Oasis under the ice: exploring the surprisingly rich world hidden beneath the Antarctic ice

International Wildlife, Nov-Dec, 1997 by Jim Mastro

Remote cameras with electronic beams are the newest tools for biologists investigating mysteries in the wild

Heavily insulated in a dry suit and loaded down with scuba gear, I slide into the dark water. My lips, the only exposed part of my body, go numb almost immediately. I give my dive partner the signal to follow, then begin to descend. It is a tight squeeze, as the hole we drilled through the thick ice is beginning to freeze in. Finally, two meters (6.5 ft.) down, I come out the bottom of our passageway into a new world.

My dive partner and I drift down, over a scattering of orange sea anemones and soft corals. Then, at 30 meters (100 ft.), we find ourselves floating over a dazzling array of sponges: long, thin yellow fingers; giant, white vases; pink staghorns; bright yellow cacti; deep, green globes; and bright red clusters. Among the sponges are other bottom-dwelling, or "benthic," creatures: tuber-like sea squirts, bryozoans (tiny animals that form delicate, lacy colonies) and groves of lavender hydroids (flower-like relatives of sea anemones). White sea cucumbers hang precariously on sponge perches. Several perfectly round, bright yellow-green mollusks lie scattered about. Spindly sea spiders crawl over the thick brown carpet of debris that covers the rocky bottom.

We are diving in McMurdo Sound, near Ross Island, Antarctica, as part of an ecology study funded by the U.S. National Science Foundation. Above us is a frozen wasteland--the world's driest, coldest and most barren desert, where hurricane-force winds scour the glaciers and lifeless volcanic rock. Yet here, just a few meters beneath the ice, we are discovering a richness and diversity of animals to rival some tropical environments. Each dive is an excursion into an unexplored world.

We can't afford to sightsee, however. Our team is investigating ways that immobile or sluggish bottom-dwelling creatures keep predators at bay by producing distasteful and toxic compounds. My job is to locate specific organisms and promising new groups of creatures. Working as far as 40 meters (130 ft.) below the surface, we have very little time to complete our task without risking a case of the "bends," or decompression sickness.

I swim over to a large colony of bright yellow "finger" sponges and pinch off a few slim pieces to take back for chemical analysis. It is somewhat like pruning a tree, and the animal won't miss the parts. Since we want to minimize our impact in this unique and largely untouched environment, we carefully avoid stirring up any bottom sediments and judiciously select bits of sponge to collect.

The complexity of the communities we are seeing points to a long history of evolution and development. In fact, the Antarctic benthic community has evolved in response to one of the most stable and predictable environments on Earth. Three key factors shape this unique ecosystem: water temperature, anchor ice formation and an annual plankton bloom.

The water temperature in McMurdo Sound is below freezing year-round. Because it is salt water, though, it doesn't freeze. Only the surface of the ocean, which is exposed to winter temperatures as low as -50 degrees C (-58 F), will turn to ice. This "sea ice" (also called "annual ice" to differentiate it from the thicker, more permanent ice shelves) forms a 1.5- to 3-meter-thick (4- to 10-ft.) lid covering the ocean.

Every spring, though, in a tiny seasonal fluctuation, supercooled water pours out from beneath the massive Ross Ice Shelf to the south and lowers the water temperature by about one-tenth of a degree Celsius. At this slightly lower temperature, ice begins to crystallize on bottom rocks, and even on some unlucky animals. This so-called "anchor ice" grows in flat, hand-sized crystals that interlock, forming a fuzzy, bluish blanket on the sea floor. This icy "blanket," which can be as thick as half a meter (20 in.), completely covers the ocean bottom down to about 12 meters (40 ft.) below sea level. From 12 to 18 meters (60 ft.), it occurs in widely separated clumps. Below 18 meters, though, water pressure keeps the ice from crystallizing.

Normally ice floats, because it is less dense than water. The only reason the ice stays on the bottom is that it is literally "anchored" onto rocks and animals. But occasionally the ice will grow large and buoyant enough to lift its anchor and break free. I have witnessed all manner of items floating under balls of anchor ice: sea stars, sponges, urchins, rocks--even a metal bucket.

The growth of anchor ice essentially scours the ocean floor and helps divide the bottom into distinct biological zones. In shallower areas where anchor-ice activity is greatest, one finds only creatures that can "outrun" the ice before it forms: sea stars, urchins, sea spiders and the occasional giant isopod. In deeper waters, where increasing water pressure impedes ice crystallization, beds of sea anemones and soft corals flourish. Beginning at about 30 meters (100 ft.) and going deeper (where no anchor ice is found), the sea floor is virtually covered with an array of sponges and accompanying organisms.