28843078 a Comp a Ration of Three Different Hydroponic Sub Systems Gravel Bed Floating and Nutrient...

8/8/2019 28843078 a Comp a Ration of Three Different Hydroponic Sub Systems Gravel Bed Floating and Nutrient Film Tech

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Abstract Murray Cod, Maccullochella peelii peelii (Mitchell), and Green Oaklettuce, Lactuca sativa, were used to test for differences between three hydroponicsubsystems, Gravel Bed, Floating Raft and Nutrient Film Technique (NFT), in afreshwater Aquaponic test system, where plant nutrients were supplied from fishwastes while plants stripped nutrients from the waste water before it was returned tothe fish. The Murray Cod had FCRs and biomass gains that were statistically

identical in all systems. Lettuce yields were good, and in terms of biomass gain andyield, followed the relationship Gravel bed > Floating > NFT, with significant dif-ferences seen between all treatments. The NFT treatment was significantly lessefficient than the other two treatments in terms of nitrate removal (20% less effi-cient), whilst no significant difference was seen between any test treatments in termsof phosphate removal. In terms of dissolved oxygen, water replacement and con-ductivity, no significant differences were observed between any test treatments.Overall, results suggest that NFT hydroponic sub-systems are less efficient at bothremoving nutrients from fish culture water and producing plant biomass or yield thanGravel bed or Floating hydroponic sub-systems in an Aquaponic context. Aqua-

ponic system designers need to take these differences into account when designinghydroponic components within aquaponic systems.

Keywords Aquaponic Hydroponic NFT Biological nutrient removal Wastewater Murray Cod Nitrate Phosphate

Introduction

Aquaponics is the integration of hydroponic plant production into recirculating fishaquaculture systems (RAS). The hydroponics control the accumulation of waste

W. A. Lennard (&) B. V. LeonardSchool of Applied Sciences, Department of Biotechnology and Environmental Biology,RMIT University, Building 223, Level 1, Plenty Road,71, Bundoora, Vic. 3083, Australiae-mail: [email protected]

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Aquacult Int (2006) 14:539550DOI 10.1007/s10499-006-9053-2

O R I G I N A L P A P E R

A comparison of three different hydroponic sub-systems(gravel bed, floating and nutrient film technique)in an Aquaponic test system

Wilson A. Lennard Brian V. Leonard

Received: 18 August 2004 / Accepted: 7 April 2006 /

Published online: 27 May 2006 Springer Science+Business Media B.V. 2006


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nutrients from fish culture (Rakocy and Hargreaves 1993), which may lower overallconsumption of water (McMurtry et al. 1997) and produce additional, saleable crops(Rakocy and Hargreaves 1993). Early researchers showed that waste nutrients couldbe stripped from fish culture waters using hydroponically grown plants (Naegel

1977; Lewis et al. 1978; Waten and Busch 1984), with the hydroponic componentgenerally using a sand/gravel/aggregate culture bed (Lewis et al. 1978; Waten andBusch 1984; McMurtry et al. 1993). Both floating (or raft) and Nutrient FilmTechnique (NFT) hydroponic plant growth components have also been indepen-dently tested within the context of an Aquaponic system (Rakocy and Hargreaves1993; Rakocy et al. 1997; Seawright et al. 1998; Adler et al. 2000a), but little liter-ature is available that compares all three hydroponic growth systems against eachother in an Aquaponic context.

