Oxygen in Water

Water doesn’t hold much oxygen, so lakes and the oceans are relatively low in oxygen, especially if the water is warm. Oxygen made in the water by growing plants tends to form bubbles that rise and escape to the air above. Aquariums often need “bubblers” to add air to the water and give the fish enough oxygen to breathe. Running water, or fast currents in the ocean, do this job in nature, picking up a little oxygen at the surface and taking it down to fish and worms and other creatures. But if the currents are slow and a lot of dead plants are sinking, the bottom of the ocean or a swamp or lake may have more plants to be “burned” than oxygen to burn them.

Sometimes “dead zones” form in ocean water above the bottom, where the decay of sinking plants uses up almost all the oxygen so that fish and other large creatures cannot live. Such dead zones are especially associated with places where runoff of human fertilizer from fields on land causes huge blooms of algae.

Video: Gulf of Mexico Dead Zones (1:33)

This NASA image shows the Mississippi River emptying into the Gulf of Mexico (the blue arrow follows the main river outlet through the Mississippi Delta). The brownish color of water as it leaves the Mississippi and enters the Gulf is mostly from sediment in the water, but also from algae. Farther from the shore much of the sediment has settled out, and the water is greenish from algae; dead zones often form below the surface in the summer here. Even farther out, much less algae grows, and the water appears dark blue.

Click here for a video transcript of "Gulf of Mexico Dead Zone".

PRESENTER: Burning plants to release the energy in them takes oxygen. Now, when plants grow in water, the oxygen often bubbles out, rises, and goes up into the air. And so the plant is down there with less oxygen than it originally produced. And when the plant is burned, it may use up all the oxygen that's available, unless currents pick up oxygen from the air and bring it down.

Now, what we're looking at here, the Mississippi River comes flowing down, and it feeds the great Mississippi Delta, which is a huge pile of mud that's been carried down by the Mississippi. Out here is the Gulf of Mexico, clear water. And you can see the mud being washed off of the continent and carried into the ocean by the Mississippi.

But mixed in with the mud, all the green you see here are plants. They're algae growing. And the algae give off their oxygen. It goes to the air. And then when the algae die, they tend to sink a little bit, and then they get burned. And that may use up all the oxygen.

And especially because fertilizer from human waste and from farms goes in the water, and that fertilizes these blooms. There are often dead zones out here in the summer, when there's no oxygen in the water. And that's the sort of situation that contributes to the formation of fossil fuel, but it also kills fish and makes poisonous gas and all sorts of other things that we don't really like.

Source: NASA Goddard Earth Sciences Data and Information Services Center

More commonly, though, oxygen is present in the water but scarce in the sediments beneath. Almost everywhere in lakes and the ocean, sediment is piling up at the bottom. This may include large pieces of rock—sand and gravel—washed into the water by rivers, or carried across by melting icebergs and dropped. Smaller pieces are more common—silt and clay, sometimes just called mud—with most of the small pieces washing into the water in streams, but some blowing in, and even a tiny bit sifting down from meteorites. This sediment also includes organic matter (dead plants and animals).

Strong currents carrying plenty of oxygen tend to carry away the small pieces of mud and the dead plants, leaving sand and gravel without much organic material, and with big spaces between the big grains that oxygen-bearing water can move through. Where currents are slow, mud and dead plants accumulate, and the tiny spaces make it hard for water to move through, carrying oxygen. As worms and bacteria start to burn the dead plants, the oxygen is exhausted and the burning stops. So, where lots of plants grow in still, warm water, dead ones tend to pile up in the mud at the bottom without being burned.

Want to know more?
Read the Enrichment called More on Oxygen in Water at the end of the module!