Author: L. A. Helfrich
Posted June 2002 
Alkalinity: recommended = greater than 20
mg/L
Alkalinity, the total concentration of bases, is a measure of the capacity of
water to neutralize (buffer) acids with carbonate and bicarbonate ions. Alkaline
(basic) pond waters are those with pH values greater than 7. In general,
alkaline pond waters (>20 mg/L CaC03), exhibit greater fish productivity than
acidic waters, and are more resistant to rapid changes in pH. Liming (applying
agricultural limestone powder) a pond is used to neutralize acid waters,
increased nutrient availability, and promote fish and plankton production.
Aluminum: less than 0.01 mg/L
Aluminum is the 3rd most abundant element the earth's soils. Some forms
(inorganic monomeric) can be very toxic to fish, particularly in soft, acidic
pond waters. The solubility of aluminum increases as the pH decreases. Aluminum
toxicity to fish (damage to fish gills) generally occurs below a pH of 5 in
ponds that receive acid rain or mine acid drainage.
Ammonia NH3 (unionized): less than 0.05 mg/L
Ammonia is a by-product excreted by fish and the decomposition of natural
substances. Unionized ammonia (NH3) is very toxic to fish, whereas other forms
of nitrogen (ammonia, NH4; nitrite, N02; nitrate, N03) are usually harmful. The
toxicity of ammonia (NH3) to fish is directly related increasing pH and water
temperatures. Although ammonia levels rarely reach toxic levels in fish ponds,
spills of nitrogen fertilizers, livestock feed lot run-off, and algae die-offs
can result in fish kills.
Copper: less than 0.01 mg/L
Copper is a toxic metal commonly used in certain herbicides, especially copper
sulfate (bluestone) which is frequently used to control algae. High
concentrations of copper ions (Cu++) are also toxic to fish, especially trout,
and shellfish (mussels) and crayfish. Copper applied to ponds with soft or
acidic water is more toxic to aquatic life than in ponds with hard or alkaline
water.
Dissolved Oxygen: greater than 5 mg/L;
over 75% saturation
Dissolved oxygen is essential for all aquatic life. It is the most critical of
all water quality parameters. Most of the oxygen in a pond is produced by water
plant (algae and rooted plants) photosynthesis during the daylight hours
(reaching a maximum value near 6 PM). Fish kills generally occur at night as the
oxygen produced during the daylight is used (minimum values occur near 6 AM).
Summer fish kills often result because warmer waters hold less oxygen than
cooler waters. Trout generally require more oxygen (6 mg/L) than warmwater fish
(5 mg/L). Fish may live at low levels, but growth, reproduction and long-term
survival will be compromised. Oxygen measurements must be made on-site. Mixing,
aerating, pumping, and spraying the water into the atmosphere with motors,
pumps, windmills, and paddlewheels will can dissolved oxygen concentrations.
Hardness: greater than
20 mg/L
The hardness of water is a measure of the total mineral content (polyvalent
metal ions) expressed as mg/L equivalents of calcium carbonate. Calcium and
magnesium are the primary causes of hardness in natural waters, but other metals
(Mn, Fe, Zn, Al) also contribute. Natural pond waters range from soft (0-75
mg/L) to very hard (> 300 mg/L). In general, hard waters are more
biologically productive than soft waters (poor in mineral content and acid
buffering capacity).
Iron:
less than 0.1 mg/L
Although some iron is essential to aquatic animals, more than 0.1 mg/L
precipitates on exposure to air and can decrease pond clarity (increase
turbidity), clog drainpipes, and favor the growth of iron bacteria which can
disflavor water and fish flesh. High levels (> 1 mg/L) can indicate mine acid
drainage.
Nitrite (N02):
less than 7 mg/L
Nitrite concentrations lethal to fish can occur in cool pond waters in the
spring and fall as nitrifying bacteria (nitrosomonas) activity is
reduced. High levels of nitrite interior with the oxygen-carrying capacity of
the blood, resulting in fish suffocation. This is characterized as "brown
blood disease" in catfish. The treatment is to add 3 mg/L chloride for
every 1 mg/L of nitrite, usually in the form of salt (NaCL =61% Cl).
Nitrate (N03): less than 5.0 mg/L
Nitrate is generally non-toxic to fish, but it is a plant nutrient and high
concentrations (> 10 mg/L) can result in overfertilization of ponds and
corresponding nuisance algae blooms, fish kills, and taste and odor problems.
Decomposing mats of algae can deplete the oxygen supplies in the pond causing
fish kills. Limit fertilizer and animal waste runoff into ponds. Nitrate levels
exceeding 100 mg/L can cause methemoglobinemia in human infants.
Nitrogen Gas Supersaturation: less than 103%
Nitrogen gas comprises 78% of the atmosphere. It can become dissolved in high
concentrations in falling water below high dams, and in groundwater supplies, or
in recirculating aquaculture systems, especially under high pressure and
temperature conditions. Total gas pressure should be maintained at less than
110% and nitrogen gas content should be less than 103%.
pH (acidity or
alkalinity):
between 6.5-9.0
pH is a numerical measure of the acidity or alkalinity of pond water on a scale
of 0 to 14, with a mid-point reading of 7 indicating neutral water (neither acid
or basic). Fish kills or stress occurs in acidic waters with a pH below 5 or
alkaline waters with a pH greater than 11. The pH of pond water increases daily
as phytoplankon consume carbon dioxide during photosynthesis (reaching a maximum
value near 6 PM), and decreases at night as they release carbon dioxide during
respiration (reaching a minimum value near 6 AM). A pH between 6.5 and 9.0 at
dawn is recommended for fish ponds. Pond with low pH values (< 5) receiving
acid rain, mine acid drainage, or acidic swamp water can be improved by liming.
Phosphorus:
Less than 1 mg/L
Phosphorus is the nutrient most needed (limiting) for plankton growth in
ponds. In general, fish production increases with nutrient levels, but high
levels of phosphorous may lead to excessive fertilization and nuisance algae and
rooted weed problems. Excessive weed growth can cause flavor and odor problems
and fish kills.
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