Each fish tank had an associated, 20L bio-filter,made from a plastic storage box. This bio-filter sat above the fish tank and was of a wet/dry trickling design. Water entered the bio-filter by way of a 20 mm airlift pipe,running from the base of the fish tank and into the top of the bio-filter. A 6 mm plastic hose delivered air to the airlift pipe via an air stone. Water from the airlift entered the top of the bio-filter via a “spray bar”, trickled across the biological filter medium and out through a series of 6 × 10 mm holes and back into the fish tank. The bio-filter had a plastic “core flute” lid to lower evaporation. This lid contained a breathing hole in one corner made from a short length of 65 mm PVC pipe to allow gas exchange within the bio-filter. Each tank and bio-filter unit had an associated hydroponic plant growth component which contained standard, washed aquarium gravel to a depth of approximately 200 mm. This component was rectangular in shape and was placed above the fish tank/bio-filter unit on a separate shelving system. A submersible water pump in the fish tank continuously delivered water to the hydroponic component via a 19 mm pipe. Water from the hydroponic component was returned to the fish tank via a 20 mm drainpipe, situated at the opposite end of the hydroponic bed from the water inlet. A continuous flow of water through the hydroponic gravel bed was used because previous experiments demonstrated that this was the most efficient method for the research-scale aquaponic system for the experimental duration applied. Water for all experimental tanks was supplied from the aquaculture laboratory water supply system. Air for all bio-filters and associated airlifts was supplied by a centralised, pressurised air supply that delivered air to the entire aquaculture lab. Lighting consisted of six x 400 W metal halide lamps. Lights were situated above the hydroponic beds at a height of 700 mm above the gravel surface, ebb and flow table with one lighting unit located at the interface between two hydroponic subsystems.
Previous researchers have noted that the nutrient make-up of the water in standard RAS fish systems is sub-optimal for plant growth in recirculating aquaponic systems. This research-scale experiment was designed to determine whether the ionic make-up of the buffer could improve the performance of a recirculating aquaponic system, allowing a controlled pH without negative impacts on fish growth and feed conversion, lettuce growth, yield and health and other test parameters. Fish mortality in all treatments was zero. Ingram obtained less than 5% mortality for Murray Cod exceeding 50 g in weight in culture trials in tanks. Therefore, the mortality in the present study is what should be expected for Murray Cod of this size in standard recirculating aquaculture conditions. In terms of feed conversion efficiency, Ingram,using feed containing 43% protein, obtained a mean FCR for Murray Cod over 150 g in weight of 1.2 therefore, the FCR values obtained in the present study are comparable with research results using industry-standard, recirculating culture methods. Whether the buffer addition regime affects fish growth is also an important question. In the present research-scale study, no significant differences in any fish growth parameter were detected between any treatments or controls,therefore suggesting that none of the pH buffer treatments tested in this study had a deleterious effect on fish growth or survival. Plant growth, yield and health are the parameters most likely to be significantly affected by the constitution of the buffer added to the recirculating aquaponic system. Whilst the products of fish metabolism supply both nitrogen and phosphorous-based nutrients to the system,the buffer that is added to control the pH drop caused by fish metabolism and bio-filter nitrification is an additional source of the macro-nutrients needed for optimal plant growth. Lettuce production as wet, leaf weight gain or yield within the four treatments in the present study followed the relationship mixed = potassium > sodium control > calcium,with a significant difference detected between all treatments, except between the potassium and mixed treatments which exhibited statistically similar results. Therefore, the ionic make-up of the buffer added to the system did affect the efficiency of the aquaponic system in terms of plant production. Yields were equal to or better than those in the studies of Burgoon and Baum,Seawright et al.,Lennard and Leonard,Lennard and Leonard,Geisenhoff et al.,Johnson et al.,Jordan et al. and Maucieri et al..
It is well established in both terrestrial plant production and standard hydroponic plant production that beyond nitrogen and phosphorous, potassium and calcium are the next most-important macro-nutrients for plant growth. Potassium plays an essential role in several functions crucial to plants, including the formation of sugars and proteins, carbohydrate synthesis, cell division, water balance and structural rigidity. Calcium also plays an important role, being a major component of cell walls contributing to the support of plant tissues, as well as contributing to enzyme activation and regulation of water movement into and out of cells. All standard hydroponic nutrient solutions contain macro amounts of both potassium and calcium,and it therefore is expected that aquaponic systems need to contain appropriate concentrations or proportions of these two macro-nutrients for efficient plant production. In the present study, whilst mixed and potassium treatments produced statistically similar lettuce growth and yields, these were statistically significantly higher than those of both the control and calcium treatment systems,with the calcium treatment producing the lowest plant yield. The results of this study therefore, do not fully agree with the findings of other aquaponic studies that contend that potassium and calcium-based buffers are the most appropriate for recirculating aquaponic systems in terms of plant growth and production, since this study indicates an advantage with a potassium-containing buffer. Net nutrient accumulation within the recirculating aquaponic system is an indicator of balance between fish waste production and plant nutrient use. In the present study, all treatment systems contained statistically similar amounts of nitrate at the end of the experiment ; therefore, no buffer addition was better than any other in terms of nitrate removal by the plants. However, results showed that both potassium and mixed treatments achieved higher plant growth and yield. Therefore, it may be inferred that, whilst the lettuce plants within the potassium and mixed treatments did not remove any more nitrate than did the plants within the calcium and control treatments, they may have used that nitrate more efficiently to achieve more plant tissue growth. Resh,Morgan and Jensen and Collins noted that potassium is essential to carbohydrate synthesis in plants.
