PACHIRI — This small Bolivian village of adobe houses sits at the end of a peninsula that juts into Lake Titicaca. Several fields are visible in Pachiri's surroundings, where women sort crops on the ground as a handful of llamas, their ears decorated with multicolored ribbons, cross the Altiplano.

The ochrous outline of the shoreline, fringed by eucalyptus, and the deep blue sky — we are at an altitude of nearly 4,000 meters (13,000 ft) — form a picture-postcard landscape. But in one quick gesture, local farmer Mario Mamani, turns it into a nightmare.

"Look," he exclaims, brandishing black roots of a reed just ripped up from the edge of the lake. "The roots are completely asphyxiated — the algae we use to dry on our fields has contaminated the soil. Fish are dying, and we can't even water our quinoa and potato crops. The lake water is filthy ... We're doomed!"

We are in the bay of Cohana, where the rivers running from El Alto finally meet Lake Titicaca, in an industrial suburb of La Paz, the Bolivian capital. The rivers, visible between banks of totoras — Titicaca's typical reeds — carry along heavy and putrid water. They've spawned endemic pollution, particularly alarming when we consider that this "small lake" is on average less than 10 meters deep.

But since last April, a new phenomenon alerted lakeside residents. Huge patches of green algae appeared, drifted and decomposed, giving off an unbearable stench. Much of the fauna, deprived of oxygen, did not survive, including renowned giant Titicaca water frogs studied by the great French underwater explorer Jacques Cousteau in the 1970s.

[rebelmouse-image 27089361 alt="""" original_size="1024x685" expand=1]

Lake Titicaca's green algae — Photo: Rocco Lucia

"On the beaches of Brittany ( France ), unfortunately, we are used to phenomena like eutrophication due to an excess of nutrient, but it's the first time that it's happened here," confirms Xavier Lazzaro, a biologist at the French Institute of Research for the Development, who has been studying the waters of Titicaca for more than 30 years.

For the researcher, this exceptional episode in an Andean tropical lake known to be very poor in organic matter, is a result of a growing population on the shores, combined with unusually long rainfalls. "The rains flowed down the coast and carried along nutrients but also waste water that discharge directly into the lake ," he explains. "This oversupply fostered the growth of algae that ended up occupying one third of the lake for more than one month. We've never seen anything like this."

Today, there are three million inhabitants in the catchment basin and no purification plant has proven effective. On the Bolivian side, the main problem is El Alto. In this sprawling and flat city — well-known destination for all migrants from the Altiplano and home to 1.2 million inhabitants — 130 factories operate, 60% of which illegally.

Action plans and promises

On the Peruvian side, the city of Puno discharges its waters without any treatment. Recent research also shows significant levels of heavy metals — mining residue in the Andes — and antibiotics. "Lake Titicaca is divided between Bolivia and Peru, which makes the compilation of data difficult," says Lazzaro. "Of all the large lakes in the world , it's the last one that doesn't benefit from regular scientific monitoring."

[rebelmouse-image 27089362 alt="""" original_size="1024x768" expand=1]

Llamas on the Bolivian shore of Lake Titicaca — Photo: Matthew Straubmuller

In any case, the green algae episode was a wake-up call for locals. Lake Titicaca, considered the highest navigable lake in the world, was until then seen as the symbol of Andean purity. That is where the founders of the Inca Empire are believed to have started their combative hike, at the foot of a cordillera, which rises to over 6,000 meters (20,000 ft). It is a mythology incompatible with images of dead fish floating in greenish water.

The presidents of Peru and Bolivia, who met on June 23, are expected to carry out a joint action plan. Lima has already planned to construct 10 treatment plants for the sum of $470 million. In La Paz, authorities mobilized hundreds of volunteers to clean the lake's tributaries , before launching a call for projects.

"We've received more than 50 proposals from research centers or organizations that have ideas for protecting the lake," says Bolivian Environment Minister Alexandra Moreira. "Titicaca is a national symbol for Bolivia, people are committed."

These announcements, which made national headlines, went unnoticed in Pachiri. Here, on the shores of a lake supposed to be one of the purest of the planet, inhabitants of Aymara origin equip the roof of their houses to capture rainwater. "We need to act quickly," explains the farmer, Mario Mamani. "After school, more and more of our youth choose to leave to start a new life elsewhere. Look at the hills around us: It's desert and dry. Without water, we can't live."

Nearby, the local village chief, a traditional staff slung over his shoulder, does not hide his fatalism. "It's true that government officials came once to make promises," he says. "They had their pictures taken and left. Don't know if we'll ever see them again."