The central basin of Lake Erie is just the right depth for a hypoxic area called the dead zone. (Cleveland Water)

It sounds like a horror movie: the dead zone.

But it happens every year in the central basin of Lake Erie, when warmer water stratifies from the colder water below and the oxygen gets used up by decaying organisms.

Fish flee because the water has too little oxygen for them to survive. Organisms that can’t move – including quagga mussels, mayfly larvae and invertebrates – die and decompose. Decomposition uses up more oxygen. And so the dead zone grows, typically from July until October, stretching as large as 10,000 square miles, possibly from Cleveland to Canada.

“It’s a snowball rolling downhill until the water temperatures equalize,” said Tory Gabriel, the extension program leader for Ohio Sea Grant.

The dead zone doesn’t affect just fish, either. It can discolor your drinking water.

At times, Cleveland’s water intake pipes could be part of the dead zone, where the lower pH allows water can be discolored because it absorbs manganese from the bottom of the lake.

Here’s how the dead zone works – and how it relates to the annual harmful algal blooms.

How long has the dead zone existed?

Likely for centuries. But it’s gotten worse in the modern era with the influx of phosphorus from fertilizer runoff, which causes the toxic blue-green algae that coats the surface of the lake in the western basin. It can be a different size and shape every year.

Why does it just affect the central basin of Lake Erie? And why not the other Great Lakes?

“Lake Erie’s central basin is the perfect depth,” said Brenda Culler, project coordinator for lake health and water quality communications at Cleveland Water.

When the warm air and sun warms the top of the lake, stratifying the water based on temperature, the layer of rapidly declining temperature in the middle is called the thermocline. Oxygen from the top cannot penetrate the thermocline. Therefore the bottom becomes hypoxic, which means low oxygen.

Culler compared the lake to a glass, with cold lemonade at the bottom and hot tea at the top. Until the temperatures equalize, the liquids won’t mix.

The central basin of Lake Erie is an average of 60 feet deep, so there’s not enough water beneath the thermocline for fish and other organisms to survive.

The western basin, around Toledo and the Erie Islands, is too shallow to form a thermocline. And the eastern basin, around Erie, Pennsylvania, and Buffalo, New York, is up to 200 feet deep, so there’s plenty of room for organisms to live below the thermocline. The rest of the Great Lakes are also too deep, though there is a dead zone in the Gulf of Mexico and other bodies of water worldwide, Gabriel said.

How big was it this year, and how bad?

Scientists are still collecting and comparing data. But Cleveland Water officials say this year was pretty mild, in part because of big storms, which mixed up the layers of water and added oxygen. All those whitecap waves you see in the lake on windy days? The white part is aeration, where oxygen is dissolving into the water, said Scott Moegling, water quality manager for Cleveland Water.

The dead zone period ended earlier than usual, by the beginning of September.

How does the dead zone affect our drinking water?

In the dead zone, the deep below the surface, where the intake pipes are for Cleveland’s water system, is colder and has a lower pH. That allows the water to absorb manganese, which is not harmful to humans but can smell bad and look yellow. Iron would make the water yellow, brown or red. cause discoloration.

But Cleveland water has two buoys in the lake, at about 3 miles and 15 miles out, to monitor for low oxygen and a bunch of other parameters. That way, officials know the low-oxygen water is coming.

“Early warning is our best defense,” said Moegling. “We can treat anything.”

How is the dead zone related to harmful algae blooms?

You might hear lake lovers refer to both in one breath, but they’re separate problems. And while drought can keep blooms in check, it can make the dead zone worse.

But the two issues are related. Because the more phosphorus and nitrogen in Lake Erie, the more toxic algae grow. And the more it grows, the more it decays, sinking to the bottom and using up oxygen – therefore creating a more pervasive dead zone, more quickly.

Warmer weather, and water, also increases the size of the harmful algal bloom, as well as the stratification of the lake.

What can we do?

Don’t dump yard waste or dead leaves into storm sewers or directly into the lake, Culler said. They decompose, using up oxygen.