Carbon dioxide (CO2) occurs naturally in water but the level varies depending on conditions.
Potential sources for CO2 in water include:
In tilapia ponds highest levels of CO2 will occur:
Carbon dioxide is a gas that dissolves in water and acts as an acid with of the formation of hydrogen (H+) ions. Also, bicarbonate (HCO3-) and carbonate (CO3=) ions are formed. All dissociations are equilibrium reactions.
1. CO2 + H2O <---> H+ + HCO3- <---> 2H+ + CO3=
As CO2 is increased hydrogen ions accumulate and the pH of the water declines. However, when the pH of water is ò 8.4, free CO2 does not exist. Algae, however, can extract CO2 from bicarbonates and carbonates until the pH reaches a level of about 10. In this way the uptake of CO2 by algae tends to shift the form of alkalinity from bicarbonate to carbonate and carbonate to hydroxide as depicted by the equilibrium reactions below.
2. 2HCO3- <---> CO3= + H2O + CO2
3. CO3= + H20 <---> 2OH- + CO2
In production tilapia culture systems in Hawaii, microbial and fish respiration are the principal source of CO2 in the culture water. However, some well or spring waters may contain high levels of CO2 and low dissolved oxygen. Carbon dioxide should be measured periodically in the source water to establish a baseline for the site. Vigorous aeration of the water before use can drive off excess carbon dioxide and increase oxygen content.
Since phytoplankton take-up CO2 for photosynthesis, carbon dioxide declines during periods of high sunlight in phytoplankton rich culture water, but will increase at night. In addition, high CO2 conditions usually follow a phytoplankto crash. Both plant and animal respiration contributes to CO2 concentration which reaches peak levels just before sunrise. Thus, carbon dioxide level will vary on a diurnal cycle in culture ponds which carry a significant phytoplankton population. This has practical application because the time of day a water sample is collected for CO2 analysis can, depending on the system, strongly influence the level of carbon dioxide measured by the analysis. A graphic representation of the pattern of CO2 change over a 24 hour period in a phytoplankton rich pond is presented below:
Carbon dioxide alone is not appreciably toxic to tilapia unless fish are exposed to a very high level (> 50 mg/L) for an extended period. However, high environmental CO2 results in low pH which reduces oxygen loading in the gills and contributes to tissue acidosis (depressed pH within organs and tissues). Moreover, hypoxia (low oxygen concentration) also can lead to acidic conditions in the tissues of fish. Thus, the combination of low dissolved oxygen and high CO2 increases the harm done to the fish. When tilapia are raised at high biomass in plankton rich water, high CO2 and low dissolved oxygen occur together (e.g. in the early morning hours, following a phytoplankton crash or during prolonged periods of heavy cloud cover and low sunlight). Signs of CO2 toxicosis include extreme sluggish behavior, depression, loss of equilibrium and death.
Knowledge of the dynamics of carbon dioxide in water is important for assessment of a possible role of high CO2 as a factor in a water quality problem affecting cultured tilapia. For example, when pH is > 8, free CO2 will be very low and CO2 need not be measured, unless the pond or container water is stratified (e.g. low pH and high CO2 may exist in the bottom water).
To assess peak concentration in pond water, samples should be collected just before sunrise or following a phytoplankton crash. In source water from a well or rapid flowing stream, diel changes are not probable and the timing of water collection should not be critical. Just as for pond water, an alkaline pH would preclude the presence of appreciable free CO2. Carbon dioxide can be measured in water using commercially available test kits. However, care must be taken in how water samples are handled during and following collection for carbon dioxide analysis. The water container should be filled to allow water to overflow to remove water in the sample that has come into contact with air. Samples transported to the laboratory must not have any air in the container.
High CO2 can be temporarily removed from water by treatment with calcium hydroxide (slacked or hydrated lime - Ca[OH]2). Carbon dioxide reacts with lime to form calcium carbonate and water. One milligram CO2 per liter is converted by 1.68mg/L of hydrated lime. This treatment is only temporary and, if the conditions are appropriate, elevated carbon dioxide will reoccur. Dosage calculations for slacked lime removal of CO2 are given in the Treatment Module.
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