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Effects on the Lake

Contamination & Eutrophication

Death by eutrophication — that’s what will happen to Lake Atitlán if nothing is done to save it.

Eutrophication, or depletion of oxygen, is caused by an excess of nutrients (nitrogen and phosphorus) that stimulates the overgrowth of algae in the lake.

When the algae dies, the resulting bacterial decomposition depletes the water of oxygen and it can no longer support life. This process kills bodies of water from the bottom up.

A common reason why people don’t fully comprehend the severity of the threat facing Lake Atitlán is that we can’t see what’s happening.  But the lake’s zero oxygen (dead) layer is rising every year, and its layer of oxygen-rich water is now only a couple hundred feet deep.

What we do see (and smell) are blooms of the blue-green algae cyanobacteria. Cyanobacteria occurs naturally in the lake, but nutrient overloading causes it to grow out of control. The worst bloom (so far) occurred in 2009 and covered a significant portion of the lake surface. Smaller blooms of various strains of cyanobacteria were also detected in subsequent years, and there was a thick bloom in 2015.



The Atitlán basin is volcanic in origin. The lake is surrounded by rock escarpments and volcanoes and has no outflow to the ocean, meaning that everything that flows into the lake remains in the lake.

Most of the excess nutrients that flow into Lake Atitlán come from three sources: (1) Sewage and organic runoff; (2) Runoff from chemical fertilizers, pesticides, and fungicides; (3) Commercial tilapia farming.

Every day, thousands of gallons of raw or improperly treated sewage flow out of the wastewater treatment plants around Lake Atitlán and into the lake itself, adding to the amount of putrefactive bacteria and other pathogens contaminating the lake.  Most of the treatment plants simply remove solids and do little to reduce the volume of pathogenic bacteria entering the lake. In some instances, outflow parameters are actually worse than inflow parameters meaning the treatment center is basically a breeding ground for pathogens.  Resources for maintenance and upgrading these facilities are lacking and of the 7 new facilities that were planned only 1 has been built.

For well over half a century, local farmers have been pressured to abandon natural farming and use chemical fertilizers, pesticides, and fungicides, which run off into the lake during the rainy season. These highly subsidized products are distributed to poor subsistence farmers and coffee growers, who are encouraged to use them copiously to make their crops grow. Consequently, greater and greater quantities of toxic runoff have polluted the lake every year. The fertilizers are particularly high in phosphorous.

Contamination of the lake due to human activity has increased exponentially in recent decades, with the above problems compounded by rapid population growth. The surrounding towns and villages have grown significantly, and Lake Atitlán’s thriving tourism industry is also a factor. Rapid population growth has led to the most alarming problem of all: that in a World Treasure like Lake Atitlán, the government has been allowing commercial tilapia farming on a massive scale. This one activity alone destroys many lakes all by itself!


Hundreds of thousands of people around Lake Atitlán depend on the lake as their primary water source for agriculture, household use, and drinking. Poor water quality is already taking a toll on people’s health and causing a high incidence of diarrheal disease in infants and young children. It is estimated that approximately 25%- 30% of the local population suffers from intestinal infections at any given time, including e-coli, giardia, and amoebas, as a result of contaminated water and poor sanitation. The level of e. coli in wastewater that leaves the treatment plants is frequently measured at tens of thousands of times above the safe limit.

Fishermen are struggling to survive on what they can catch in the lake, and much of the soil is so contaminated with chemicals that it can no longer support farming without the use of yet more toxic chemicals.

Cyanobacteria blooms release toxins that are poisonous to humans, as well to as the fish and invertebrates in the lake.

Fear about the health effects of pollution has a negative impact on the tourism industry, which is important to the local economy and which has suffered particularly severely during cyanobacteria blooms. In the end, if the lake is not saved, tourism will stop (who wants to visit a green, stinky lake?), hotels will close, restaurants will close, the local economies will collapse and land values will go to pennies on the dollar of what they are now. Lake Atitlán is sitting on not only the brink of ecological collapse but also on the brink of economic collapse.  Dark days lie ahead if the problems aren’t fixed…..