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 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.
Please watch the following video:
Gulf of Mexico Dead Zones (1:33)
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 mud at the bottom without being burned.
Want to know more?
Read the Enrichment called Oxygen in Water!