Lake Baikal

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 In early March, snow still blankets lake Baikal, its deep waters sealed beneath three-foot-thick ice. Over the next two months, under bright springtime sun, the ice will slowly thaw in a process punctuated by cracking sounds not unlike the sharp report of guns. As long as the ice remains in place, though, scientists can set up camp right in middle of this 400-mile-long, 5,000-foot-deep lake.

Russia says "Sacred lake" is 25 million years old-the oldest lake on the planet. It is also the deepest lake, holding more water than all of North America's Great Lakes combined. It's aquatic life comprises more than 1,500 animal species and 1,000 plant species, two-thirds of which are endemic. According to marine scientist Andy Rechnitzer, Baikal is more biologically diverse than other lakes because oxygen-rich water circulates from its surface to its deepest depths, a process likely related to geothermal vents.

One of the most interesting animals in the lake is the Baikal seal, or nurpa, the world's only fresh water seal. Nurpas use their sharp claws to carve dens for their families while ice is still forming. Finding their dens is relatively easy: look for air bubbles trapped in the ice after being exhaled by nurpas. You can also look for small breathing holes poked into snowdrifts by the seals.

For a diver to get into a den is another story. First, a diving crew member must use a small metal saw to cut a small hole in the ice. Then a circular, manhole-size opening is cut with a chain saw, and long poles are used to push the round slab under the ice. To keep the hole from freezing over, it must be constantly raked. A team effort, indeed. Under the ice, the water is warmer than the air [36 degrees F], but it is still very cold for scuba diving. Every 30 seconds or so, divers must tug on safety lines attached to their wrists to let the crew above know that they are all right.

Seen from an underwater perspective, the seal's den is an intricate ice carving, complete with tunnels and an igloo-like canopy that functions as an air pocket. Nurpas are shy, and pups resting on a bunk bed of ice quickly dive into the water when startled by a visitor.

In June, conditions at Lake Baikal are much different. Although the water temperature is about the same as it is in spring, the air temperature is usually in the 60s. Visibility underwater is perhaps 200 feet, many times greater than that in most lakes. The "Great vis" at that time of year is caused by the water's relative lack of minerals and by countless small crustaceans eating the algae, plankton, and bacteria that can cloud fresh water and salt water alike. Clarity does not last long, however. By mid July, an algae bloom produces pea-soup conditions.

Except for the numbing cold that pains their ears, face, and fingers within minutes of entering the water, divers exploring the shallows of Baikal might feel as if they are hovering over a meadow on a sunny day. Looking up from a depth of 50 feet, they can see clouds in the sky. Looking down, they sea fields of fluffy green algae.

The greens spires of three-foot-tall candelabra sponges poke through the algae. Such large sponges, which get their color from algae living symbiotically in their tissues, are not rare in saltwater, but in other freshwater lakes they have no parallel. The sponges are homes for amphipods, alien-looking shrimplike creatures that are as small as specks or as large as human thumbs. And the waters of Lake Baikal hold 240 species of them.

Hiding among the sponges and algae are sculpins, bottom-dwelling fish that are masters of camouflage, their patterned bodies blending in with their surroundings. These ancient fish, like most cold-water species, don't move fast; it's just too cold here to make quick moves. So, the lake's 40 species of sculpins, comprising 80 percent of Baikal's fish biomass, rely on camouflage for protection against larger fish. Pear

Near the lakes northern and, at a depth of approximately 1,350 feet, a geothermal vent provides warmth for the community of sponges, snails, worms, and fish living in the pitch-dark environment. The existence of this vent confirms that Baikal is a place where continental masses are being pulled apart. Photographer Emory Kristof, who has visited the site for the National Geographic Society, explains: "The communities of life resemble organisms normally found in an ocean, which gives weight to the theory that Baikal is an ocean in the making."

One rarely seen creature is the omul, a delicious fish endemic to the lake. Its scarcity indicates Baikal is ecologically out of balance, a result of the destructive effects of industrial development and logging nearby. Vadim Fialkov, of the lake Baikal Limnological Institute, reports that "local environmental groups have put pressure on the government to reduce the amount of effluents that are dumped into the lake. With some luck, we'll get Baikal back to its pristine state and keep it that way. "To help the effort, UNESCO has recommended that the lake and its watershed be designated a World Heritage Site."

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