part one - added Nov. 11/97
- see part two for Pond Turnover and Winter Kill
The Oxygen Factor
(in ponds)

by Dr. Schramm

Looking at a body of water from the surface it's hard to imagine the dynamic world below. Yet, in this mysterious, watery environment, fish are governed by many biological forces. Oxygen is among the most critical, as well as the least understood.

First, a few facts about fish and oxygen. Oxygen in water is measured in parts per million (ppm). The amount of oxygen dissolved in water is usually in the range of 3 to 10 ppm. Commonly accepted lower limits for fish survival are 5 ppm for most warm-water fishes (bass, carp, crappie, catfish, etc.) and 7 ppm for cold-water species ( salmon, steelhead, trout and sturgeon).

Oxygen concentration is inversely related to temperature---cooler water holds more oxygen than warmer water. But because fish metabolism, and therefore oxygen requirements, increase with temperature, fish need more oxygen in warmer water. In other words, just when fish need the most oxygen, it is least available.

Besides temperature, other factors affect oxygen concentration. Oxygen dissolves in water as it mixes at the surface and through underwater plant photosynthesis.

In standing water, most oxygen comes from photosynthesis of plants, both rooted aquatic plants (macrophytes) and algae. Algae includes readily visible growths on substrates such as rocks and wood, as well as suspended microscopic phytoplankton. Flowing and turbulent water contains higher levels of oxygen because the water is constantly mixing with the oxygen in the atmosphere.

Just as oxygen is added to water, it can also be removed. Respiration of fish and other aquatic animals depletes the oxygen in water. Decomposing organisms and aquatic plants, whether macrophytes (green plants) or phytoplankton, also deplete oxygen. During the day, photosynthesis exceeds respiration and oxygen levels increase; at night, or when light cannot penetrate the water, oxygen levels decline as respiration and decomposition continues.

Two final notes, seemingly exceptions to the above information about flowing water, are worth remembering. First, water from underground springs has no oxygen*. Where springs enter a lake or stream, no oxygen is present unless that spring water has been aerated by flowing above ground before entering the stream or lake.

Second, inflowing water receiving animal wastes or industrial discharges, or water flowing through a shallow marsh, will likely contain little oxygen. Such waters, because they add oxygen-consuming organic matter, may further reduce the oxygen in the receiving lake, stream, pond or river.

* Water from deep springs and drilled wells will contain no oxygen but will have a high level of dissolved nitrogen which will kill trout within a few hours. (added by D.H.)

Rainbow Trout Hatchery & Fish-Out Pond

Added March 17, 1998

Part two

Water Turnover, Winter Kill
and Low Dissolved Oxygen Concentrations
in Ponds Without Incoming or Mechanical Oxygenated Water
by David Hedley

"Water turnover" in itself does not kill fish. Fish kills are most frequently caused by low dissolved oxygen concentrations in pond water. The following article will attempt to explain the causes of these events and the relationship between them.

Pond turnovers usually occur during early summer after a cold, hard rain. Turnovers can also occur following the first cold weather of fall. When there is a sudden cooling of the surface water temperatures, the surface waters will "sink" because it is more dense than the waters near the pond bottom. Warmer, less dense, bottom waters are forced to the surface creating a mix or a "turnover." When a sufficiently large volume of oxygen deficient waters dominate the bottom of the pond then, the entire pond may become oxygen starved as the waters mix.

Normally a pond is in a "layering effect" or thermo levels when the cooler water having the greater density remains near the bottom, while the less dense (warmer) water migrates to the surface. During the hottest months of summer there is little mixing of the pond's water unless there was unusual heavy cold rain.

Deep water remains cool if there is an absence of sunlight to warm it. A decrease in light penetration is usually due to suspended clay particles and/or "blooms" which are composed of large populations of microscopic algae.

Almost no oxygen is produced in the cold layers of water, due to the absence of sunlight which prohibits plant and algae growth. Furthermore whatever oxygen remaining in these cool waters is usually consumed by decomposing organic matter. Organic matter is caused by plants and animals that have died and sank to the pond bottom.

Unless the pond has dense aquatic plant growth, microscopic algae "blooms" generally produce most of the dissolved oxygen in ponds and lakes. Algae and plants produce oxygen during daylight hours, but consume oxygen during the night.

Healthy algae bloom and aquatic plant populations will produce enough dissolved oxygen to support life in a pond throughout a 24 hour period. However, dying populations may consume more dissolved oxygen at night then they produce during the day.

When algae bloom or aquatic plants release less dissolved oxygen during cloudy days than they consume at night, low dissolved oxygen conditions may occur. The oxygen consumed by the constant decay of algae, vegetation and other organic matter will further deplete dissolved oxygen concentrations in the pond.

Low dissolved oxygen conditions often occur during the night or just before dawn, particularly when water temperatures are warm. Please remember: warm water holds less oxygen than cold water. Increases in altitude and salinity will also lessen the ability of water to hold dissolved oxygen.

Extreme dissolved oxygen depletions may kill all fish, invertebrates and often microscopic algae populations. However severe, these conditions typically last only a few days. Due to the dying algae bloom, the water color may change from green to black or gray with black streaks.

Human activities such as livestock pasturing, over fertilizing lawns, and drainage from septic systems will encourage aquatic plant growth which will eventually die and consume dissolved oxygen. Those ponds that contain less organic matter such as leaves, and aquatic plants are less likely to experience fish kills from water turnover.

All fish can tolerate low concentrations of dissolved oxygen for short periods. However, these conditions may result in disease causing stress. Partial fish kills may occur by selecting the pond's largest fish or a particular species which has a high oxygen requirement (i.e., trout). Other fish may be seen at the water's surface "piping" or gasping for air. Warm-water fish (bass, bluegill, catfish) require 5 mg/L of dissolved oxygen to maintain good health while cold water fish (trout, salmon) typically require 7 mg/L.

Low oxygen conditions may also occur in ponds with prolonged ice and snow cover. Not only does the water beneath the ice have no access to air but, the snow-covered ice will prevent sunlight penetration. The lack of sunlight, if prolonged, will eventually cause the death of algae and plant growth. Decaying algae blooms and plant material will further lower the dissolved oxygen concentrations, and eventually a fish kill may occur. This is known as "winter kill"

To prevent this type of fish kill, ponds may be partially cleared of snow by shoveling long strips across the pond to allow some sunlight to penetrate the ice. Clearing snow from frozen ponds should only be done when the ice is 4 inches thick or greater. Aeration devices may be used to prevent ice formation.

To a lesser extent, oxygen is also absorbed from the atmosphere at the water's surface. Wave action or other disturbances will increase the water's dissolved oxygen concentration by expanding the surface area for oxygen to enter.

In short, abrupt changes in weather conditions and water temperatures can rapidly affect the pond environment.

Typically, ponds that may be affected severely from water turnovers are those that are older, have depth, shallow sloping sides with abundant aquatic vegetation and/or filamentous algae and sheltered from the prevailing winds.

Proper aeration, water circulation and water quality management will reduce the severity of "water turnover" and prevent dissolved oxygen-related fish kills.

Rainbow Trout Hatchery & Fish-Out Pond

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