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This file photo taken on May 25, 2020, and provided on July 3, courtesy of the charity National Park Rescue, shows the carcass of one of the many elephants that have died mysteriously in the Okavango Delta in Botswana.-/AFP/Getty Images

Investigators say the mysterious deaths of 330 elephants in Botswana were caused by toxic blooms of cyanobacteria, a dangerous phenomenon that has been increasing worldwide because of climate change.

The elephants died between March and June this year in areas around the famed Okavango Delta, one of Africa’s greatest tourist attractions and conservation successes. Many of the animals had seemed to walk dizzily in circles near waterholes and then dropped suddenly dead.

Hundreds of carcasses were spotted by aerial surveys in Botswana as the crisis worsened.

Their tusks were untouched, making it unlikely that poachers had poisoned them in an illicit hunt for ivory.

Botswana, with an estimated 130,000 elephants in its territory, is home to almost a third of Africa’s elephant population, which has gone into severe decline because of widespread poaching and habitat destruction, adding greater urgency to the search for the cause of the Botswana deaths.

Investigators spent months studying the hundreds of baffling deaths, sending test samples to specialized laboratories in Canada, South Africa and other countries. They announced Monday that the deaths were caused by neurotoxins in the waterholes from a species of cyanobacteria, a blue-green algae.

“Our latest tests have detected cyanobacterial neurotoxins to be the cause of the deaths,” said Mmadi Reuben, principal veterinary officer at the Botswana Department of Wildlife and National Parks, at a news conference. “These are bacteria found in water.”

The deaths seemed to stop by the end of June, as waterholes and water pans were drying up. Other animals were unharmed by the phenomenon; officials believe the elephants may have been affected more severely because they ingest larger quantities of water and because they drink at deeper levels in the waterholes, closer to the silt where toxins are concentrated.

There is no official indication yet that the Botswana deaths may be linked to the unresolved deaths of about 25 elephants near a major national park in neighbouring Zimbabwe last month.

The warming of water sources in Southern Africa could be a key factor in the growth of cyanobacterial blooms in the region, where temperatures are rising at twice the global average, scientists say.

A global review of the expanding number of harmful cyanobacterial blooms, published in 2018 in the journal Nature, concluded that the trend was linked to climate change. The review was written by scientists from the Netherlands, Scotland, Switzerland, China and the United States. It did not mention evidence from Africa, but cited examples from many countries worldwide, saying the toxic blooms can kill or injure fish, birds and mammals.

“Cyanobacteria have inhabited aquatic ecosystems throughout much of Earth’s history,” the review concluded. “There is mounting evidence, however, that harmful cyanobacterial blooms have increased on a global scale during recent decades, and they are likely to expand further in coming decades owing to continued eutrophication, rising atmospheric CO2 concentrations and global warming.”

The review said the blooms are “increasing in frequency, magnitude and duration globally” and need to be countered with mitigation strategies.

“This trend is of great concern, as it may have negative effects on the biodiversity and functioning of aquatic food webs and threatens the use of affected waters for drinking water, bathing, fishing and other recreational uses,” it said.

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