The unusually hot temperatures could be perfect conditions for algal bloom to grow. The toxic scum can cause illness or death for people and animals.

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High temperatures and a holiday weekend may be the perfect conditions for cooling off in one of the region’s many lakes, but water lovers might want to think twice before diving in.

Algal bloom — scum along lake shores that can be toxic — thrives on sunny days, and the ripe conditions are leading officials to warn that the bloom could be greater this year.

They’re carefully watching Green Lake, which has had a history of algal bloom, and Lake Washington, which has had blooms in it since the winter. Lake Union, which had some blooms last fall, has a history of pollution and sewage dumping in its early days at the center of an industrialized city.

But swimmers don’t seem to mind.

The rush to the beaches is actually heavier than usual, said Seattle Parks and Recreation spokesman David Takami. He said there are about twice as many people at city-swimming beaches, which opened June 20, as there were this time last year — most likely due to the temperature spike.

In areas designated as swim beaches and monitored by the King County’s Water and Land Resources (WLR) Division, swimmers are safe, as long as they keep an eye out for the signage. The county tests these spots weekly from just before Memorial Day to just after Labor Day. For areas not marked as swimming beaches and without lifeguards, including Lake Union, King County’s message is clear: Swim at your own risk.

Toxic blooms can cause illness or even death for people and animals.

King County monitors levels of fecal bacteria, temperature and two toxins produced by blue-green algae near swim beaches to determine whether the areas are safe. Officials are directing lake visitors to websites tracking algae levels and water quality and any signs around the beaches.

“We’re really, really concerned that people understand that although the lake may not have ‘closed’ signs all over the place, there can be places around where it’s not safe,” said Sally Abella of WLR.

Algal blooms have been increasing in Washington lakes in the past few years, said Lizbeth Seebacher, a wetland and aquatic biologist at the state’s ecology department. She said experts think the increase in algal blooms are caused by more days with higher temperatures leading to warmer waters and nutrients coming into the lakes — especially phosphorus. At some lakes, blooms can seem to appear out of the blue, Seebacher said.

Last year, Green Lake closed its swimming area around Labor Day, Abella said. The pattern in the past few years has seen the scum accelerate in mid-August, but it could be earlier this year with the hotter days, she said. Seattle just finished its hottest June on record.

Seattle is pouring about $1.8 million into cleaning up Green Lake in 2016 with a treatment called alum, Takami said. Alum, or aluminum sulfate, would prevent the algae from growing back again, Abella said. The last alum treatment to Green Lake was done in 2004. It worked, Takami said, and there weren’t blooms in the lake until recent years.

Lake Washington is already blooming; it started in December and there were toxic blooms in January, Seebacher said. Another was found in the north end of the lake last week. There hadn’t been recorded blooms on the lake since 2007 — and even then, they weren’t toxic.

This year’s could have been caused by the water staying warm in the milder winter instead of cooling, Abella said, but there’s no way to know what that means for algal bloom levels in Lake Washington this summer.

While swimming beaches in the county are tested weekly, other spots in Lake Washington, Lake Sammamish and Lake Union not designated as swimming beaches are tested every other week in the summer and for change in water quality over time — not water quality for swimming, Abella said.

The city doesn’t have any designated swimming beaches along Lake Union because there’s no gradually sloping shoreline and easy access, Takami said, except for Gas Works Park. But near the park are sediments beneath the surface that are dangerous when disturbed, he said.

The top of Lake Union is mostly clean, because of water flushed in from Lake Washington, said Mike Brett, a professor at the University of Washington’s department of civil and environmental engineering. But the bottom is full of pollutants, which could include industrialized waste from the factories that used to line Lake Union in the 20th century.

Green Lake, meanwhile, still looks green, despite its swimming beaches and the more frequently tested waters there. Brett said it’s because the water can’t be diluted the way Lake Union’s can, so that anything — pet or goose waste, for example — gets washed right in, and stays there.

The county notes that blue-green algae has been growing in the lake since 2011, Abella said, and 2014 had “substantially more” high toxin levels. It’s usually the case that most of the lake is safe except for one or two areas, she said. The swimming beaches at Green Lake typically don’t have high levels of blooms, but last year, blooms were occasionally present there.

Seattle, Renton, Kirkland and Kenmore have had signs up on and off since December warning about conditions, Abella said. Other lakes in the area are in pretty good shape, she said.

But all bets are off in places like Lake Union. Though the middle is sampled twice a month and monthly in the winter, that’s to check water quality over time and not for safety to swim.

“It looks clean,” said Nick Udod, 33, of Federal Way, after taking a dip in Lake Union earlier this week. “It doesn’t stink when you swim in it.”