An Accidental Experiment
When Brazil built its new capital, Brasília, in the late 1950s, an artificial lake was part of the master plan. The damming of the Paranoá River in 1959 created Lago Paranoá, a 37.5 square kilometer reservoir intended to increase the region's humidity, generate some hydroelectric power, and provide recreation for the city's inhabitants. The city's population grew much faster than planners anticipated. This rapid, uncontrolled urbanization led to massive quantities of raw and inadequately treated sewage being discharged directly into the new lake.
By the 1970s, the lake had become a textbook example of cultural eutrophication. The immense load of nutrients, particularly phosphorus and nitrogen from the sewage, fed explosive algal growth. Water transparency, measured by Secchi disk, dropped to as low as 0.65 meters. The phytoplankton community became dominated by the potentially toxic cyanobacterium Cylindrospermopsis raciborskii. These blooms turned the water a murky green, created foul odors, and led to frequent, large-scale fish kills as the decomposing algae consumed all the available oxygen in the water. During this period, which lasted into the 1990s, the lake was largely considered unfit for any recreational use.
The Road to Recovery
The turning point for Lago Paranoá came in the early 1990s with a committed effort to clean up the wastewater entering the lake. The local water and sanitation company, CAESB, invested heavily in upgrading the city's two main wastewater treatment plants (WWTPs), known as ETE Sul (South) and ETE Norte (North). These plants were retrofitted with advanced tertiary treatment systems specifically designed to remove phosphorus, which had been identified as the limiting nutrient for algal growth.
The results were dramatic and surprisingly swift. After the upgrades became fully operational in 1993-1994, the external load of phosphorus to the lake plummeted. In the years following, water quality showed significant improvement. Chlorophyll-a concentrations, an indicator of algal biomass, dropped by a factor of ten between 1999 and 2011. Water transparency increased, and the dominance of cyanobacteria gave way to a more diverse and healthy phytoplankton community. By the early 2000s, the lake was once again safe for swimming and water sports, and in 2017 it was even approved as a source for the city's drinking water supply. The restoration of Lago Paranoá is now considered a success in urban lake management.