Why does algae bloom create low-oxygen dead zone?

Help fight my ignorance about this:

I’ve often seen articles pointing out that excessive nutrients in ocean water (runoff from farms, etc.) cause algae blooms, which leave “dead zones” in the ocean due to oxygen starvation. Example I just saw tonight:
Lawsuits against EPA target nutrients in US waters

Why does an algae bloom deplete oxygen in the water? Everyone knows that green stuff soaks up H[sub]2[/sub]O and CO[sub]2[/sub] and rearranges them to form organic compounds, with leftover O[sub]2[/sub] being given off. And algae grow in the water – so why doesn’t this oxygen stay dissolved in the water? Why does an algae bloom de-oxygenate water instead of oxygenate it?

Unfortunately his is one of those cases where what everybody knows is almost completely wrong.

“Green stuff”, as you point out, liberates oxygen when it is producing carbohydrate. IOW it only liberates oxygen when it is physically growing larger.

But the other half of the equation is why “green stuff’ forms those carbohydrates. And quite simply they primarily do it for the same reasons that animals eat carbohydrate: because it provides something that they can burn to produce usable energy. IOW “green stuff” respires exactly like you do. They absorb oxygen, combine it with carbohydrates, and expel carbon dioxide.

Two sides of the same coin. The problem is that while respiration and the consumption of oxygen goes on continually, 24/7, the liberation of oxygen only occurs while the plant is physically getting larger.

As soon as the light levels drop, such as at night or when the water becomes silty, the plants stop liberating oxygen. But they continue to absorb oxygen at the same rate.

As soon as the nutrient levels in the water decline, the plants stop liberating oxygen. But they continue to absorb oxygen at the same rate.

As soon as the water temperature becomes too high or too low, the plants stop liberating oxygen. But they continue to absorb oxygen at the same rate.

As you can imagine, when you have umpteen thousand cells per milliliter of water, and they are all actively respiring and consuming oxygen and nothing at all is producing oxygen the water becomes anoxic incredibly fast.

The problem is compounded when the plants consume so much oxygen that they literally suffocate themselves. The same occurs when the plants grow so fast that they consume all the nutrients and start to die. The dead bodies are a convenient source of nutrients for bacteria, many of which are facultative anaerobes. So the algae consume all the oxygen in the water, and then die. Then the bacteria start digesting the algal corpses anaerobically. That anaerobic digestion releases all sorts of partially oxidized organics into the water column, and they chemically bind up any scrap of oxygen that might still be left. But the bacteria have trouble digesting the more recalcitrant organics in the algal corpses without oxygen, so there growth rate slows. But they facultative anaerobes. They can survive without oxygen. Then as soon as the oxygen levels start to rise even slightly the bacteria consume the oxygen and use it to digest the remains of the corpses. That process ensures that the oxygen levels can’t return to normal until the corpses sink to the bottom and are covered by other sediments. That can take weeks, and during that time oxygen levels are perpetually depleted.
It’s a popular belief that plants produce oxygen, but the reality is that, to within tiny limits, plants consume just as much oxygen as they produce. IOW plants don’t actually produce oxygen, they just cycle it.


If they’re just making sugar during the day, then eating it at night, the whole thing has to net out to zero unless, as you say, there’s growth going on.
If they grow, then die and are buried in silts or swamps, the hydrogen and carbon gets sequestered (ultimately perhaps becoming coal or oil) and there’s a net oxygen input into the atmosphere.

I’m a little confused by this.
According to multiple reputable sources, cyanobacteria (blue-green algae) are thought to be responsible for most of our atmospheric oxygen.

And cyanobacteria are what usually make up algal blooms (even though they aren’t classified as algae any more, according to wiki).

overall the algae do produce oxygen.

in localized areas where there is an imbalance, like an injection of nutrients, other things can happen.

They only make a net contribution to atmospheric oxygen if they sequester whatever the oxygen was originally combined with (i.e. water and Carbon Dioxide) - that is, if they grow, then die and are buried, in most cases. This is a question of conservation of matter, really.

Plus there are all of the marine organisms that eat the algea. If you dump any sort of organic carbon into the water, organisms will consume it and use oxygen in the process. So, accompanying the boom in photosynthetic algae, there is also a boom in heterotrophic plankton. Eventually the algae will run out of nutrients and stop growing, but the plankton will continue to gobble up all of the algae until they either run out of food or oxygen.

This is in addition to everything Blake said above – algae will consume their own carbon stores whenever they can’t make more.