Imagine for a moment what life would be like for us terrestrial creatures if the oxygen content of our air was constantly changing, 24 hours a day, seven days a week.

Visualize living in an environment where oxygen concentrations fluctuated with available sunlight, air temperature, or the number of people breathing nearby. What if oxygen levels began to suddenly drop in your home or office, causing everyone to scurry from room to room searching for pockets of “breathable” air? What if there were none available and you had to leave the building all together in search of this valuable life-sustaining gas?

Where would you go? What would you do?

While this little mental exercise may seem absurd, it’s a daily reality for fish and other aquatic animals. Dissolved oxygen is indeed a precious commodity in the underwater realm, a place where things can change quickly with very little warning.

In Florida’s warm-water lakes, oxygen-related problems are one of the most common causes of fish kills. While such events are upsetting to many people, the good news is that most are the result of naturally occurring processes. And, as you are about to learn, many of these processes are inextricably linked with the presence of plants.

We’ll be discussing these processes and much more in the following sections, but first we’ll start with a quick review of how oxygen finds its way into water.

Oxygen in Water

Oxygen is a natural element. No animal can live without oxygen in its gaseous state. Fish and other aquatic animals are no less dependent on gaseous oxygen dissolved in the water than people are dependent on gaseous oxygen surrounding us in the air.

In fact, many people are surprised to learn that fish and other aquatic organisms don’t actually use oxygen from water molecules themselves (H₂O). This is because the single oxygen molecule (O) is bound to the two hydrogen molecules (H₂) and is not “available” for use. Instead, aquatic organisms are dependent on dissolved oxygen gas (O₂), a colorless, tasteless and odorless substance that is continuously entering water from plants and the atmosphere above.

Oxygen from Plants

Using nothing more than carbon dioxide, water and light energy, earth’s innumerable plants — both aquatic and terrestrial — are continuously generating new cells and repairing damaged ones using a process known as photosynthesis. As “luck” would have it, dissolved oxygen gas is continually being released as a by-product.

In aquatic environments, free-floating microscopic plants known as algae and larger submersed plants (macrophytes) release oxygen directly into the water where it is used by myriad animals and organisms including the plants themselves.

Oxygen from the Atmosphere

In addition to plant-generated oxygen, the earth’s atmospheric pressure is constantly “pushing” tiny molecules of dissolved oxygen gas into the surface waters of our lakes, ponds, oceans, swimming pools — even that glass of water on your kitchen counter. The process is known as diffusion and it’s a never-ending cycle. As oxygen gas is being pushed into water, excess oxygen from the water is simultaneously being released back into the air. Wind and wave action or man-made aerators can accelerate diffusion by creating more surface area for oxygen to enter the water.

Water’s unique ability to hold and release oxygen makes it possible for fish and other animals to breathe or “respire” underwater. The downside is that oxygen concentrations in aquatic environments are rarely stable. When the sun is shining and aquatic plants (including algae) are photosynthesizing at full capacity, there’s plenty of oxygen to go around. However, after the sun sets each evening, photosynthetic activity is greatly reduced and is oxygen concentration. Under “normal” circumstances, this is not a problem because there is usually enough of a dissolved oxygen buffer available in the water to last until morning, when the process begins all over again. However, if something should alter that pattern, things can go awry.

Oxygen levels in flux throughout the day

Weather patterns are a common culprit, particularly in Florida’s subtropical climate. Several consecutive days of cloudy weather reduces the amount of sunlight available for algae and plants to use for photosynthesis. Meanwhile, the algae, plants and animals are still using up a dwindling oxygen supply. Once the buffer is used up and oxygen levels dip below 2 mg/L, fish and other animals become stressed, increasing the chances for illness or, if conditions last long, a fish kill can occur. This usually happens in the early morning hours or just before dawn (4 to 6 am) and often during hot weather when there is less oxygen in the water to begin with. But that’s just one example.