The choice of hydroponic growing system within an Aqauponics context maybe based on the independent advantages conferred by that particular hydroponic

component. For example, sand/gravel systems may remove the requirement for aseparate biofilter, as the substrate will also act as a substrate for nitrifyingbacteria, and therefore replaces conventional biofilters (McMurtry et al. 1997;Seawright et al. 1998; Dontje and Clanton 1999). Similarly, the gravel/sand sub-strate may also act as a solids filtering medium (McMurtry et al. 1997; Seawrightet al. 1998). On the other hand, proponents of floating or raft hydroponic com-ponents argue that sand or gravel substrates are excessively heavy and may easilyclog, leading to water channelling, inefficient biofiltration and inefficient nutrientdelivery to plants (Rakocy and Hargreaves 1993; Rakocy et al. 1997). NFT, whichuses a thin film of water typically flowing down a narrow channel, with plantroots partially in the water film, may confer advantages over gravel bed andfloating systems, such as the ease and economic advantages of construction andthe substantially lower weight of the components. However, NFT has not yetreceived much research attention in aquaponics systems, despite it requiringlower water volumes and being one of the most often used and best under-stood hydroponic growing systems (Graves 1993; Morgan 1999; Adler et al.2000a).

This experiment was devised to test whether alternative hydroponic components(gravel bed, floating or NFT) conferred advantages in nutrient stripping, water

consumption, plant yields or fish growth in aquaponics systems. The fish species usedwas the Australian native Murray Cod, Maccullochella peelii peelii and thehydroponic vegetable was lettuce, Lactuca sativa (Green Oak variety).

Materials and methods

Fish origin and holding

Murray Cod were obtained from Australian Aquaculture Products Pty. Ltd.,

Victoria, Australia. Fish were held indoors at the RMIT University AquacultureAnnex and ranged in size from 120 g to 220 g. All fish were kept in 1000 l, cylindricaltanks receiving flow-through water at a flow rate of 3000 l day)1, until required forexperimentation. Water was of domestic origin, carbon filtered and heated toapproximately 22C.

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Experimental aquaponic system

The experimental aquaponic system consisted of 12 individual, identical aquaponicunits, allowing replication of experimental treatments. Each Aquaponic unit con-

sisted of fish holding tank, an associated biofilter and a hydroponic sub-system(Fig. 1). The fish tank was a round 100 l, opaque, white plastic tank (570 mmDiameter 460 mm Deep). As well as fish, the tank contained an airlift pipe (to thebiofilter), a submersible water pump (to the hydroponic bed) and a 100 W ther-mostatically controlled electrical resistance aquarium heater. A plastic core flute(3 mm) lids covered the tank to lower evaporation and stop fish from jumping fromthe tank.

Each tank had an associated, 20 l biofilter (360 mm l 330 mm W 290 mm D),made from a plastic storage box. This biofilter sat above the fish tank and was of awet/dry trickling design. Water entered the biofilter by way of a 20 mm airlift pipe

(at an average of 250 l h)1), running from the base of the fish tank and into the top ofthe biofilter. A 6 mm plastic hose delivered air to the airlift pipe via an air stone.Water from the airlift entered the top of the biofilter via a spray bar, trickledacross the biological filter medium (polystyrene bean bag beads at approx.300 m2 m)3: area/volume) and out a series of 6 10 mm holes (drilled in the bottom,front area of the biofilter) and back into the fish tank. The biofilter of each replicatewas pre-inoculated with nitrifying bacteria from experiments performed previouslyto the one reported here, and therefore, was efficiently converting ammonia andnitrite to nitrate.

Each tank and biofilter unit had an associated hydroponic plant growth compo-nent which was configured depending on the test treatment (see Experimentalmethodology below). This component was rectangular in shape (780 mm l 670 mm W 220 mm D) and was placed above the fish tank/biofilter unit on aseparate shelving system. A submersible water pump (Rio 1700, 1200 l h)1 at 1.2 mhead) in the fish tank continuously delivered water to the hydroponic component viaa 19 mm pipe. Water from the hydroponic component was returned to the fish tankvia a 20 mm drainpipe, situated at the opposite end of the hydroponic bed from thewater inlet.

Fig. 1 Schematicrepresentation of theAquaponic test system

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Water for all experimental tanks was supplied from the Aquaculture lab watersupply system. This water was carbon filtered and heated to approximately 22C.

