Salem, Ore. • One of the hottest and driest months of May on record may have contributed to the growth and spread of the toxic algae bloom wreaking havoc in Salem’s drinking water.

The algae bloom was originally spotted in Detroit Lake on May 8 and reached highly toxic levels around May 21, according to officials.

The record heat and dry conditions apparently kept the bloom strong, allowing the toxins to spread from the reservoir into the North Santiam River and finally into Salem’s drinking water for the first time at dangerous levels.

Salem issued its second do-not-drink alert Wednesday for vulnerable populations after the discovery, for the second week in a row, of high levels of cyanotoxins in the drinking water.

“We have a toxic algae bloom at Detroit Lake just about every year at this time,” said U.S. Forest Service Detroit district ranger Grady McMahan. “In most years, we get some rain that helps dissipate the bloom and kind of clear out the lake. But this year we just didn’t get rain — it was sunny and dry for an entire month, which probably helped it.”

The month of May was parched by every standard. It was the fourth-driest and sixth- hottest May in records dating back to 1892, National Weather Service officials said.

In a normal May, the Willamette Valley and Cascade Foothills would get 2.5 to 3 inches of precipitation. This year, only a quarter-inch of rain fell, NWS officials said.

Hot and dry conditions can fuel the growth and potency of toxic algae, said Rebecca Hillwig, natural resource specialist with the Oregon Health Authority.

Hillwig and McMahan said toxic algae blooms have been more common, perhaps suggesting a link to the string of abnormally hot and dry spring months Oregon has seen in the past four years.

“I think it’s fair to say that factors associated with global warming — hotter and drier conditions and a rapid snowmelt — could definitely increase conditions that cause algae blooms,” Hillwig said.

“There’s a lot of factors to consider, but it’s fair to say that we have the potential for more of these type of issues in the future.”

The first evidence of an algae bloom at Detroit Lake occurred May 8, said Lacey Goeres-Priest, Salem’s water quality supervisor.

The city regularly tests for toxins at Detroit and “we had good results for many days following” the first evidence of the bloom, Goeres-Priest said.

In other words, the bloom hadn’t turned toxic yet.

As the weather warmed, the bloom grew and developed toxins — specifically, the cyanotoxins known as cylindrospermopsin and microcystin.

Water samples taken May 21 came back with results May 23 that revealed toxin levels high enough to trigger a health advisory for Detroit Lake.

There is, again, nothing particularly unusual about that.

Locals often reference the wildflowers and algae blooming at the same time of year — it’s become part of life in the small tourist town.

Salem officials were confident enough they issued a news release on May 23 titled “City of Salem drinking water remains safe to drink.”

Even so, there were hints something was different about this bloom.

Instead of one bloom — as was the case in 2017 — testing revealed toxic algae in three locations at the lake: Blowout Arm, Heater Creek Arm and near the dam.

More striking, the toxin levels were high in Blowout Arm — the liver toxin microcystin was tested at 48.21 parts per billion. A health advisory is triggered when levels are 4 parts per billion.

“It’s not the highest level of toxin that we’ve ever seen, but it is high,” Hillwig said. “If you get a high enough level, the toxins can travel a long way downstream, so it could make sense that that’s what happened here.”

The toxins at Detroit Lake led to increased testing by city officials, including at Geren Island Treatment Facility on the North Santiam River.

Those tests showed concerning data.

Tests from May 23 showed the toxin cylindrospermopsin at 6.9 parts per billion — above the safe threshold even for adults, according to OHA officials.

By the May 25 test, levels had dropped to 1.9 parts per billion — safe for adults but not for small children.