Toxic cyanobacteria, also known as blue-green algae, has been found at Harku Lake in Tallinn's Haabersti district, the Health Board said on Tuesday. Swimming is not advised.
Water samples show the safe level of cyanobacteria has been exceeded, the agency said. Swallowing or staying in the water too long can be potentially dangerous.
Those who continue to swim should wash off the bacteria as soon as possible after bathing, ideally, using fresh water and soap.
It is important to ensure that small children and pets do not swallow lake water, the Health Board said.
People exposed to cyanotoxins may experience stomach pain, headaches, muscle weakness, or dizziness.
Those who feel ill after swimming can call the agency's information line 16662.
"Cyanobacteria is a frequent problem in the Harku Lake during the summer," said Oleg Siljanov, the mayor of Haabersti. "Now people have to be careful, follow the Health Board's recommendations, and wait for the results of new water analyses."
Editor: Merili Nael, Helen Wright