BioEd Online

The remote and lightless deep-sea floor has long been thought to be protected from events on the surface, such as global warming. But it now seems that climate change impinges on the rhythm of life on the seabed after all.

Henry Ruhl and his colleagues at the Scripps Institution of Oceanography in La Jolla, California, regularly visit a site off the California coast to study the ocean floor, 4,100 metres below.

They retrieve samples in sediment traps and measure the amount of organic matter, such as dead or dying plankton and faeces, that drifts to the bottom. And they send down a camera on a sled to photograph which animals are present.

At the bottom of the ocean, the sea cucumber is king. These animals live off the gentle rain of organic particles and come in species of various colours, including some that look like purple balloons.

The researchers report in this week's Science¹ that different species are more prevalent at different times, and that these population fluctuations correlate with food availability and major climate events, including the El Niño weather system.

For example, Elpidia minutissima, an unprepossessing sea cucumber that is the colour of sediment, showed up in many photos in the years before El Niño, when food was scarce. But it practically disappeared when disturbances wrought by the system apparently increased the food supply. By contrast, a white cucumber that is normally rare, Scotoplanes globosa, thrived in plentiful times.

Vulnerable community

The work shows that the abyss is not protected from climate change by the mountain of water above it, and that it does not take long for effects to show up. And Ruhl believes that El Niño is unlikely to be the only event to affect the deep sea. "The climate may be affecting deep sea communities as rapidly as terrestrial or shallow-water systems," he says.

The work is "solid", according to Ron Kaufmann, a deep-sea biologist at the University of San Diego. But he points out that, so far, the researchers have just shown a correlation, rather than any causal link. "It is always tempting to say climate change causes things," says Kaufmann. "The reality is that the actual links have not been completely established."

However, he hopes the data will be enough to convince others that the deep sea is vulnerable, and interesting. "Because it's remote, people think it's just rocks, dirt and a few worms," he says.