The Importance Of Proper Oxygen Levels Year-Round
Article reprinted from Pondtrade Magazine, May/June 2014 by Demi Fortuna
First, let's go over why proper oxygen levels are critical, in both summer and winter, to keep your ponds and lakes clean and clear. Of course, all fish, animals and plants require oxygen to live, so keeping oxygen levels as high as possible keeps fish, frogs, plants and other life happy and healthy.
The amount of oxygen that water can carry goes down as temperatures go up, so the heat of summer can be stressful, especially when there's no breeze to help mix the air into the water. The lowest oxygen levels occur on still summer nights just before dawn, when photosynthesis reverses and plants pull oxygen (O2) out of the water and exhale carbon dioxide (CO2) into it. The warmth of the water also increases metabolic rates of plants and animals, so fish require more oxygen just as levels decline.
The major algae blooms that occur in summer threaten O2 levels at night, and they can remove all the oxygen if there's a mass die-off, as the algae decompose. To make things worse, the warm blanket of low-oxygen surface water can create a thermocline, the boundary between the warm upper and cooler deeper water, which seals off deeper waters from wind and wave action and keeps O2 from mixing into the water below. Fish retreating below the thermocline to deeper, cooler water consume all the oxygen and can suffocate. An air diffuser at the bottom of the pond in the summer is a simple, cost-effective solution that mixes the layers of water, distributing life-giving oxygen throughout, night and day.
But aeration isn't just important for animals and plants - they're only two thirds of the story. Bacteria, the third group of organisms in the aquatic cycle, convert wastes and toxins to nutrients that feed plants and animals alike at the very base of the food chain ... and once again, oxygen plays a critical role.
Bacteria such as the nitrogen-converting bacteria that turn toxic ammonia to plant-feeding nitrates are aerobic; they require oxygen. Other bacteria have the faculty of digesting wastes either way, with or without oxygen, which makes them facultative heterotrophs ... but even if they can work anaerobically, they metabolize wastes far better with a ready supply of oxygen.
Think of bacteria as engines that work tirelessly converting ammonia and organic wastes to nutrients for higher organisms. Oxygen is just as essential to their performance as it is in an internal combustion engine, and for the same reasons. A gasoline or diesel motor may still fire up with a clogged air filter, but it will run more slowly and inefficiently, smoking and stinking - and the same goes for bacteria. Without oxygen, aerobic bacteria can't "burn" ammonia, and heterotrophic bacteria consume wastes more slowly and produce smelly, toxic methane and hydrogen sulfide.
On the other hand, given enough oxygen, aerobic bacteria will convert ammonia from animal wastes into nitrates that then get consumed by plants. Heterotrophic bacteria break down the sludge at the bottom of the pond or lake, removing nutrients that would otherwise fuel algae blooms. Aeration supercharges these reactions, increasing both ammonia conversion and sludge digestion exponentially. The rising bubbles of a well-designed and well-placed aeration system can even create powerful currents that bring the anaerobic sludge at the bottom up into the water column, allowing half a foot of muck per year to be converted to carbon dioxide and just bubble away! Sure beats shoveling that sh- ... uh, stuff!
One caveat: mixing in too much low-oxygen bottom water or anaerobic sludge all at once could drop the total dissolved oxygen levels too quickly, so aeration and the addition of aerobic bacteria should be started slowly to avoid fish kills when more than one-sixth of the volume of the pond or lake lacks oxygen.
Using Aeration Effectively
Now that we understand why we want to use an aerator year-round, let's look at how to use subsurface aeration most effectively to get the most oxygen in and the most carbon dioxide out.
The dynamics of aeration depend as much on surface area and circulation as air volume, and the depth of the water makes a big difference. The deeper the water, the greater the fetch; that is, the more water each rising bubble will displace upward as it makes its way to the surface. The greater the total volume of water that gets pushed upward, the wider the water will spread out when it reaches the surface. And all that water being lifted upwards needs to be replaced, by a counter current that sweeps along the bottom inward to the diffuser, to be lifted upward in its turn.
A relatively modest amount of air can move tremendous volumes of water under ideal conditions. The best-case scenario is a fine bubble air diffuser set just off the bottom in a deep, bowl-shaped depression with smooth sides. The rising column of air bubbles carries deeper, colder, denser water from the bottom up toward the surface, moving up to 10 times its own volume of water - especially remarkable because of how much more water weighs than air. It's not a perfect ratio, because the heavier bottom water tends to slip off to the sides of the column as it's moved into warmer, lighter surface waters, decreasing the efficiency somewhat. And wind and thermoclines can increase that slippage, but the bubbles keep moving water upward regardless. The "boil" of rising water at the surface lifts water a few inches, and the water spreads out over the surface, as much as 100 feet outward. Manufacturers reliably claim that up to 10 acres can be aerated with a single three-quarter horsepower air pump and properly placed diffusers, as long as the lake is deep enough.
Shallow bodies of water (under four feet in depth) are actually harder to aerate. Although they aren't usually as stratified and rarely suffer from thermoclines, shallow ponds have little vertical room for bubbles to rise. Diffusers don't get a chance to move as much water on the way up, and they won't affect as much water laterally either. Aerating shallow lakes and ponds effectively usually requires more diffusers spaced much more closely together, which increases the cost per unit area somewhat. But low-pressure air pumps and blowers suitable for shallow water cost less to run per cubic foot per minute (CFM) of air than high-pressure, deep-water compressors, so the choice of the proper source of air is important. Surface units such as fountains and paddle-wheel aerators can be effective in shallow water, but the paddle wheels are noisy and generally less attractive, while the surface fountain types look wonderful but don't do much mixing beyond a very limited area, offering more show than D.O. (dissolved oxygen).
Regardless of the type of system you choose, aeration in conjunction with a maintenance program that includes regular applications of heterotrophic bacteria can cut long term maintenance costs drastically. Not only can drain-down cleanups be eliminated, a properly sized aeration system will also provide excellent water quality and clarity, eliminating odors and increasing customer satisfaction. Contact us for further information regarding system selection and sizing.