There are a number of factors and/or “events” that can influence the amount of dissolved oxygen found in water. Altitude and the temperature of the water are two of the main influences:

Altitude, in Florida, is not really a factor as most of the state is barely above sea level. However, for lakes located in northern latitudes — and higher altitudes — the rule goes something like this: as altitude increases, the amount of oxygen in a lake or waterbody decreases. This is due to the fact that at higher altitudes, there is less atmospheric pressure available to push oxygen molecules into the water. We can determine the amount of dissolved oxygen in a lake at any given altitude using Correction Factors for Lake Altitude supplied in Table 1.

Water temperature, however, is another matter altogether; aside from photosynthesis, it is probably the most influential factor in determining oxygen concentrations in water. A few examples:

Temperature determines water's ability to hold oxygen. For instance, cool water generally holds more dissolved oxygen. However, once water temperatures drop below freezing and the water becomes a solid (ice), oxygen becomes unavailable to most organisms. At the other end of the spectrum, warm water can also be problematic; at full saturation, 90-degree (F) water holds only 7.4 mg/L of dissolved oxygen whereas 45-degree water can hold 11.9 mg/L.

Differences in water temperature can create a layering effect throughout the water column, with warm water on top and cooler water below. This thermal stratification can limit oxygen’s ability to “mix” or move between the layers, resulting in less oxygen at the bottom even though the water is cooler. If the two layers should suddenly become mixed from strong wind/wave action or a cold rain— there is the potential for oxygen concentrations to average out to a low level, leaving dangerously low oxygen concentrations throughout.

For more on thermal stratification and the potential for lake turnovers, see
LAKEWATCH Information Circular 109, page 13.

Higher temperatures cause aquatic animals to behave differently; as water becomes increasingly warmer, fish and other organisms tend to become more active which results in more oxygen consumption at an even faster rate. If the oxygen is used up faster than plants and algae are able to produce it (via photosynthesis), problems can occur.

Other factors that can decrease dissolved oxygen in water:

Rainstorm events can wash heavy inputs of organic matter such as leaves, twigs and grasses into a waterbody and cause a chain reaction that uses up oxygen quickly. Once the debris is washed into the water, billions of microorganisms including bacteria and zooplantktonkick into high gear as they work to decompose the vegetation. If a large amount of material is introduced at once, the increased activity can accelerate oxygen consumption, sometimes with dire consequences. Dissolved substances (tannins) from decaying vegetation in the lake or from wetlands surrounding a lake can also leach into the water after a heavy rain and have a similar affect, as can wastes from animal feedlots or septic tank wastewater.

For more on dissolved substances, see pages 7-9 of A Beginners Guide to Water Management/ Information Circular 108.

Large-scale loss of algae or plants can also deplete oxygen in much the same way. This has been a problem in lakes where algicides or herbicides are used as an efficient way of eliminating unwanted vegetation. When used at the recommended doses, the chemicals are designed to affect plants only. However, once the algae or plants begin to die en masse and sink to the bottom, decomposition is increased, accelerating oxygen consumption and sometimes resulting in a fish kill. During warmer weather, when dissolved oxygen concentrations are already less than optimal, chances of such problems are increased. That's why , when using chemicals, it is now recommended that treatments be staggered over time so that large amounts of plants are not all dying at once.

Anthropogenic (man-made) oxygen problems are less frequent than naturally occurring events, but they do happen. One rather unusual event involved an explosion at a whisky factory near Lawrenceburg, Kentucky. The Subsequent release of thousands of gallons of bourbon whiskey into a nearby river triggered a feeding frenzy as billions of microbes quickly began devouring the whisky and consequently, depleted oxygen concentrations in the water. hundreds of thousands of fish died in that incident. other man-made influences include spills of chemicals such as formalin that remove oxygen directly from the water column.

Measuring oxygen concentrations in water

See U.S. EPA Volunteer Lake Monitoring Methods Manual (Chapter 5)

Helpful Link:

Dissolved Oxygen for Fish Production
Ruth Francis-Floyd
http://edis.ifas.ufl.edu/FA002