Researchers Document Significant Changes in the Deep Sea

Although it covers more than two-thirds of Earth’s surface, much of the deep sea remains unknown and unexplored, and many questions remain about how its environment changes over time.

A new study led by scientists at Scripps Institution of Oceanography at the University of California, San Diego, has shed new light on significant changes in the deep sea over a 14-year period. Scripps Institution’s Henry Ruhl and Ken Smith show in the new issue of the journal Science that changes in climate at the surface of the ocean may be impacting communities of larger animals more than 13,400 feet below the ocean surface.

Important climatic changes such as El Niño and La Niña events are well known to affect regional and local areas, but Ruhl and Smith describe how such changes also can extend to the deep ocean, one of Earth’s most remote environments.

“Large animals, the kind you would be able to see if you were standing on the bottom of the ocean, may be impacted by climate just the same as animals in shallow water or terrestrial environments,” said Ruhl.

In 1999, Smith and colleague Ronald Kaufmann showed that seafloor-dwelling animals were experiencing a long-term food shortage. The new study indicates that food supplies have since increased and that climate, food supplies and the abundance of large animals on the seafloor are linked.

Since 1989 members of Smith’s laboratory team have studied a deep-sea location in the eastern North Pacific Ocean approximately 136 miles west of Point Conception off the central California coast. “Station M,” as the location is known, has been the site of one of the longest time-series studies of any abyssal area in the world.

“It’s important to study these places on a long timescale because you can’t predict what is going to happen by just studying it once,” said Smith, a research biologist in the Marine Biology Research Division at Scripps. “If you have changes such as these in such a large portion of the globe, you’ve got to pay attention to it.”

Ruhl and Smith use time-lapse photography, sediment traps and a host of other equipment to capture basic ecological information related to the seafloor community.

The Science paper illustrates a stark contrast in the community structure of the 10 most dominant mobile animals before and after the powerful 1997-1998 El Niño/ La Niña event. Animals examined as part of the study include deep-ocean sea cucumbers, urchins and brittle stars.

While numbers of some animals decreased when food supplies were low during the 14-year period, certain other species seemed to thrive on such conditions. For a number of possible reasons, some of these animals may have a competitive advantage during food shortages.

During their many trips to Station M, the researchers worked aboard the Scripps research vessel New Horizon. Each expedition began with a 30-hour trip out of San Diego heading northwest covering 300 miles. The researchers typically remained at Station M for a week or more to complete the various tasks necessary to retrieve, maintain and deploy instrumentation.

One of the key pieces of equipment they used is a camera mounted on a “sled” that moves across the ocean bottom. Once lowered overboard, the device takes nearly two-and-a-half hours to reach its more than 13,000-foot-deep destination. A small animal-collecting net also makes the trip so the scientists can retrieve and inspect the various animals seen in the photography.

The camera records about one photograph every five seconds. One hour of images can lead to weeks of analysis for the scientists. Forty-eight such photo transects (one transect can be nearly one mile across) were analyzed as part of the study.

“The ocean is a source of food for human populations, but it’s also a place of waste disposal,” said Smith. “It’s important to consider how you impact the deep sea. In that view it’s puzzling that we don’t study the deep sea in more detail.”

Funding for the study was provided by the National Science Foundation.