Lake Baikal

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

Most of the world's large lakes are geologically young and comparatively shallow, carved by glaciers or dammed by rivers within the last few tens of thousands of years, and most are also doomed on a long enough timescale, since sediment washing in from surrounding rivers slowly fills a lake basin in until it becomes a marsh and then dry land. Lake Baikal, in southern Siberia, breaks both patterns at once. It sits inside an active continental rift, a place where the Earth's crust is being pulled apart rather than compressed, and the same rifting that opened the basin roughly twenty to twenty five million years ago is still slowly widening it today, deepening the lake faster than sediment can fill it in. The result is a lake old enough that scientists study its sediment layers, hundreds of meters of them accumulated undisturbed over millions of years, as one of the most complete continuous climate records anywhere on the continent, and deep enough, at a maximum of about 1,700 meters, to hold roughly a fifth of all the unfrozen fresh water on the surface of the Earth in one basin.

That combination of age and isolation has made the lake into something closer to an inland ocean than an ordinary lake, biologically speaking. A body of fresh water that has remained stable and separated from other water systems for tens of millions of years gives evolution enough time and enough isolation to produce endemism on a scale freshwater lakes almost never reach, comparable in kind, if not in scale, to island archipelagos like the Galapagos. More than half of Lake Baikal's recorded species, by most estimates, exist nowhere else, spanning everything from sponges and flatworms to specialized deep-water fish adapted to the near-freezing, high-pressure conditions of the lake's lower depths, conditions closer to those in parts of the open ocean than to those in any other lake on the continent.

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

Seals are, as a group, thoroughly marine animals, built for salt water and, with only a handful of exceptions, found nowhere far from an ocean coastline. Lake Baikal holds the exception that has puzzled naturalists since it was first formally described in the nineteenth century: the Baikal seal, known locally as the nerpa, the world's only seal species that lives its entire life in fresh water, in a lake sitting more than a thousand kilometers from the nearest sea by any waterway a seal could plausibly have swum.

The leading explanation researchers have converged on is that an ancestral population of ringed seals, a species still found today across Arctic seas, made its way up the river systems connecting the Arctic Ocean to the Baikal basin at some point during the repeated sea-level and river-connection changes of the ice ages, when the drainage pattern of the region looked considerably different than it does now, and became isolated in the lake as those connections later closed off. Cut off from the ocean population for a very long stretch of time by the reckoning of a mammal's evolutionary clock, though a comparatively short one against the lake's own twenty-five-million-year age, the isolated population diverged into what taxonomists now recognize as its own distinct species, adapted entirely to fresh water and to prey, chiefly a small, oil-rich fish called the golomyanka found only in Baikal itself, unlike anything its Arctic relatives eat.

The golomyanka connection is itself part of the reason the nerpa could establish itself in the lake at all: nowhere else does a freshwater lake support a food web rich and stable enough, across a large enough water volume, to sustain a top predator built on a seal's energetic needs, one more downstream consequence of Lake Baikal behaving, ecologically, more like a small ocean than like an ordinary lake. The population today, estimated at somewhere in the range of eighty to one hundred thousand individuals by various surveys, though as with several other Baikal population figures precise agreement across sources is limited, remains a live subject of population monitoring and a widely used indicator species for the health of the wider lake ecosystem.