Aquaculture Water Quality for Small-Scale Producers - Aquaculture and the Environment: A Shared Destiny - page 130

2.2.2 Nitrite
The presence of nitrite inside an aquaculture pond is often caused by ammonia biological
oxidation (Hargreaves, 1998). The stoichiometric reaction is the following:

+1.5 → +2 +
As a result, there is a free energy yield (Δ G ) of - 65 kcal mole -1 from the ammonia oxidation.

This reaction is carried out by bacteria that naturally growths in the aquacultural
environment. Bacteria of the genus Nitrosomonas are the main responsible for nitrification in
aquaculture, but there are other important genus involved in this processes (Hovaneck &
DeLong, 1996; Ebeling et al., 2006).

The physiological effects of nitrite in the fish health are mainly caused by a chemical reaction
on the haemoglobin. The role of this protein is the oxygen transportation all over the blood
stream. In the presence of nitrite, haemoglobin becomes methaemoglobin, a non-efficient
oxygen transporter (Jensen, 2003). As a result of oxygen deficit, the fish blood became brown,
and a gasping behaviour can be observed. If nitrite intoxication remains unattended, massive
fish dead became after a short time caused by hypoxia (Masser et al., 1999).

High nitrogen concentrations in ponds occurs more frequently in the fall and spring, when
low and fluctuating temperatures cause decay rates on phytoplankton and bacteria
metabolisms (Durborow et al., 1997b).

2.2.3 Nitrate
The presence of nitrate in aquaculture water is water is delivered as a waste product of
organic bacterial activity in the pond. The reaction involves the presence of nitrite and
oxygen. The following stoichiometric reaction shows the overall process (Wheaton, 1982).

+1.5 →
In total, when a nitrite molecule is oxidized, a free energy yield (Δ G ) of -18 kcal mole -1 is
released. In aquacultural ponds, the most representative bacteria genus that can perform the
nitrite conversion to nitrate is Nitrobacter , but other genus of bacteria are commonly
presented during the nitrification (Camargo et al., 2005).

The nitrate is commonly controlled in aquaculture systems by dilution. In intensive
recirculation systems between 5 and 10% of water are removed and replaced every day. In
systems with low technification, the daily water exchange usually is more than 10%.
A natural pathway to remove the nitrite in an aquacultural system is done by denitrification.
The reaction is carried out by bacteria in absence of oxygen and in presence of methanol as a
carbon compounds. The general reaction is done in two steps (Van Rijn et al., 2006):

+ 5

6 → 1 2 + 5 6 + 2 3 +

2.2.4 Carbon dioxide

Carbon dioxide (CO 2 ) in aquatic systems is very important, because its presence is required
in some chemical and biological processes. For example, CO 2 interacts with water to form a