No two ponds are ever exactly alike. Even ponds in the same watershed
and built very close to each other can be very different in appearance,
and differences in watershed and soil characteristics are particular to
each pond. Differences affecting management are those associated with water
quality, plankton, and fish populations.
Water quality factors of temperature, pH, alkalinity, hardness, and dissolved
oxygen affect fish health and production. These factors are rarely constant
in a pond. Temperature, dissolved oxygen, and pH will change or cycle each
day, and alkalinity can change over longer periods of time.
Oxygen Cycle
Oxygen is dissolved in water from two sources--air and photosynthesis.
Oxygen dissolves into the pond water from the air as the two are mixed
together through wind and wave action. Mechanical aeration using pumps,
sprayers, and paddle wheels can be used to increase dissolved oxygen levels
during periods of low oxygen.
Photosynthesis is the other source of dissolved oxygen. In this
process, plants produce oxygen while making food from carbon dioxide and
water in the presence of sunlight. Algae release this oxygen directly into
the water during photosynthesis. Since photosynthesis is driven by the
energy of sunlight, oxygen production occurs during daylight. Therefore,
dissolved oxygen concentrations in ponds tend to rise throughout the day.
At night dissolved oxygen slowly declines as fish, insects, zooplankton,
bacteria, and algae consume oxygen through respiration. Under normal conditions
dissolved oxygen concentrations should not fall below 3 or 4 parts per
million (ppm). Oxygen concentrations below 3 ppm stress fish, and many
fish will suffocate at concentrations below 2 ppm.
Alkalinity, Hardness, And pH
Alkalinity and hardness are important in providing adequate natural
food and in maintaining a healthy fish population. The pH of the pond cycles
daily because of respiration and photosynthesis. Carbon dioxide released
from respiration reacts with water, producing carbonic acid. The pH scale
measures the acidity; therefore, as carbonic acid is formed the pH is lowered
or the pond becomes temporarily more acidic. Algae use carbon dioxide for
photosynthesis during daylight hours and the pond water becomes less acidic
with the decline of carbonic acid. Because of this, a pond pH normally
fluctuates between 6.5 and 9. If the pH drops below 5, as it does in ponds
that receive acid runoff, or rises above 10, as in low alkalinity ponds
with excessive algae blooms, fish will be stressed and can die. The only
practical method to manage for abnormal pH changes is to increase the alkalinity
of the pond.
Alkalinity is a measure of bases in the water. Bases
react to neutralize acids and, therefore, directly influence pH. As bases
react with the hydrogen ions present, they buffer or suppress pH changes.
Some alkalinity is necessary for good algae production. An alkalinity of
20 ppm or more is necessary for proper algae growth and, therefore, good
fish production.
Hardness is a measure of calcium and magnesium ion. Hardness concentrations
are usually similar to alkalinity (if derived from limestone) but can be
different, especially in coastal areas. A lack of hardness can reduce plankton
production and increase the likelihood of muddiness.
Blooms And Pond Color
Plankton is a term used for all microscopic and near microscopic
living things that float in the water. Plankton includes both tiny aquatic
plants called phytoplankton or algae and animals called zooplankton.
Planktonic algae serve as the base of the food chain. Zooplankton and aquatic
insects feed on algae, and they in turn are eaten by small fish (fry).
Small fish are then eaten by larger fish. Directly or indirectly, algae
provide almost all the basic food for the pond except for a small quantity
of insects and worms that fall or wash into the pond. Managing planktonic
algae is essential in providing the food to produce an abundant and healthy
fish population.
Changes in pond water color can be related to planktonic
algae concentrations, called "blooms," or to suspended sediments
and organic matter. Water that is good for fish production is green water;
the green color comes from billions of suspended microscopic algae. Water
color changes if these algae blooms "die-off" rapidly, turning the
water brown, black, milky, or clear. When this happens, decomposition of the
dead algae consumes oxygen, leading to possible stress, suffocation, or disease.
Algae die-offs are common in deep hill-type ponds or in fish ponds receiving
too many nutrients. Mechanical aeration may be necessary after algae die-offs
to keep fish alive.
Sediments washed into ponds after heavy rains will also change pond color.
Color should return to normal within a few days as settling occurs. Heavy
sediment loads can stress fish by irritating the gills and reducing oxygen
production. Ponds that receive sediments from surrounding fields may need
a wide sod strip around the pond to help trap the sediments before they
enter the pond (see Pond Construction). A pond that receives sediment only
during heavy rains may need a diversion ditch built around it to channel
excess water away from the pond. Many chronically muddy ponds need lime
to reduce acidity and to settle suspended clay. If your pond is always
muddy, contact your county Extension office for help. The office is listed
under your county name in the telephone book.