Lighting (for plant growth in the hydroponic sub-systems) consisted of 6 400 Wmetal halide lamps. Lights were situated above the hydroponic beds at a height of

700 mm above the gravel surface, with one lighting unit located at the interfacebetween two hydroponic sub-systems. Lights were controlled by a digitally timedelectrical switch.

Experimental methodology

This experiment was designed to compare the three hydroponic growing sub-systemswithin the Aquaponic unit(s). These three hydroponic sub-systems consisted ofgravel bed culture (the hydroponic component was filled with approximately 80 l of7 mm, washed river gravel, with the rate of the return water flow set to produce a

water level in the hydroponic component that completely saturated the gravelmedia), floating or raft culture (the drain point was configured so the hydroponiccomponent became a flooded tank containing approximately 48 l of water whichsupported a 4 mm, polystyrene raft, with holes for plant support, was floated ontop of the water) and NFT culture (the drain point was configured so only a thin filmof water, approximately 3 mm deep, flowed across the bottom of the hydroponiccomponent, and a 4 mm, polystyrene sheet, with holes for plant support, was fixedinto the bed so only the lower area of plant roots hung in the water). Four separatetreatments, each with three replicates, were tested:

1. Control: fish in tank, no plants in the hydroponic component (gravel); this wasa control treatment to compare nitrate and phosphate accumulation in theabsence of plants.

2. Gravel: fish in tank, plants and gravel in the hydroponic component.3. Floating: fish in tank, plants in the floating raft hydroponic component.4. NFT: fish in tank, plants in the NFT hydroponic component.

The lighting regime for plant growth was 10 hs on14 hs off, with lights comingon at approximately 08:30 am (AEST) and going off at 18:30 pm (AEST).

At the initiation of the experiment, systems were flushed and refilled with fresh,

Aquaculture system water to 100 l and initial nutrient levels (nitrate and phosphate)were recorded. Fish were added to each system up to the treatment biomass ofapproximately 1000 g (individual tank fish biomass was recorded). Twenty lettuceplantlets (Lactuca sativa Green Oak variety) were planted using an evenly distrib-uted planting scheme in each of the replicate hydroponic beds. The individual initialweight (biomass) of these 20 plantlets was recorded (weight with attached soil plug).Because plantlets had attached plugs of soil, initial leaf weight was estimated byrecording the weights of 15 plantlets with and without attached soil plugs. Theseweights were used to establish a mean ratio of leaf only to leaf + plug weight. Thisratio was then used to estimate the initial leaf only weight of the tested plantlets.

Fish were fed at a percentage of the total initial fish biomass per day (for 6 out of7 days per week) with a 9 mm, sinking pellet (43% protein) (Skretting Classic SS,Skretting Pty. Ltd., Australia) (Table 1). Feeding rates were set at 1.0% of fishbiomass (per day) for the first 5 days, then adjusted to 1.5% for the remaining15 days of the experiment.

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Six out of seven-days a week (at the same time every day9:30 am, AEST;Monday to Saturday, inclusive), the amount of fish feed fed (g), air temperature(C), water replaced per tank (L) (to adjust for evapotranspiration), sodium bicar-bonate added (to adjust pH between 6.80 and 7.00; added directly to the fish rearingcompartment) (g), pH, temperature of the tank water (C), conductivity (lS cm)1)

and dissolved oxygen (mg l)1

) were recorded. Twice a week, tanks were sampled forammonia (mg l)1) and nitrite (mg l)1), whilst once a week, tanks were sampled fornitrate (mg l)1) and phosphate (mg l)1). All samples for analysis were taken fromthe fish-rearing compartment of the Aquaponic unit(s).

