Q: What causes oxygen to be used up in deep waters?This answer is focused on coastal ocean waters, but generally applies to deep waters in estuaries, ponds, lakes, and slow-moving stretches of rivers.

A: Bacteria use up the oxygen in the process of breaking down organic matter, like the waste from tiny animals and dead algae. Here's how it works...

Under Review

Tiny organisms, known as algae or phytoplankton, use sunlight to make food in a process called photosynthesis. 1

The energy they get from the sun enables them to multiply.2

Algae, like plants, need nutrients such as nitrogen (N) and phosphorus (P), to grow. They also need a suite of trace nutrients, like iron (Fe), as well as vitamins and other growth factors. 3

N P

Fe vitamins

When algae are eaten by little animals, some of the waste from the animals is exported into deeper waters within their fecal pellets, or waste. 4

exportedwaste

yikes!yum!

Some algae settle into deeper waters on their own, where there is insufficient light to continue growing. 5

I'm sinking... Me, too!

In deeper waters, tiny bacteria break down the fecal pellets and algae that have settled. These bacteria consume oxygen that is dissolved in the water in a process called “aerobic respiration.” 6

oxygen

Typically, some factor controls how rapidly algae multiply, or grow. It may be insufficient light. It may be too little of a macronutrient like N or P, or a micronutrient like Fe. 7

N P

Fe vitamins

I need N!

Algae “bloom” when they have everything they need to grow and multiply. This is typical during the spring, when nutrients are plentiful and there's lots of sunlight. 8

During a bloom, or soon after, lots of organic material settles into deeper waters. This drives more bacterial activity, which consumes more and more oxygen. 9

oxygen

Blooms occur naturally, but they also can be triggered by human activities. Prime examples include “point sources,” such as sewage treatment plants, and “non-point sources” of nutrients, such as runoff of fertilizer from farm fields. 10

The longer a bloom lasts, the more organic matter that settles into deeper waters. 11

0 3 days 15 days

The more organic matter that settles, the more bacterial activity there will be. Eventually, the bacteria can use up most of the oxygen. 12

oxygen

When oxygen runs out, fish that can will avoid the low-oxygen, or “hypoxic,” waters. Shellfish and other critters that require oxygen but cannot avoid or escape from the area will die. 13

oxygen

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