Dispatches from the Deep: Light and Dark in the Sea

Part of the Deep Sea Vents Curriculum Collection.

It's a glorious sunny morning out at sea—I have to wear sunglasses up on deck from the glare off the water, and plenty of sunblock on my nose! In about five hours, the ALVIN will return from a day at the vents, and I can’t wait to hear about the latest data the researchers have collected.  

The researchers in ALVIN certainly don’t need to wear sunglasses. It’s dark down there at the bottom of the sea—darker than you can probably even imagine! Let me explain...

The ocean is very, very deep; light can only penetrate so far below the surface of the ocean. As the light energy travels through the water, the molecules in the water scatter and absorb it. At great depths, light is so scattered that there is nothing left to detect. Only the very top layers of the ocean get enough light to support plants, and most of the truly abundant animal life is crowded into the top 200 meters. This upper region is called the photic zone; almost all of the marine plants and tiny microscopic marine organisms that engage in photosynthesis can thrive only in the photic zone.

Fish & Snal
Hydrothermal vent teeming with life. Photo © University of Washington, American Museum of Natural History, and Pennsylvania State University.

Two hundred meters is a lot of ocean, right? Maybe, but the depth of the photic zone is only a tiny fraction of the ocean’s total depth. (The depth of the ocean varies greatly depending on where you are; here at the Juan de Fuca Ridge, the depth below me is about 2,300 meters (~1.5 miles). The greatest ocean depth ever measured, at the Mariana Trench in the Western Pacific, is about 11,000 meters, almost seven miles!) Below the photic zone, from 200 to 1,000 meters, is the aphotic (a meaning without and photic meaning light) zone. In the aphotic zone; all that’s left of sunlight is a dim, dark, blue-green light, too weak to allow photosynthesis to occur. There is food to be had, though; detritus, bits of decaying plants, and animal waste falls from above to feed the organisms in the aphotic zone.

After the aphotic zone, there’s complete darkness. From 1,000 meters below the surface, all the way to the sea floor, no sunlight penetrates the darkness; and because photosynthesis can’t take place, there are no plants, either. Animals that live in the abyssal zone feed on detritus raining down from above—or on each other. And sometimes they make their own light; certain species of deep sea fish and jellyfish have special light-producing cells.

Obviously, organisms who live at the deep sea vents can’t rely on the Sun; instead, many of them rely on the chemicals that come out of the vents—the process they use to create food is called chemosynthesis instead of photosynthesis. Pretty amazing, right? But how can we even see them from the ALVIN if it’s so dark down there?  

Inside ALVIN
Conditions inside the ALVIN are very cramped. Three people, usually two scientists and a pilot, can barely fit. A total dive in these tight quarters might last eight to ten hours. Visit the Web site for the Deep Submergence Operations Group at Woods Hole Oceanographic Institution to learn more about the ALVIN. Photo © Woods Hole Oceanographic Institution, Deep Submergence Operations Group.

As the ALVIN reaches the bottom of the ocean, there is no natural light. The pilots plot the position of the sub with both standard navigation equipment and with special topographical (or 3-D) maps that are made by side scan sonar. To find a specific site, the pilots use the 3-D maps, but they must also rely on visual clues. Visual work is difficult because they only have the light of the sub. Imagine looking for rock samples on land in total darkness, with only a flashlight—things seem to suddenly loom out of the blackness, and it’s hard to see enough to find what you’re looking for. These folks are often trying to find a single small probe, only a couple meters in length, or a specific spot they’ve visited before. Talk about a needle in a haystack! So it’s quite helpful to have a pilot who knows the area well; it’s a bit like when you hike—you look at the map, but even the best map doesn’t show the individual trees that you recognize as you become familiar with a specific place.

To see what’s around the sub, the ALVIN pilots use the spotlights on the outside of the sub and the video cameras mounted on the front of the sub to see what’s around it. There is also a small porthole for a direct view of the outside. The lights of the sub penetrate about 30 feet into the darkness. Video cameras constantly record what comes into view, and the tapes are later played back on board ship to document major events of the dive. All the electronics run on batteries and there is no tether connecting the sub to the research vessel 2,250 meters (over one mile) above it. In other words, the ALVIN—with researchers and crew inside—is totally on its own once it’s launched. Alone in the dark!

But I’m not scared of the dark; I would love the chance to go down to the vents inside  i. Soon we’ll be drawing straws to see which REVEL teacher gets this once-in-a-lifetime opportunity—I hope it’s me!

Yours truly high above the aphotic zone,