The amount of feed provided and the bicarbonate added per tank (to maintainpH) was measured using a top loading balance (A.N.D. HL-200). The amount ofwater replaced per tank was determined by re-filling the tank to a pre-measured100 l mark and recording the amount of water added (water added was freshaquaculture annex water to replace evapotransporation). Temperature (tank water),pH, conductivity and dissolved oxygen were determined using a TPS 90-FL multi-

parameter meter and associated probes. Ammonia, nitrite and nitrate were deter-mined using a Hanna, C203 Multi-parameter ion specific meter (H025463) andHanna Ammonia LR reagent (HI 93700-01), Hanna Nitrite LR reagent (HI 93707-01) and Hanna Nitrate HR reagent (HI 93728-01), respectively. Phosphate wasdetermined using a Merck spectroquant color reagent test (code: 1.1482.0001), read

Table 1 Murray Cod wet weight gain, specific growth rate (SGR), food conversion ratio (FCR) andfood consumption; lettuce mean biomass gain and mean yield; mean net phosphate and nitrateconcentrations, mean weights and removal rates for Control, Gravel, Floating and NFT treatments atthe end of the 21 day trial

Parameter Control Gravel Floating NFT

FishWet weight1 (g/rep.) 220.0a 16.1 206.7a 13.3 266.7a 29.6 250.0a 25.2SGR1 (%/rep./day) 0.90a 0.05 0.89a 0.06 1.13a 0.13 1.09a 0.10FCR1 1.01a 0.08 1.07a 0.07 0.85a 0.10 0.90a 0.08Feed fed (g/rep.) 220.0 220.0 220.0 220.0LettuceBiomass gain1 (g/rep.) 2639.4k 28.9 2338.1m 14.5 2159.0n 9.8Yield1 (g plant)1)) 131.97k 6.46 116.91m 3.24 107.95n 2.20Yield1 (kg m)2) 5.05k 0.25 4.47m 0.12 4.13n 0.08Nutrients

Phosphate1

(mg l)1

) 7.15a

1.03 3.42b

0.11 3.47b

0.94 3.91b

0.37Nitrate1 (mg l)1) 51.23a 1.58 4.63b 2.85 2.60b 1.84 15.70c 2.57Phosphate (g/rep.)y 0.80 0.38 0.51 0.40Nitrate (g/rep.)y 5.74 0.52 0.39 1.62Phosphate removal (%)y 52.5 36.3 50.3Nitrate removal (%)y 90.9 93.2 71.8

1Values are means SE

k, m, n: values showing the same letter are not significantly different (P > 0.05, n = 60) (ANOVA)

a, b, c: values showing the same letter are not significantly different ( P > 0.05, n = 3) (MannWhitney)

y: values are calculated from mean final nutrient concentration per unit volume of test system

replicate

SGR: specific growth rate (% day)1): [(ln final wt. ) ln initial wt.)/(time (days))] 100

FCR: food conversion ratio: feed fed/(wet weight gain)

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against a standard curve using a spectrophotometer at 400 nm (Varian Cary 50 BioUV-Vis).

The entire experiment ran for 21 days from water flushing and planting to har-vesting. At the end of the experiment, fish biomass was determined by wet weight

(A.N.D. HL-200) and plant (leaf only) biomass was determined by wet weight(A.N.D. HL-200). Gains in both fish biomass and plant biomass per tank (orhydroponic component) were determined by the difference between initial and finalwet biomasses recorded. Fish biomass was determined on a per tank basis, whilstplant biomass (leaf only) was determined on an individual, per plant basis.

Comparisons between treatments and controls at the end of the experimentalperiod for Fish biomass, Fish FCR, Fish SGR, nitrate and phosphate were analysedusing MannWhitney, two independent population, non-parametric analysis. Com-parisons between all other parameters were analysed using ANOVA and LeastSignificant Difference (LSD) post-hoc analysis where appropriate. All statistical

analysis was performed using SPSS (Version 10.0) software.

