to survive and build reefs. So how has an extensive reef been thriving in some of the muddiest waters of the world – the plume of the Amazon river?

‘The plume is huge,’ says Patricia Yager, a Professor of Oceanography and Climate Change at the University of Georgia, who co-authored the paper on the reef discovery. ‘It covers many million square kilometres of the northern tropical Atlantic Ocean. Imagine that huge discharge forming a ten-metre thick layer, which spreads out across the sea.’ The plume is the dense haul of the Amazon river, which carries 1,200 million tons of sediment from the South American continent to the ocean every year. It billows from the mouth of the river, usually spreading northward and skimming ‘like oil on vinegar’ over the saltwater realm of the corals on the sea floor.

“Because the reef stretches across the width of the river mouth, in and out of the plume, it offers a spectrum of light-to-dark living conditions”

A coral of many colours, dredged from the Amazon reef (Image: Moura)

‘The reef animals are not within the plume water, they are beneath it. However, the plume is dark “tea” coloured – from the organic matter and sediment picked up in the Amazon.’ In other words, light doesn’t penetrate it very well and anything living beneath will struggle to photosynthesise, ‘which is where it gets interesting,’ says Yager. ‘The plume does not shade the reef equally: in the north it is completely covered, while in the south it is covered for just a few months of the year.’

Because the reef stretches across the width of the river, it offers a spectrum of light-to-dark living conditions, she says. ‘The more southerly communities there included hermatypic corals (reef-building corals, that form symbiotically with algae) and rhodoliths (calcareous algae that also photosynthesise). As you move along the the shelf to the north, the community transitions to sponges and filter-feeding invertebrates, until there are no living corals. According to Yager, the northern, darkest communities are likely feeding on detritus floating down from the plume. ‘Though the entire reef is home to many other animals, especially baby fish that use the reef as a nursery ground.’

There is a sense of urgency in studying the species variation across this surprise find, as around 80 leases for oil exploration have been delineated and sold in and around the reef system. ‘Some are are already producing oil,’ says Yager, ‘many of the leases are right on top of where we think the reefs are located. I hope the publicity surrounding the paper will encourage the companies and government agencies involved to take another look at the environmental impact of all human activities around these newly appreciated ecosystems.’