Coral reef at Amazon River's mouth surprises scientists

Scientists have discovered a huge, coral reef hidden in a place they didn't think coral reefs could live – under the muddy waters at the mouth of the Amazon River.

The massive reef covers about 9,500 square kilometres, stretching from the Brazil-French Guiana border in the north to Brazil's Maranhao State in the south, report Brazilian and U.S. researchers in a recent paper in the journal Science Advances.

'Oh my gosh, it is just full of the most amazing, colourful animals I have ever seen on any kind of an expedition like this' - Patricia Yager, University of Georgia

Despite its huge size, it was unknown to science until very recently.

That's largely because it's in an environment where no one would have thought to look for a coral reef, said Patricia Yager, an associate professor of marine sciences at the University of Georgia, who was part of the scientific team that discovered and studied it during expeditions in 2012 and 2014.

Amazon plume

The University of Georgia's Patricia Yager, left, and Debbie Steinberg of the Virginia Institute of Marine Sciences hold up a sample of water collected at the mouth of the Amazon River. (Jason Landrum)

Where the Amazon River flows into the Atlantic Ocean, it generates a huge "plume" – a muddy mixture of fresh water from the nearby mouth of the Amazon River and salty seawater from the Atlantic Ocean, covering about 1.3 million square kilometres. The researchers explored it aboard the research vessel Atlantis.

"The ocean was chocolate brown. It kind of looked like a latte," recalled Yager in an interview with CBC's Quirks & Quarks that airs Saturday.

Scientists have traditionally thought that coral reefs can't thrive in dark, muddy environments like that.

Amazon Plume

Where the Amazon River flows into the Atlantic Ocean, it generates a huge 'plume' – a muddy mixture of fresh water from the nearby mouth of the Amazon River and salty seawater from the Atlantic Ocean, covering about 1.3 million square kilometres. (Lance Willis)

So Yager was skeptical when a Brazilian collaborator, Rodrigo Moura of the Federal University of Rio de Janeiro, mentioned he wanted to check out a report of possible reefs in the area that he found in a journal article from the 1970s.

"To be honest with you, my eyebrows went up, and I thought, 'Have you ever been there?'" she said.

Her own studies were of the plume itself, but once those were complete, the team visited a spot that Moura had chosen. It was at the southern end of the plume, in an area that is only covered by the plume for six to nine months of the year.

The team dredged the area below the boat for about two minutes.

"All the oceanographers at this point are hanging over the side trying to see what's going to come up," Yager said. "Everyone's really excited. Partly, none of us believe that he's going to find anything."

Corals, sponges, fish

When the dredge came back up, they were shocked.

"Oh my gosh, it is just full of the most amazing, colourful animals I have ever seen on any kind of an expedition like this – pink and red and blue and yellow," Yager recalled. "At this particular spot, there were corals, lots of sponges, beautiful little fish, a big snail that's the size of your fist covered with barnacles… just really, really beautiful animals."

RV Atlantis

The University of Georgia's Patricia Yager, centre, oversees ocean sampling equipment on board the RV Atlantis before it's lowered into the Amazon River plume. (Lance Willis)

Yager says she thinks the coral reef is able to exist because it sits below the plume, which floats because fresh water is less dense than seawater. The fast current in the region carries food past the reef for animals to snatch as it goes by, but prevents sediment from settling and burying living things.

Now that the coral reef has been discovered, the researchers are concerned about potential threats in the area, including climate change, fishing and oil and gas exploration.

Yager said she saw a recent map of oil and gas exploration leases in the region. "And oh my gosh, they're just right there on top of where we think the reefs are."

She says the oil and gas companies need to be encouraged to look and see what's on the sea floor before they begin drilling.

Coral samples

Brazilian graduate student Nara Oliveira and Rodrigo Moura of the Federal University of Rio de Janeiro sort through the reef animals brought up by the dredge during the 2012 expedition on board the RV Atlantis. (Federal University of Rio de Janeiro)

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