Results

Fish

Survival of Murray Cod in all replicates (all treatments) was 100% for the 21 daytrial. Table 1 shows the increase in fish biomass, specific growth rate (SGR) and foodconversion ratio (FCR) for all treatments. No significant differences (P > 0.05,n = 3) were detected between any treatments in terms of any fish growth parametersmeasured (Table 1).

Lettuce

Lettuce production values (gram per treatment replicate) for Gravel, Floating andNFT treatments are represented in Table 1. There was a significant difference (P Floating > NFT. Yields averaged 5.05, 4.47 and 4.13 kg m)2 for

Gravel, Floating and NFT treatments respectively (Table 1).

Metabolites, nitrates and phosphates

Ammonia (NH3/NH4+) and nitrite (NO2

)) were recorded twice weekly to ascertainbiological filter conversion efficiency. All replicates in all treatments showed unde-tectable ammonia concentrations (0 mg l)1) over the duration of the experiment.Nitrite levels remained undetectable (0 mg l)1) for all replicates in all treatments forthe duration of the experiment.

Final net (finalinitial) mean phosphate (PO43)) concentrations, mean phosphate

weights (per treatment replicate) and phosphate removal rates are represented inTable 1. A significant difference (P< 0.05, n = 3) was detected between the Controltreatment and all other test treatments for final net phosphate concentration, whilstno significant difference (P> 0.05, n = 3) was detected between any of the three testtreatments (Table 1).

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Final net (finalinitial) mean nitrate (NO32)) concentrations, mean nitrate weights

(per treatment replicate) and nitrate removal rates are represented in Table 1. Asignificant difference (P < 0.05, n = 3) was detected between the Control treatmentand all other test treatments for final net nitrate concentration. A significant dif-

ference (P < 0.05, n = 3) was also detected between the Floating and NFT testtreatments, whilst no significant difference (P > 0.05, n = 3) was detected betweenGravel and Floating treatments and Gravel and NFT treatments in terms of final netnitrate concentration (Table 1).

Physical/chemical parameters

Air temperature was measured daily and remained steady at 24.0C. Control,Gravel, Floating and NFT tank temperatures averaged 22.08 0.03C,21.94 0.05C, 22.03 0.03C and 21.96 0.03C, respectively (Data not shown).

Mean daily dissolved oxygen (D.O.) concentrations did not differ significantlybetween any treatments (P > 0.05, n = 39) (Table 2). D.O. concentrations droppedover the length of the experiment in all replicates, but remained above 7.0 mg l)1 forall treatment replicates (Table 2) (final and initial D.O. shown).

Bicarbonate additions were used to maintain pH levels between 6.70 and 7.00,hence, bicarbonate addition and pH are integrally linked. The level of sodiumbicarbonate additions required for test treatment pH maintenance followed theorder Control > Gravel > NFT > Floating (Fig. 2). Control treatment replicatesrequired significantly higher (P< 0.05, n = 51) sodium bicarbonate additions than alltest treatments, whilst the Floating treatment exhibited a significantly lower (P 0.05, n = 51) in terms of sodiumbicarbonate requirement was detected between the Gravel and NFT treatments orthe Floating and NFT treatments (Fig. 2).

Final and initial (and the difference) mean conductivity readings are representedin Table 2. A significant difference (P < 0.05, n = 54) in conductivity was detectedbetween the Control and all test treatments. No significant difference (P > 0.05,n = 54) was detected between the NFT treatment and the two other test treatments,whilst a significant difference (P < 0.05, n = 54) was detected between the Gravel

and Floating treatments (Table 2.)Table 2 Mean initial, final and difference recordings for dissolved oxygen (D.O.), conductivity andmean water use for Control, Gravel, Floating and NFT treatments

Parameter Control Gravel Floating NFT

D.O. Initial1 (mg l)1) 7.77a 0.07 7.84a 0.03 7.96a 0.06 7.93a 0.07D.O. Final1 (mg l)1) 7.23a 0.04 7.20a 0.01 7.28a 0.03 7.28a 0.01D.O. Difference (mg l)1) 0.54a 0.64a 0.68a 0.65a

