Oxygen Saturation – We’re Approaching Sensitive Seasons

As biologists, we collect natural facts and use the information during treks into the field.  That benefits us, but we don’t often share many of the technical, yet important points. Here are some of those boring facts which impact every body of water on the planet…especially yours.

Water is an amazing substance.  Its temperature affects its density.  Cooler water is heavier than warm water, to a point.  Water is most dense at 39-degrees F.  Water’s affinity for oxygen rises as its temperature drops. What about oxygen saturation? Water at 50-degrees holds approximately 11.5 parts per million oxygen.  Water at 86-degrees holds slightly less than 8 parts per million. Most fish need a minimum of 3 ppm to survive.  So, how do oxygen depletions occur?  There are many ways, especially when water is warmest during summer months.  Plants use oxygen, as fish do.  During daily dark hours, plants take-up oxygen and give-off carbon dioxide.  Fish breathe taking-up oxygen.  With big biomass and hot water, sometimes a pond can’t keep up with the demand.  The result is oxygen depletion, almost always at sunrise.

At the other end of the spectrum, here’s a memorable Texas pond checked at 4 p.m.  Oxygen levels were 13 ppm.  It was August and water temperatures in the low 90’s.  Saturation was less than 8 ppm, so how could the test kit read 13 ppm?  Supersaturation.  How could that be?  Simple. Those same plants, which can contribute to oxygen depletion, also can contribute excessively high oxygen levels. During sunlight hours, plants take-up carbon dioxide, use it to create food for growth, and give-off oxygen. Big amounts of plants actually can produce more oxygen than water can physically hold.  While water acts as a sponge, soaking up large amounts of oxygen, it also works to release as much oxygen as possible to reach an equilibrium, saturation.  It simply can’t release excessive oxygen into the atmosphere as fast as aquatic plants photosynthesize it.

So, next time you stroll past your pretty pond, pause a moment and admire all the fascinating things about the wet stuff, even if you can’t see what’s going on in it.