Effects of aeration on lettuce (Lactuca sativa) growth in deep water culture aquaponics

The main objective of this thesis was to investigate the effects of increased aeration on lettuce (Lactuca sativa) growth in deep water culture aquaponics systems. Oxygen levels play an important role in plant growth, especially in soilless systems a sufficient amount of oxygen needs to be supplied to the root zone to prevent oxygen depletion. Aeration, on the other hand, also represents a cost factor; therefore it is necessary to strike a balance between supplying enough oxygen to ensure good growth and avoiding energy wastage by supplying too much. Research on aeration of growbeds in aquaponics systems is very limited and the results of this study could help growers maximize outputs and resource efficiency. The experiment was conducted in the TAMK greenhouse where a deep water culture aquaponics setup was installed. In the test setup lettuce (Lactuca sativa) was grown in 3 consecutive trials for a period of 6 weeks each. For the experiment six separate growbeds were available. Three were fitted with air supply systems pumping air into the beds at rates of 275 L/h, 550 L/h and 825 L/h while the other three beds served as control without additional aeration. Dissolved oxygen as well as other important water quality and environmental parameters were monitored throughout the experiment. The lettuce yield and several growth indicators were monitored biweekly and compared for different treatments. All treatments showed dissolved oxygen levels above 5 mg/L, which should rule out oxygen depletion affecting the results. The average yield at harvest was 29% lower in the aerated beds compared to the controls but only 12% lower after drying the samples. The lower yield could be partly attributed to the higher pH readings that were accompanied by the added aeration. The results show that heavy aeration can cause a pH shift in the system and even be a limiting factor to plant growth. Therefore it is highly recommendable to monitor and manage dissolved oxygen levels in deep water culture aquaponics