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OKLAHOMA CITY (KFOR) – Folks around Lake Hefner witnessed a bizarre and unpleasant site Monday morning – a potentially toxic blue-green algae bloom surfaced at the area’s main recreation and drinking water reservoir.

“I thought it was a massive paint spill or something. It’s nasty. I never saw it look this bad,” said OKC resident, Sam Cagnolatti.

“Pretty gross, pretty nasty looking,” said fisherman, Chris Gibson.

In some parts of the lake, a think aqua blue matt of gunk extends yards from the shore.

In other parts, the water is so thick and green it looks like a health-food smoothie.

“We have a blue-green algae bloom. This is one of the bigger ones that we’ve had in the last ten years,” said Leigh Ann Kitsmiller of the OKC Water Quality Dept.

Basically, there are billions of microscopic plants are having a party in the lake.

“When you have warm temperatures, when you have sunlight and you have stagnant shallow water, there is a potential for them to bloom,” said Josh Campbell of OSU-OKC.

Experts say fertilizer runoff from recent rains has also contributed.

They say that there are many different types of blue green algae in lakes – some are toxic, some aren’t.

So what does that mean for our drinking water?

“We don’t pull water from the surface. We pull water down at a 20 ft depth, there are different screens at the intake tower,” said Kitsmiller.

And that water is then treated at a plant to kill any bio threats.

The city says currently, boats are laying down chemicals designed to knock down the algae but still safe for the environment and citizens.

“Yeah, it’s pretty disgusting because it comes back on your lure and stuff,” said OKC fisherman, Donald Terry.

As for recreation at the lake, officials say to avoid contact with the water.

Swimming is already illegal at Lake Hefner.

They also say to keep pets from getting near the water, as drinking it can be deadly to them.

Experts say there are no hard and fast rules for eating fish coming out of the murk but…

“It’s better to be safe than sorry,” said Campbell.

Experts say the chemicals should knock back the mats in 2-3 days, but with rain this week and hot temperature next week, the blooms could be back soon.