Jump­ing from the high board of sci­en­tific re­search ves­sel Ti­tov into the frigid waters of Lake Baikal knocks the wind out of me. It’s June, but as 37-de­gree wa­ter seeps un­der my thick neo­prene hel­met, my lips be­come numb and the mem­brane of my reg­u­la­tor starts to rat­tle with the cold. Ti­tov’s en­gine roars loudly and un­pleas­antly, but it be­comes a bea­con on our way back. A sci­en­tific team from the Baikal Lim­nol­ogy In­sti­tute and I check our

equip­ment and start to de­scend into the depths, where the white flour­ish of a tran­sect is vis­i­ble, a marked rope that helps to com­pare con­di­tions in dif­fer­ent years at the same spot.

The wa­ter is ab­so­lutely clear, but the view is far from nor­mal. Green slime is ev­ery­where, a car­pet stretch­ing to the hori­zon; to­day only 13 per­cent of the lake shal­lows is free from it. We in­spect the bot­tom near the town of Listvyanka, on the south side of the lake, in Rus­sia’s “Baikal Riviera,” just north of the bor­der with Mon­go­lia; the tourist town is the most pop­u­lar place on Baikal. The wa­ter is still as trans­par­ent as air, so guests here have no idea that the lake is not as clean as it seems.

The sci­en­tific team be­gins tak­ing sam­ples of wa­ter and al­gae and mak­ing pho­to­graphs. Dur­ing our three­week ex­pe­di­tion, they will do from three to five dives per day, spend­ing about five hours a day in the chilly waters, rac­ing to com­plete their re­search of in­va­sive Spirogyra al­gae be­fore funds and warmer weather run out.

Back on Ti­tov, they will have a cou­ple of hours to eat and have a nap be­fore the next dive. While they rest, their field lab­o­ra­tory in­ves­ti­gates the sam­ples, work­ing to com­plete their tasks be­fore divers bring them an­other batch.

For two years, Baikal Lim­nol­ogy In­sti­tute has de­voted it­self prin­ci­pally to ex­plor­ing the is­sue of Spirogyra, a genus of fil­a­men­tous, free-float­ing green al­gae that is stran­gling the world’s largest fresh­wa­ter lake by vol­ume.

The root of the prob­lem is grow­ing tourist ac­tiv­ity. Most set­tle­ments and camp­sites on the lakeshore have no sewage-treat­ment fa­cil­i­ties. Those that do don’t meet estab­lished stan­dards. As a re­sult, the lake takes in too much hu­man waste and de­ter­gent. For Spirogyra, it’s a par­adise of ni­trate and phos­phate fer­til­iz­ers.

Lake Baikal has a spe­cial com­po­si­tion: Its waters con­tain very lit­tle or­ganic mat­ter and a lot of oxy­gen, even more than in hu­man blood by per­cent­age. The slime Spirogyra stops the cir­cu­la­tion of wa­ter at the bot­tom of the lake; oxy­gen in the bot­tom layer is quickly used up, and the spot be­comes un­in­hab­it­able for na­tive species.

Baikal is vast — the nearly 400-mile-long lake con­tains 20 per­cent of the world’s fresh wa­ter not locked in ice, more than the U.S. Great Lakes com­bined — and densely pop­u­lated by var­i­ous an­i­mals, but the num­ber of “hot spots” of bio­di­ver­sity in the lake is fewer than the fin­gers on a hand. Two per­cent of the lake’s area con­tains 90 per­cent of its species de­scribed to date, and all of those places are over­grown with slime. In­creas­ing num­bers of tourists means jobs and money for the re­gion, and dis­as­ter for the lake.

Pro­fes­sor Dmitry Shcherbakov, the head of Baikal Lim­nol­ogy In­sti­tute’s ge­net­ics lab­o­ra­tory, has been work­ing on Baikal since 1989. “It seems the alien al­gae has such a per­fect ge­netic com­po­si­tion that it thrashes other lake in­hab­i­tants like a bully on the dance floor,” he says.

“In my opin­ion, a phase tran­si­tion is oc­cur­ring. Ecosys­tems can change their com­po­si­tion very quickly and very dra­mat­i­cally,” Shcherbakov says. “Where will it hap­pen, and when? I’m not sure that there is a sim­ple math­e­mat­i­cal model that could pre­dict that. But it is pos­si­ble that an ecosys­tem can sud­denly — bang! — change to a new one. This slime might be­come truly a Baikal species, but it will be a com­pletely dif­fer­ent Lake Baikal.”

Sci­en­tist Igor Khanaev takes sam­ples of biofilms cov­er­ing a Baikal sponge near the town of Listvyanka, Rus­sia.

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