An Introduction to Oxygen

For terrestrial (land-dwelling) animals, oxygen is readily available. Approximately 20% of the air around us comprises oxygen. Because that percentage remains nearly constant, there is an ample supply of oxygen in the air for us to breathe.

The story is a bit more complicated for aquatic organisms that breathe the oxygen within water. Liquid water holds less oxygen than air. Also the concentration of oxygen in water is not constant; there are chemical, physical and biological factors which affect the amount of dissolved oxygen in water. To understand this, let’s start with how oxygen gets into the water in the first place.

When we talk about dissolved oxygen, we are talking about free oxygen (O2) in the water that is available for animals to take in for respiration, not the oxygen that is already bound with hydrogen in water molecules (H2O). Free oxygen enters the water through two main avenues: aquatic plants and the atmosphere.

Plants are producers. They take in water and carbon dioxide and use energy from sunlight to produce sugar and oxygen (as a byproduct). This process is called photosynthesis. Plants use the sugar as fuel and release the waste oxygen into their environment. Dense patches of submerged aquatic vegetation (SAV), algae and other types of phytoplankton (photosynthetic plant drifters) in the Hudson produce much of the valuable oxygen for fish and other aquatic organisms in the river.

Oxygen can also enter the water through contact with the atmosphere. Waves act like an aerator, stirring up the water and increasing its contact with the air. As the water mixes with the air, oxygen from the atmosphere is dissolved into the water. This wave action is one reason why there is typically more dissolved oxygen in the shallow water along a natural shoreline.

The amount of dissolved oxygen in water is measured in milligrams of oxygen per liter of water (mg/L). Fish require at least 5 mg/L for optimum health. Most species of fish will be distressed when levels fall below 4 mg/L and mortality can occur below 2 mg/L.

Oxygen levels in the Hudson vary with changes in several chemical, physical and biological factors. Learn more about which factors affect dissolved oxygen levels in our next edition of The BZK Edu Net.

You can keep tabs on the dynamic dissolved oxygen levels in the Hudson with our monitoring data website. The DO reading shows the amount of dissolved oxygen in the water. DOSat shows the percent saturation, or the amount of oxygen in the water compared to the maximum amount the water can hold. Check it out and you too can breathe easy knowing that the fish in the Hudson are getting all the oxygen they need.