Pictures: Green Slime Invades World’s Deepest Lake

Russia’s Lake Baikal has a new threat that worries scientists.

The world’s deepest lake was long known as one of the most pristine. But in recent years, a bizarre invasion of green slime has taken over areas of Russia’s Lake Baikal, leaving scientists looking for possible sources of pollution.

Located in far eastern Russia, Lake Baikal “has got to be one of the top five most beautiful places in the world,” says Stephanie Hampton, a lake ecologist at Washington State University who has spent a decade studying Baikal. “But these dramatic algal blooms need attention,” she adds.

Scientists from Russia and elsewhere have been observing dense mats of filamentous algae on the bottom of the lake each fall since 2008. A recent study found the algae belong to the genera Spirogyra and Stigeoclonium.

The mats, resembling green shag carpets, have been popping up in some areas just offshore of towns, which suggests human pollution may be to blame, says Hampton.

Scientists are looking for a cause, but they suspect nutrients are washing into the lake from human sewage or from animal waste used as fertilizer. Such nutrients can cause a bonanza of algal growth, particularly in a lake normally characterized by high oxygen content and low nutrient levels.

If a source of pollution is identified it should be corrected as soon as possible, before it causes wider impacts in the lake, says Hampton. “The good news is we know how to fix sewage problems, with treatment,” she adds.

Baikal is known for its high endemic species diversity, including hundreds of species of fish and invertebrates. The ecological impact of the algae is unknown, although scientists have measured a sharp decrease in oxygen above the mats, which might spell trouble for aquatic life.

Scientists also are concerned that Baikal has been warming due to climate change. Hampton’s research suggests that this has triggered changes in plankton—the base of the food web—leading some to wonder how long the lake will retain its reputation as a deep, clean oasis. (Learn more about warming lakes.)

A version of this article appeared in National Geographic's Russian edition.