Jungle Down There: What’s a kelp forest doing in the tropics?

Biologists say that they’ve found dense, underwater forests of kelp, a cold-water lover, off the tropical Galápagos Islands.

LEAFY LUNCH. Marine creatures such as this Galápagos iguana feed on productive beds of kelp. Tropical kelp grows at least 25 meters deep. S. Connell

And a new computer model predicts that many more of these richly productive ecosystems could lie undiscovered in low-latitude oceans, say Michael Graham of Moss Landing (Calif.) Marine Laboratories and an international team of collaborators.

Marine biologists had known that at least three species of the supersize brown algae called kelp turn up sparsely in a few spots in the tropics, says Graham. But the new model identifies a total of 23,500 square kilometers of potential sweet spots for kelp in the tropics around the world. The first real-world test of those predictions led divers to eight uncharted Galápagos patches, Graham and his colleagues report online and in an upcoming Proceedings of the National Academy of Sciences.

Kelp forests and other marine-algae beds are cold water’s answer to tropical coral reefs. The stands of giant kelp off California, for example, grip the ocean bottom and send stalks sprouting as much as 30 meters toward the water’s surface. These forests house abundant marine creatures, from vulnerable fish larvae to sea otters.

A recent flood of data about the oceans inspired the kelp-habitat model, says Graham. He and his colleagues used low water temperature as an indicator of dissolved nitrogen adequate to support kelp. It needs cold, nutrient-rich water welling up from the deep plus enough sunlight and a seafloor suitable for anchoring.

Graham says that the possible habitats identified by the model included all the tropical spots where sparse kelp had been collected to date, even a spot in the Philippines that he hadn’t previously known about. One of his collaborators, Louis Druehl of the Bamfield (British Columbia) Marine Science Center, had kept that location secret from other team members as a test of the model’s power. Only when Graham mentioned that the model predicted kelp in the Philippines did Druehl reveal a decades-old Russian-language article reporting a few kelp specimens there.

In the Galápagos, biologists already knew some locations for Eisenia galapagensis kelp, but the modelers hoped to find more.

After the researchers lost two remotely operated underwater vehicles on the first day, they had to explore by diving. When he and a student made the first dive to a predicted kelp-friendly spot, Graham reports that “I went down, cleared my mask, and there was kelp right in front of me.”

Brenda Konar of the University of Alaska, Fairbanks says that the highlight of the work for her is its suggestion that deep tropical kelp forests have served as stepping-stones allowing the forests’ species to spread around the world.

James Leichter of the Scripps Institution of Oceanography in La Jolla, Calif., points out that biologists already know of other kinds of algae patches growing in deep water near coral reefs, such as that at a site he studies off the Florida Keys. He calls the Graham group’s paper “an indication of the extent to which we still know surprisingly little about basic ecosystems” at these depths in the ocean.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.