Daily pH Cycle and Ammonia
Toxicity
World Aquaculture, 34(2): 20-21.
(view also as PDF)
William A. Wurts, Ph.D.
Senior State Specialist for Aquaculture
Ammonia is a nitrogen waste released by aquatic animals into the production pond environment. It is a primary byproduct of protein metabolism. Ammonia is excreted directly from the fish gill into the water. Ammonia concentrations are usually at their highest late in the production season when biomass of the cultured species and the amount of protein fed are greatest. Ammonia is toxic to aquatic life and toxicity is affected by pond pH. Ammonia-nitrogen (NH3-N) has a more toxic form at high pH and a less toxic form at low pH, un-ionized ammonia (NH3) and ionized ammonia (NH4+), respectively. In addition, ammonia toxicity increases as temperature rises.
The measure of whether water
is acidic, basic (alkaline) or neutral is known as pH. A scale of 1 to 14 is traditionally used,
which represents the negative logarithm of the hydrogen ion concentration. A pH of 7.0 is neutral; above 7.0 is basic
and below 7.0 is acidic; close to 7.0 is weak and far from 7.0 is strong. It is a common perception that the pH of
water is neutral and constant at a value of 7.0. In an environment free of carbon dioxide,
aquatic life, and compounds other than H2O; pond pH would remain 7.0
or neutral. However, this combination of
conditions is unlikely to occur on our planet.
The pH of water is naturally acidic because the atmosphere contains
carbon dioxide (CO2). Carbon dioxide readily dissolves into water,
raindrops and other sources of water exposed to air, forming a weak acid (H2CO3, carbonic acid). Therefore, events in the aquatic environment
that affect CO2 concentrations also affect pH. There are minerals in soil that can dissolve
in water to create acidity and alkalinity as well.
Photosynthesis and Respiration
Pond CO2
concentrations and pH, are affected by respiration and photosynthesis. Carbon dioxide is released during respiration
and consumed for photosynthesis. As a
result, pond pH varies throughout the day (Fig. 1).
The plant members of the
pond plankton community, phytoplankton, absorb CO2 for
photosynthetic production of sugar. As
daylight progressively intensifies, the rate of photosynthesis increases and so
does the uptake of CO2. The
removal of CO2 reduces the concentration of carbonic acid, and pond
pH rises. Late in the production season,
high waste nutrient concentrations can promote dense phytoplankton blooms
which, in turn,
can
remove all of the CO2
from pond water during photosynthesis.
This can cause the water to become alkaline with pH levels greater than
9.0. Pond pH is highest late in the
afternoon -- a few hours before sunset.
After sunset, photosynthesis
and CO2 uptake stop. However, respiration continues day and
night. During respiration, plants and
animals consume oxygen to free the energy stored in food. The end product of respiration is CO2,
which is released directly into the water.
As photosynthesis is halted
by the absence of light, CO2 begins to
accumulate and the carbonic acid concentration increases. The rising concentration of carbonic acid
causes the pH to fall. Toward the end of the production season, the biomass and
respiration of cultured animals and phytoplankton is high. Nighttime
concentrations of CO2, and therefore carbonic acid, can become
excessive, lowering pH below 7.0. As
such, pond pH would be lowest an hour or two before sunrise.
Effects of pH
on Ammonia Toxicity
The daily interplay of photosynthesis and respiration creates a cyclical change in pond pH. Pond water becomes most acidic just before the period of darkness ends and most alkaline after several hours of daylight. The presence of un-ionized ammonia, the toxic form, increases as pH rises and decreases as pH falls which causes ammonia to become more ionized. The concentration of un-ionized ammonia in production ponds is lowest just before dawn and highest late in the afternoon.
This has significant
implications for water quality monitoring, especially several weeks prior to
harvest when fish biomass is greatest.
For example (Table 1), a producer measures water quality at 0400 hr. The total NH3-N concentration is 2.7 mg/L,
pH is 7.0, and water temperature is 28 oC. The farmer then cross-references these values
with a standard, pH-temperature table and calculates the concentration of
“un-ionized” NH3-N to
be 0.019 mg/L. The producer decides to check water quality
again at 1600 hr and finds that total
NH3-N is still
2.7 mg/L. But, pH
and water temperature have risen to 9.0 and 30 oC. After checking the reference table, the
farmer discovers that the un-ionized NH3-N concentration is now 1.2
mg/L. An un-ionized NH3-N level of 0.019 mg/L would
be considered acceptable for
channel catfish production. However, the un-ionized NH3-N concentration of 1.2 mg/L
recorded at 1600 hr could be lethal to channel catfish within several hours. Over a 12-hr period, the un-ionized ammonia
concentration increased approximately 63-fold.
The temperature change accounts for less than 10% of the increase in
toxicity while the rise in pH from 7.0 to 9.0 is responsible for more than 90%.
Table 1. Amount of total ammonia-nitrogen
(Tot/NH3-N) present as un-ionized ammonia-nitrogen (UI/NH3-N),
for early morning and late afternoon pH and temperature measurements in a
hypothetical production pond.
|
Time |
Tot/NH3-N
(mg/L) |
Temp
°C |
pH |
UI/NH3-N
(mg/L) |
|
0400 hr
1600 hr |
2.7
2.7 |
28
30 |
7.0
9.0 |
0.019
1.2 |
Measuring pH and Ammonia
Photosynthesis and respiration have significant effects on
pond pH. Because those processes affect
pH, ammonia toxicity is influenced also.
When monitoring water quality, it is important for producers to
understand the daily shifts in pH and their impacts on un-ionized ammonia
concentrations. First, for
ammonia-nitrogen measurements to be useful, pH and NH3-N must be measured at the
same time. Second, a morning pH
determination is not meaningful for assessing whether daily ammonia
concentrations have reached unsafe levels in ponds. To have any practical value for pond
management decisions, NH3-N and pH should be tested late in the
afternoon. A solid grasp of the pH cycle
and its interrelationship with NH3 is critical for the successful culture of
any aquatic species.
For related information
click on the topics below:
(pdf) SUSTAINABLE AQUACULTURE
IN THE TWENTY-FIRST CENTURY
2000. Reviews in Fisheries Science, 8(2): 141-150.
SUSTAINABLE
CHANNEL CATFISH FARMING
Low management production through modified
stocking and feeding practices.
World Aquaculture, 26(3): 54-59.
LIMING PONDS FOR AQUACULTURE.
Southern Regional
Aquaculture Center, Publication No. 4100.
World Aquaculture, 33(1): 16-17.
(pdf) INTERACTIONS
OF PH, CARBON DIOXIDE, ALKALINITY AND HARDNESS IN FISH PONDS.
Southern Regional Aquaculture Center, Publication No. 464.
UNDERSTANDING WATER
HARDNESS
World Aquaculture, 24(1): 18.
(pdf)
MANAGING AMMONIA IN PONDS.
Southern Regional Aquaculture Center, Publication No. 4603.