By Jason Moak
Researchers at Phinizy Center monitor the oxygen levels in the Savannah and Ogeechee Rivers to better understand how they function and ensure the health of aquatic organisms that live there. Most of us know that oxygen is one of the essential requirements for human life. Oxygen in our bodies is a vital component of many life-sustaining chemicals and processes. The same is also true for most organisms that live in our lakes, rivers, and streams. Just as we humans depend on the oxygen present in the air we breathe, aquatic organisms depend on the oxygen present in the water they “breathe.” As those organisms breathe, they use up the oxygen in the water and it must be replenished. But how?
One source of oxygen in water is diffusion from the atmosphere. Based on the laws of physics, there is a maximum amount of oxygen that water can contain. To understand this, imagine if you were to add a couple of tablespoons of sugar to water and stir. After some time, the sugar granules would dissolve in the water. You could add a couple more tablespoons of sugar and stir and the sugar would again dissolve in the water. At some point, though, once the water becomes full or saturated, the sugar you keep adding would no longer dissolve. The same holds true for how much oxygen can be dissolved in water. If water does not contain the maximum amount of oxygen it can hold (it is undersaturated), then oxygen present in the air above the water can dissolve or diffuse into the water, just like the sugar. This diffusion process is pretty slow, but it can be sped up by turbulence, such as when water falls over a dam or passes through steep, rocky sections of a river or stream.
The other source of oxygen in water is photosynthesis. On land, plants absorb the energy from the sun and carbon dioxide from the atmosphere, use them to grow, and put out oxygen as a byproduct. Similarly, organisms like algae and phytoplankton that live in the water emit oxygen into the water during the day as they grow. Sometimes, these photosynthetic aquatic organisms can be so productive that they actually supersaturate the water with oxygen. When this happens, there are only two ways for that extra oxygen to get out of the water: diffusion (also called off-gassing), or respiration. Diffusion in this case is simply the reverse of what was described above, except here the excess oxygen dissolves from the water into the atmosphere. The other process, respiration, occurs when aquatic organisms like fish or bacteria “breathe” in the oxygen in the water using it for their own growth.
Based on state and federal laws, the waters in our local rivers and streams are required to have a daily average of at least 5 milligrams per liter of dissolved oxygen to ensure that they remain healthy. If you are interested in the dissolved oxygen levels of the places Phinizy Center monitors, check out our website at https://phinizycenter.org/projects/river-monitoring/.