Conductivity initial1 (lS cm)1) 212.3k 5.2 189.7m, p 3.2 171.7m, q 1.2 155.0m 1.0Conductivity Final1 (lS cm)1) 857.0k 20.8 360.0m, p 22.6 416.3m, q 34.0 377.7m 13.3Conductivity Difference (lS cm)1) 644.7k 170.3 m, p 244.7m, q 222.7m

Daily Water Use1 (L replicate)1) 1.83a 0.10 1.73a 0.10 1.83a 0.10 1.97a 0.10

1 Values are means SE

a, b: values showing the same letter are not significantly different (P > 0.05, n = 54) (ANOVA)

k, m: values showing the same letter are not significantly different (P > 0.05, n = 54) (ANOVA)

p, q: values showing the same letter are not significantly different (P > 0.05, n = 54) (ANOVA)

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Water was replaced daily to compensate for evapotranspiration (Table 2).Overall, daily water replacement averaged 1.83 l replicate)1, 1.73 l replicate)1, 1.83 lreplicate)1 and 1.97 l replicate)1 for the Control, Gravel, Floating and NFT treat-ments, respectively, with no significant difference (P > 0.05, n = 51) detectedbetween any treatments (Table 2).

Discussion

The question of which hydroponic sub-system is best suited to integration withrecirculating fish culture (known as Aquaponics) is a complex one. Fish growth,plant growth, net system nutrient accumulation and other water quality parametersmay be used as indicators of suitability. This study was set up to compare threecommon hydroponic sub-systems within an aquaponic context to try and identify anydifferences between them.

For any aquaculture system, fish survival and growth parameters are paramount.In the present study, fish mortality in all treatments was zero. Ingram ( 2002) ob-tained less than 5% mortality for Murray Cod exceeding 50 g in weight in culture

trials and therefore, the mortality in the present study is what should be expected forMurray Cod of this size (250400 g) in standard recirculating aquaculture. In termsof food conversion efficiency, Ingram (2002) (feed containing 43% protein) obtaineda mean FCR for Murray Cod over 150 g in weight of 1.2, therefore the FCR valuesobtained in the present study (Table 1) are comparable with research results usingindustry standard recirculating culture methods. Whether the hydroponic sub-systemeffects fish growth is also an important question. In the present study, no significantdifferences in any fish growth parameter (biomass gain, SGR or FCR) were detectedbetween any treatments or controls (Table 1). This suggests that none of thehydroponic sub-systems tested in this study had a deleterious effect on fish growth or

survival. However, it must be noted that this study ran for only 21 days, beingdesigned predominantly to test nutrient stripping efficiency by alternate hydroponicsystems, in advance of further experiments on aquaponic systems. Fish would haveto be in the system for a substantially longer period to grow to market size. Fishresults therefore, are only a short-term indication of how the fish reacted to thesystems tested. While long-term experimentation may be required to gain a more

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1.00

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Treatment

Bicarbonate(g/Treatmentreplicate/Day)

Control (a) Gravel (b)(x) Floating (b)(y) NFT (b)

Fig. 2 Mean daily bicarbonate additions (per treatment replicate) for Control, Gravel, Floating andNFT treatments. a, b; x, y: treatments showing the same letter are not significantly different(P > 0.05, n = 51). Error bars represent standard errors

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complete analysis of the suitability of the system to culture fish to market sizes, theseinterim results look promising.

Plant growth is another criteria on which to base the suitability or efficiency of ahydroponic sub-system. Lettuce production as wet, leaf weight gain or yield (weight

gain per unit area) within the three test treatments in the present study followed therelationship Gravel bed > Floating > NFT (Table 1). The choice of hydroponic sub-system therefore, did affect the efficiency of the entire aquaponic system. The GreenOak lettuce in the gravel bed system of this study reached the standard marketable size(leaf weight only) of approximately 130 g. However, plants in both the floating andNFT treatments would require additional time to reach this time in the systems tested.

