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Making Waves: Episode 71 (March 17, 2011)

You're listening to Making Waves. I'm Troy Kitch.

I happened to come across a study last week about seamounts from researchers at the Zoological Society of London and New Zealand’s National Institute of Water and Atmospheric Research.

The scientists who penned this new paper combined high-resolution maps created from ship soundings with lower-resolution satellite-gravity data to generate what may be the best estimate yet of how many of these huge underwater mountains are hidden beneath the waves.

Why should we care? Well, in the past, these geologic structures were seen as little more than navigation hazards. Today, seamounts are hotbeds of research because we now know that these mysterious places are biological hotspots that host an amazing amount of sea life. But because these underwater mountains are hidden under the sea and can be hard to detect, we only have a rough idea of how many there are.

This new research gives us a better idea. The team behind the new study found that there are around 33,000 seamounts covering the ocean floor. They estimate that these underwater mountains cover about 17. 2 million square kilometers.

What does this have to do with NOAA? Well, last May, Peter Etnoyer, a marine biologist with NOAA’s Center for Coastal Environmental Health and Biomolecular Research, led a study that also estimated the area of seamount habitat using a different technique. And his research led to a similar estimate of around 18 million square kilometers of ocean floor for 33,000 seamounts. That’s very close to the findings of the newest study … and it offers independent confirmation that the NOAA research was close to the mark.

Let’s revisit that interview today. 

Last May, Peter Etnoyer and several colleagues published a report in the journal Oceanography that takes a fresh look at just how many seamounts might be hidden underneath the waves. In their report, they tallied up and compared the areas of all the major marine and terrestrial biomes on Earth, which Peter said had never been pieced together before all in one place. I reached Peter by telephone at his office in South Carolina.

[Etnoyer] “Technically, biomes are defined as major life zones with similar animals and a similar physical environment no matter where you go on Earth. So coral reefs are one good example. Tropical forests are another. Seamounts are a biome because they’re all deep, and dark, and cold with steep volcanic sides populated by fishes and corals and sponges and they do occur around the world.”

[Etnoyer] “What we found when we compared the seamount biome to all the others -- terrestrial and marine -- is that seamounts are larger than any single terrestrial biome like tropical forests or deserts or tundra, and this makes sense in a way because oceans cover 70 percent of the Earth’s surface, we just never divvied it up before into its constituent habitats.”

The difference between the seamount biome and more well-known biomes like tropical forests, coral reefs, deserts, or tundra is that seamounts are invisible. They’re hard to find. While Peter said there are a lot of ways to find a seamount, one of the newer technologies that’s making it easier to find them is to search from space using what is known as satellite altimetry.

[Etnoyer] “Satellite altimetry looks for little variations in ocean height and where the satellite sees bumps on the sea surface, we can expect to find a large seamount feature below. This is partly due to gravity, because seamounts are so massive that they actually gather up sea water around them, and the slope of this little hill -- although it’s invisible to the naked eye -- satellites can detect a one centimeter deviation from the norm.”

They definition of what a seamount is has changed over time. In fact, Peter said that a new definition of what a seamount is was proposed in the same issue of Oceanography where his report appeared last month.

[Etnoyer] “It was originally defined in the geologic sense as an isolated, submerged volcanic feature with steep sides of a thousand meter vertical relief or more, but lately more and more biologists are getting involved in that definition and biologist see this 1,000 meter cut-off as somewhat arbitrary, because smaller features of 300 or 400 meters are also hotspots of abundance and productivity. So they have seamount-like characteristics. So the new definition recommends that seamounts be defined as elevated features of 100 meters relief or more, and this would distinguish those smaller seamounts from mounds or banks, or anything less than a hundred meters.”

Now, if you just take into account the big seamounts over 1,000 meters high -- that’s about .62 miles, Peter and his colleagues found that there are around 45,000 of them in the ocean. Add in the smaller-sized seamounts, and you can see how these features collectively form one of the largest biomes on the planet.

[Etnoyer] “We were surprised that when you add them all up, we’re estimating 45,000 seamounts in the world larger than a kilometer -- so that’s just the big ones -- and when you add them up, that’s 28.8 million square kilometers of seafloor. It’s about the size of Africa, so I think of it as almost a submerged continent.”

Why are there so many seamounts out there? Well, it’s because we live in a volcanically-active world.

[Etnoyer] “If you look at a map of the tectonic plates, you’ll see that they occur throughout the oceans. We have a few instances of volcanoes recently in Iceland and we had Mount St. Helens before that, but really this kind of volcanic activity is happening all the time undersea, especially along the perimeters of these tectonic plates, but also in hot spots where magma erupts through the sea floor.”

What we haven’t directly addressed yet is what make seamounts so special. Peter said it has to do with how nutrient rich waters flow from the depths of the ocean and are carried by currents up the steep slopes of the seamounts towards the sunlit surface. This flow of nutrients make these places:

[Etnoyer] “…a riot of life above in the water column and below in the benthic habitats. I think of them as Hawaii, but underwater. You know, fishes, there’s whales, dolphins, will surround the sub as you come up from an expedition from the seafloor, and then in the benthos -- in the sediment -- there’s sponges more than a meter tall, there’s corals, and the corals are covered with brittle stars and shrimp and crabs and fishes nestled in their branches. So everything you might see around Hawaii in the shallow waters is something you’d also see around a seamount in the deep, cold, dark waters.”

Many people are surprised to learn that corals, in particular, are found in such deep, dark, and cold places. Peter said that these slow-growing, fragile animals can live for extraordinary lengths of time. Some of the black corals found on the Cross Seamount in Hawaii, for example, may be up to 4,000 years old. The bad news, he added, is that the seamounts that host these corals and so many other creatures are under increasing threat.

[Etnoyer] “The primary threat that’s caused by man is bottom trawling for commercial fisheries like orange roughy. These bottom trawlers will just decimate deep coral habitat. They have been modified with truck tires to bounce and roll over rocks and corals, and sometimes they bring up these corals, and they can be more than a meter tall. You know that these fish are dependent on these kinds of habitats for their survival and reproduction, so it’s disappointing to think that in these days and times that we rely on these kinds of destructive technologies.”

On a positive note, though, he said that these types of problems are being worked on in the global community.

[Etnoyer] “There are a lot of groups who are actively trying to improve our fisheries and trying to regulate these destructive fishing gears, and now we have new laws on the books like Magnuson-Stevens that establish these corals as essential fish habitat, and let us manage these kinds of habitats and protect them from destructive fisheries.”

The important thing Peter stressed was the need for continued, coordinated study of seamount habitats. While the recent report that he co-authored in Oceanography conservatively estimates that the total number of large and small seamounts in the ocean may collectively cover about 28.8 million square kilometers, he said that only around 250 individual seamounts have been biologically sampled. And each one of these seamounts are truly massive. Peter said that once he overlaid the trajectory of a one kilometer dive he made in the Alvin submersible over a large map of one seamount. The seamount was so huge, that he couldn’t even see where he had been on the map.

[Etnoyer] “It’s like trying to describe a seamount is like trying to describe a Monet painting from three bristles of a brush. We couldn’t hope to understand its enormity if we dove on it every day for a year. We still would have sampled a very small amount. So these are truly massive features, and that’s why when you add them all up, they are larger than the continent of South America. It’s just an enormous and unknown habitat.”


And that’s all for this week.

If you have any questions about this week’s podcast, about the National Ocean Service, or about our ocean -- or if you have an ocean fact you’d like answered -- send us a note at

Now let’s bring in the ocean …

This is Making Waves from NOAA’s National Ocean Service.