Thirteen tonnes of dead fish float in the Rodrigo de Freitas lagoon, creating an unpleasant odor in a high-end Rio neighborhood. Biologists believe that extreme heat is the culprit. The weather phenomenon known as El Nino promotes high temperatures, blocking cold fronts and causing coastal waters to become stagnant. The proliferation of phytoplankton that consumes a large amount of oxygen in the sensitive ecosystem can also cause fish kills.