If potassium levels are too low, carbohydrate and sugar synthesis are blocked, leading to lowered overall plant growth. It seems reasonable therefore, that treatments containing no additional potassium supplementation may have exhibited lower plant growth and yield than those treatments containing potassium,due to this potential blocking of sugar synthesis. Net accumulation of nitrate within all treatments and ranged from 7.80 ± 2.20 mg/L to 13.77 ± 2.23 mg/L. This accumulation is comparable to previous research results using the same aquaponic system with a similar constant flow regime through the gravel plant-growing bed of 11.80 ± 1.78 mg/L over the same time period. Delaide et al. achieved nitrate accumulations of 58 mg/L in their small-scale, deep-water culture aquaponic system growing lettuce and basil. Hasan et al. achieved average nitrate accumulations of approximately 40 mg/L after three weeks growing Sangkuriang Catfish and Nile Tilapia with Water Spinach and Lettuce. Dediu et al. observed nitrate accumulations of 34.52 ± 6.26 mg/L and 32.25 ± 7.06 mg/L in an aquaponic system applying high and low hydraulic retention times growing Bester Sturgeon and Lettuce. Therefore, nitrate accumulations in the current study compared well with other studies. In terms of final treatment phosphate concentrations, control and potassium treatments were statistically similar,whilst calcium and mixed treatments removed more phosphate than did controls. However, potassium treatments were statistically similar to both calcium and mixed treatments, and calcium and mixed treatments were statistically similar to each other. From these results it can be inferred that phosphate removal is a complex process. Adler, Harper, Takeda, et al. argued that when other macro-nutrients are in limiting supplies in hydroponic systems, plants will remove phosphate only to certain levels. The only way to get plants to remove further phosphate from the system is to supply those nutrients that are known to be limited. From the results in the present study, it may be interpreted that some other phosphate removal mechanism may have been involved. Because the control treatment removed statistically no more phosphate than did the potassium treatment, the addition of potassium to the system had little effect on system phosphate removal by plants.
However, it is evident from the results that the addition of calcium to the buffering system had a positive statistical effect on system phosphate removal; systems containing calcium removed more phosphate than did those not contain calcium. This does not necessarily mean that the addition of calcium to the system, via a calcium-based buffer,flood table allowed plants to remove more phosphate. It is known that when excess calcium is added to aquatic systems, it can form a complex with the available phosphate, which then has the ability to precipitate out of the system water, thus lowering available system phosphate levels. This may be the reason why, in the present study, more phosphate was removed from those treatments with calcium supplementation, even though no precipitate was noticed. Net accumulation of phosphate within all treatments and ranged from 2.60 ± 0.11 mg/L to 3.92 ± 0.33 mg/L. This is comparable to previous research results using the same aquaponic research system with a similar constant flow regime through the gravel plant-growing bed of 3.87 ± 0.71 mg/L over the same time period. Makhdom et al. observed phosphate accumulations in an aquaponic system growing Pearl Gourami and Cherry Tomato at the highest planting density of approximately 15 mg/L after 30 days. Liang and Chien achieved phosphate accumulation of 38.1 mg/L when growing Red Tilapia and Water Spinach in an aquaponic system testing feeding frequencies and photoperiods. Therefore, phosphate accumulations in the current study compared well with those of other studies. Dissolved oxygen concentrations show that the buffer added had no effect on the ability of the system water to maintain dissolved oxygen concentrations. D.O. was maintained at levels above the minimum requirement for lettuce,warm water, native Australian fishes and nitrifying bacteria. Previous experiments demonstrated that the inclusion of plants in the research-scale, recirculating aquaponic system led to an outcome whereby buffer additions to control pH may be lowered due to the ionic exchange mechanisms that are prevalent when plants are actively assimilating nitrate and phosphate ions. When nitrate and phosphate ions are assimilated by plants, negative ions are released in order to maintain homeostatic, cellular pH levels within the roots. It is the negative ion portion of the buffer salt added that directly impacts the buffering capacity of the recirculating water.
However, different negative ions have differing capacities to counteract acidification and to buffer pH to desired levels. Because the buffers used in the present study possessed different negative ion constituents,it is difficult to directly compare the amounts of the relative buffers used. However, results suggest that the amount of buffer required was variable and dependent on the negative ion content of the buffer. There was no significant difference in the amount of buffer required between the two buffers that used bicarbonate as the pH-buffering component. However, significantly less buffer was required in both calcium and mixed treatments. This is because these treatments used the hydroxyl ion as the ion to counteract acidification, and less hydroxyl ion is required to maintain a similar pH in a similar system than bicarbonate ion. This explains why pH levels were more difficult to maintain with the bicarbonate-containing buffers, and why these treatments required significantly higher additions than those treatments using hydroxyl-containing buffers. Therefore, whilst the positive ion component of the buffer used had effects upon plant growth, the relative amounts of the negative ion component of the buffer was the determining factor in pH buffering and maintenance.