by Bob Cordello – PondBoss.com
There has historically been a misunderstanding about what it means to aerate a pond. When I strike up a conversation with a pond neophyte they will commonly talk about aeration being a priority for their new pond.
The most common comments are that a pond needs aeration so the fish can breathe, and that the best ways to do this are with a fountain or a diffuser. Bubbles equal oxygen, right?
Not so fast…
Let’s start with some simple water physics.
Things diffuse into and dissolve into water. Water is the universal solvent. Air dissolves into water.
The percentage of oxygen in air is 20.9476 %
Oxygen will move across the “air/water interface” (remember this term ) until it reaches saturation.
Saturation is the measure of how much oxygen water will hold at a certain temperature before it starts to spit oxygen back out into the atmosphere.
Salinity also affects how much oxygen it takes to saturate water. Higher salinity means water holds less oxygen.
If water is less than saturated it wants to take up oxygen out of the atmosphere and this is, in turn, good for fish.
When you put a diffuser on the bottom of your pond and bubbles start to come out of it, the fish really aren’t interested in gulping the bubbles. What they really want is the pure oxygen that’s diffusing out of the bubble across the “air/water interface”.
In high school physics class we learned that lots of smaller bubbles have a lot greater surface area than a few big bubbles. That’s why a good diffuser makes the bubbles as tiny as possible. Lots more little bubbles means more surface area, which means more “air/water interface”, which means more oxygen diffused into the water for the fishies to use.
Make sure to get a good diffuser. This helps. But what a bottom diffuser aerator system really does is brings a big column of water up from the bottom so that the unoxygenated bottom water gets a chance to gulp some oxygen from the gigantic “air/water interface” at the surface of the pond. This produces a lot more usable oxygen than the bubbles themselves.
How else can we increase this “air/water interface”?
Waves are a good way. I’m sure it is intuitively correct to all of you that a wavy surface has a lot more surface area than a flat surface. Big ponds and lakes usually have more wave action. This means they are not as susceptible to universally low oxygen levels as a pond. That’s why we rarely need to oxygenate a 200 acre body of water. There are lots of 200 acre water bodies that would benefit from aeration–just not as many critical oxygen crashes as there would be with a one acre system.
Waves are good.
What’s another way of increasing the “air/water interface”?
How about circulation? If you stir water by using, for example, a circulator, the oxygen is constantly jumping into the water from the atmosphere to saturation, then that surface layer is pulled down and replaced by another layer of less than saturated water, which then becomes saturated, again and again and again.
This brings up an important point.
If you have a pond that is completely devoid of oxygen because of an algae crash or some other such disaster, the water at the very, very, very surface is actually saturated with oxygen. That’s why fish pipe at the surface to try to survive. The problem is that the saturated layer is only about one molecule thick. Trust me on this one. I found this out from a college instructor who told me that this isn’t enough for a fish to survive. Everything below this is robbed of oxygen so the fish probably won’t make it. That fish either needs plants to be producing oxygen from below, or enough oxygen mixing from above to adequately support their life processes.
That water either needs mixed from the big vortex formed by a nice bottom diffuser system, or a circulator or a thriving plant community.
