Botswana is home to a third of Africa's declining elephant population.
The alarm was raised when elephant carcasses were spotted in the country's Okavango Delta between May and June.
Officials say a total of 330 elephants are now known to have died from ingesting cyanobacteria. Poaching has been ruled out as a cause of death.
Cyanobacteria are toxic bacteria which can occur naturally in standing water and sometimes grow into large blooms known as blue-green algae.
Scientists warn that climate change may be making these incidents - known as toxic blooms - more likely, because they favour warm water.
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