3.2. Nitrogen transformations in aquaponics
DO concentrations, pH and temperatures were measured daily
immediately after fish were fed, and the results are presented in
Table 2. Diurnal variation of DO, pH and temperature were also
monitored and are illustrated in Fig. S2 (see Supporting
information). Similar DO concentrations, pH and temperatures
were maintained in both aquaponics.
Fig. 1 shows the variation of nitrogen compounds concentrations
during the study period. The study period was composed of
three phases, and each phase represents a batch of pak choi. Accumulation
of TAN, followed by NO2
and NO3
was observed right
after fish stocking. In both aquaponics, the highest TAN and NO2
concentrations were detected around day 7 and 28, respectively.
This was explained by the slow growth of nitrifying bacteria. In
aquaponics, nitrogen was introduced to the system daily through
protein in fish feed and was excreted to aquaculture water in the
form of ammonia after metabolized by fish. Ammonia can be oxidized
subsequently to NO2
and NO3
through nitrification. During
the startup period of aquaponics, there were not enough nitrifying
bacteria in the system to remove all of the TAN produced by fish,
and accumulation of TAN was observed. With the growth of
AOB, the accumulated TAN was gradually oxidized to NO2
. The