Deep sea paradox: little food, tons of life

When Craig McClain was a young boy he dreamt of piloting the NASA space shuttle into unknown corners of the Milky Way. As an adult, he explores a different unknown — one that lies in an opposite direction from the space shuttle’s launch trajectory: the deep sea.

McClain is a marine biologist with the National Evolutionary Synthesis Center (NESCent) in Durham, N.C. where he is associate director of science. He spends his days mulling over ecological and evolutionary conundrums of the deep, like why the nearly food-barren deep sea floor is riddled with pockets of biodiversity rivaling that of coral reefs and terrestrial rain forests.

On Tuesday, McClain gave a talk about his work at Sigma Xi in Research Triangle Park. To put in perspective the depth of the marine habitats he studies, consider that the average ocean depth is equal to about 14 Eiffel towers stacked one atop the other. The deep sea is anything below one Eiffel tower tall. This translates to study depths ranging from about 1,050 feet to an average depth of 14,875 feet. Places this deep are really, really hard to reach, which means they are hard to study.

McClain uses submersible robots called ROVs, remote operated vehicles, to probe his deep sea study sites. “They are basically robots on a tether,” McClain joked. At $5 million per ROV, and $50,000-per-day operating costs, these are expensive water toys. But they offer unparalleled sampling opportunities: they can take sediment cores, store samples in a variety of drawers and shoot pictures and video. And the research team does not have to leave the comforts of a terrestrial operating room. Watch this YouTube video to see how they work:


McClain said the deep sea looks like this. No sunlight penetrates its depths.

One of the first things a deep sea novice has to grasp is how limited food and light are in the habitats where McClain travels via his ROVs. No sunlight reaches the deep sea, and so McClain flashed an image of blackness framed in gold leaf (below, left) to drive home his point.

Nutrients rain down from above in a constant drizzle of “marine snow” – mostly whitish particulate matter that is largely carbon. (Marine snow is visible in the video.) About three percent of primary production that occurs at the ocean surface in the form of growing and replicating plankton – microscopic plants and animals – reaches the deep sea.

“It’s a constant snow of detritus at the bottom,” McClain said, “And most of it has passed through the rectum of at least two different organisms as it traveled down the water column. What arrives to the seafloor is a mixture of carbon, inorganic carbon, and other ‘stuff’. The carbon material passes through a lot of guts which both reduces the quantity and quality of the material.”

It may be constant, but it’s also completely random where this manna from heaven may land. And if you are a small worm living here, you want it to land near you. This randomness leads to an odd pattern of where certain species are found.

“Well really there is no pattern,” McClain said, explaining work he conducted using core samples taken in a gridded pattern meant to untangle the relationship of species biodiversity and distance. In one study, his team pulled sediment cores every 50 meters along an axis. He expected to find a pattern explaining why some areas of the deep sea floor harbored certain sets of species while other patches harbored different sets.

“But believe me, there is no pattern, I have looked and looked,” he said. “One spot can be barren, but you revisit it weeks later it’s full of all sorts of organisms.”

He suspects this may be due to two factors. One is the random pattern of marine snow fallout, which organisms flock to and consume. The second is the random nature of how sea urchins, sea cucumbers, seastars, big worms and scavenging crabs plow through an area leaving a wake of relatively species-vacant, aerated mud.

“They basically mass-ingest sediment and extract the nutrients from it,” McClain said. With the loss of nutrients, the many snails and worms and miniature shelled creatures that lived in the mud die or move elsewhere – their mud buffet is cleaned out and it’s time to move on.

McClain said he believes this relationship of the marine snow and the sediment-tilling species churning the sediment in a process he calls “bioturbation” result in a patch mosaic sea floor community. It’s a mosaic because the species set composing each patch vary widely over short distances.

“But it’s really not truly patchy, it’s more like a patch-work quilt,” McClain said. The biodiversity pattern, in other words, is constantly shifting. If you were to lie a one-meter grid on top of the seafloor – as ecologists are wont to do – you’d see shifting patterns of the abundance and distribution of life within the squares. What is going on in one square tells you absolutely nothing, in a predictive sense, about what is going on in the adjacent square. It’s completely random, and ever-changing. It’s the paradox of the deep sea.

In a different study, McClain sought to explain what he called the “conundrum of more food.” In this puzzle, as the availability of food increases in a given area, the biodiversity of species peaks and eventually declines. Which puzzles scientists, who wonder, Why doesn’t more food lead to more biodiversity? While he was a post-doctoral fellow at Monterey Bay Aquarium in 2006, McClain began studying the Monterey Canyon, a marine canyon leading out to sea from the continental shelf just off shore from the aquarium.

“This is like the Grand Canyon of the deep sea, it’s just vast and huge,” McClain said. Because depth acts as a surrogate for food — there is less food available at ever-greater depths — he chose three sites along the canyon spanning from about 1000 meters to 2500 meters deep. Within each site, the oxygen levels were the same but productivity varied widely.

These canyon habitats typically include a vertical cliff face that acts like a giant net capturing marine snow, like rain on your windshield, that slides downward and pools. Giant puddles of food accrue at the juncture of cliff face and sea floor, attracting hordes of sea urchins and snails. And it was in these pooled food puddles that McClain thinks he found the answer to the “conundrum of more food.” By sampling sediment on a transect leading away from the cliff faces, his team found that biodiversity tended to increase away from the cliff faces, where food was comparatively more scarce. Why?

“The urchins basically swarmed the food source, mass-ingested sediment, and then left nutrient poor sediment in their wake,” he said. “So you have this extreme monopolization of the food resource, which either excludes other species or changes conditions so that they can’t live there.”

McClain’s work also delves into the evolution of body sizes, and he is attempting to work out why some species that are small in shallow seas grow to comparatively huge body sizes in the deep sea. And why some species that are small in the shallows balloon up to preposterous sizes in the deep ocean. Here, he invokes the “Island Rule” an ecological truism stating that when animals colonize an island environment, small animals tend to get larger (think: moa birds), and large animals tend to get smaller (think: dwarf deer). He believes deep sea habitat could be delineated as food-limited islands which force species to converge on an optimal body-size that allows them to resist starvation.

“They need to be Goldilocks-sized,” McClain said. “Small enough that they don’t need much food, but large enough to move to a new area to find food. They need to be just right.”

Many people, it seems, want to peer into the deep sea with McClain as he charts new discoveries. He started the blog Deep Sea News in 2005 and after a brief stint in Science Blogs, it now receives about 100,000 to 200,000 hits per month as a stand-alone site. You can keep up with his ideas and projects there, or on his research homepage.

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