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What You Should Know About Water Quality and
Ponds John M. Jemison, Jr., Extension water
quality and soil specialist
What Temperature Should the Water in My Pond Be? The best temperature for your pond will depend on the kinds of fish present. For example, trout are very sensitive to high temperatures. If you want trout to survive in your pond, the maximum temperature in the summer months should be no greater than 70 degrees F. This requires a relatively deep pond with a good source of fresh, cold water and plenty of shade. Fish such as bluegill, bass and catfish are considered warm-water fish, and will grow best at water temperatures between 64 and 90 degrees F. These fish will survive in shallow ponds, and it is not as important that the pond have a reliable source of fresh, cold water. What Is pH, and What Does It Have to Do With My Pond? pH is a measure of the hydrogen ion (H+) activity
in water; the more hydrogen ions in the water, the lower the pH, and the more
acidic the water is. Normal pH ranges from 0 to 14. In general, the best pH for
aquatic life ranges between 6.5 and 8.0. Fish can survive at pH levels outside
of this range; however, growth is slowed, and spawning often will not occur. You
can test the pH of your pond using inexpensive equipment, or you can have your
pond's water tested by one of many water testing laboratories in the state. Be
careful when testing your pond, however, as pH changes through the day. During
the day, when aquatic plants are photosynthesizing, the pH will rise as carbon
dioxide is used by plants. At night, plants release carbon dioxide, and pH
falls. In most cases, problems occur in Maine ponds when
the pH is too low. If the pH of your pond does not rise above 6.5, you should
consider treating your pond. Apply agricultural lime at a rate of one to two
tons per surface acre. After a few weeks, test the water again. You may need to
apply more lime at this point.
Should I Be Concerned About Dissolved Oxygen? Dissolved oxygen (DO) is the free oxygen
available for fish and other aquatic life. The amount of dissolved oxygen in the
pond varies throughout the year and throughout the day. Water has a greater
capacity to hold DO at lower temperatures. Therefore, DO concentrations in water
are generally greater in winter than summer. Since plants release oxygen only in
the presence of sunlight (during photosynthesis), the DO content of water will
be greater during the day than at night. DO will reach its lowest level in a
pond just prior to sunrise, since no oxygen has been released by plants during
the night, and oxygen has been used during the night by plants and animals. DO
concentrations in ponds commonly vary from 10 parts per million during the
mid-afternoon to four ppm or less at sunrise. No specific statement can be given as to the
minimum dissolved oxygen concentration required to support fish life. This is
because fish vary in their oxygen requirements due to age, water temperature,
the concentration of other materials in the water and other factors. However, a
minimum DO level of five ppm should be sufficient to keep fish healthy. Water can be tested for dissolved oxygen using relatively inexpensive test kits. DO should be checked just prior to sunrise, or if fish are swimming at the water's surface and "gulping air." If the DO level is less than two ppm, steps should be taken to add oxygen to the water. Possible methods for adding oxygen to the water include:
What Is “Turnover”? Pond water experiences an annual cycle of
seasonal temperature changes. The density of water is greatest at 39.2 degrees F
(4 degrees C). Such water is found at the bottom of a pond when the pond's
surface is covered with ice. The colder water above the bottom is less dense and
the lightest water (about 32 degrees F) is just under the ice. During this winter stagnation period, the upper
portion of the pond contains enough oxygen for aquatic life, but there may be
little oxygen in the deeper parts due to consumption by bacteria decomposing
dead plants and animals. At this time, there may not be enough dissolved oxygen
in the deeper water to support fish and other aquatic life. Some oxygen may be
produced by microscopic plants (algae) in the upper region of the pond just
under the ice, where enough light is available for photosynthesis. However, snow
covering the ice can reduce light intensity to the point where oxygen cannot be
produced. It may be a good idea to try to use a pump to keep part of the pond
open all winter if fish production is an important management goal. Also, one
can pump oxygen into the pond through a bleeder hose. As the
weather becomes warmer, the pond's surface warms from 32 degrees F to its
maximum density at 39.2 degrees F, and the heavy surface water sinks. The
surface water mixes with deeper water and the lighter, colder water rises from
the depths, becoming oxygenated by spring winds. This spring turnover mixes the
pond water so that water temperature is almost uniform at all depths and all the
water has a high oxygen content. A pond turnover may also be caused by a cool rain
shower with wind and wave action forcing the surface layer to one side of the
pond. When this happens, the cooler surface water falls to the bottom and forces
the bottom water to the upper portion of the pond. Upon mixing of the two
layers, dissolved oxygen may become so low throughout the pond that a fish kill
results. Also, gases (such as hydrogen sulfide, methane and carbon dioxide) from
the decomposition of organic matter by bacteria can also be particularly toxic. During summer, temperatures of the surface waters
increase rapidly and become much less dense than the deeper, cooler water. This
warm surface water stays at the top of the pond throughout the summer (due to
its lightness) and does not mix with the cooler water below. In deep ponds, fish
are mainly confined to the warm, upper levels of the pond during this summer
stagnation period since the colder depths usually contain inadequate oxygen
supplies. In autumn, the surface water cools toward the temperature of the deeper water. As this water cools and becomes heavier, it sinks and mixes with the deeper water, until all the water has the same temperature and density. This is the fall turnover, and its effect on fish depends on the relative size of the well-oxygenated top layer. As vertical circulation takes place, stagnant water moves to the surface. Here it releases CO2 and other undesirable gases and is recharged with dissolved oxygen. A fish kill can result when this occurs, especially if it occurs rapidly when the surface layer is low in dissolved oxygen. © 2002 Published and distributed in furtherance of Acts of Congress of May 8 and June 30, 1914, by the University of Maine Cooperative Extension, the Land Grant University of the state of Maine and the U.S. Department of Agriculture cooperating. Cooperative Extension and other agencies of the U.S.D.A. provide equal opportunities in programs and employment. Call 800-287-0274 or TDD 800-287-8957 (in Maine), or 207-581-3188, for information on publications and program offerings from University of Maine Cooperative Extension, or visit www.extension.umaine.edu.
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Last
Modified:
04/29/08
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