All treatments in the present study (yields of 5.05, 4.47 and 4.13 kg m)2 for gravel,floating and NFT, respectively) compared well with other Aquaponic studies interms of plant growth. Yields were equal to or better than the studies of Burgoonand Baum (1984) (lettuce yields of 3.34.5 kg m)2) and Seawright et al. (1998)

(Romain lettuce yield of 2 kg m)2). Wren (1984), in a comparison of gravel culturewith NFT culture in an Aquaponic test system, produced 34.5 kg of cucumber fruitin the gravel bed treatment system (over 50 days), compared with no (zero) fruitproduction in the NFT treatment system, again suggesting that gravel bed subsys-tems in an Aquaponic context are more efficient, in terms of plant or fruit yield, thanNFT sub-systems.

Net nutrient accumulation within the entire aquaponic system may be anothercriterion used to assess the suitability or efficiency of the integrated hydroponic sub-systems. In the present study, floating raft hydroponic replicates removed morenitrate from the fish culture water than any other test treatment (Table 1). Final netnitrate concentrations (for the entire aquaponic system) followed the associationFloating < Gravel < NFT < Control (Table 1). However, the three different treat-ments contained different water volumes, therefore, to compare them directly, totalfinal mean nitrate-nitrogen weights within each treatment system were calculated.Table 1 shows that mean final nitrate removal was significantly lower in NFT rep-licates, suggesting that plants in NFT replicates were approximately 20% less effi-cient at nitrate-nitrogen removal than the other two treatments.

The lower nitrate removal of the NFT treatment may be due to the fact that theplants in the NFT treatments had a root-water contact area of less than 50% (typical

in NFT systems; Graves 1993). Plants in both Gravel and Floating treatments had100% of the root area inundated, probably providing more opportunity to assimilatenitrate from the respective culture waters. Wren (1984) also found that NFT culturewas less efficient in terms of nutrient removal than Gravel bed culture. In his testsystem, 31% of nitrate was removed when using gravel beds and 20% of nitrate wasremoved using an NFT system (Wren 1984). Wrens (1984) nitrate removal effi-ciency values are below the present study and this difference may be due to a lowerplantfish ratio within his aquaponic system. Nonetheless, Wrens (1984) studysupports a similar conclusion to the present study, that gravel hydroponics is moreefficient than NFT hydroponics in removing nitrate from fish culture waters.

Phosphate has also been recalculated from concentrations to total grams perreplicate to allow for water volume differences across different treatments (Table 1).These results suggest that, unlike plant nitrate assimilation, plant phosphate assim-ilation was not simply dependent upon the amount of root area available to thewater column, since Floating replicates removed less phosphate than the other twoplant containing treatments, even though the roots in the Floating treatment were

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completely inundated. Also, the NFT treatment removed a large amount of phos-phate (50%) relative to the Control, even though the roots were only partiallysubmerged. Adler et al. (2000b) state that a plants productivity is determined by thenutrient(s) present in lowest supply and that phosphorous uptake may be increased

by supplying those nutrients in shortest supply. In the present study, the Floatingtreatment system contained the highest volume of water (148 l), compared toControl (112 l) and Gravel treatments (112 l) and the NFT treatment (103 l). Allsystems were fed the same daily amount of fish food and contained approximatelythe same biomass of fish, which exhibited statistically identical FCRs, so the totalnutrient production from the fish (nitrate and phosphate) in each different treatmentsystem should have been approximately equal. However, because Floating treatmentreplicates had a higher total water volume, due to the larger volumes within thehydroponic component, then other macronutrients (e.g. Ca, K) and micronutrients(e.g. Fe, Mg, Mn, S, Mo) may have been less concentrated in the Floating treatment.

If so, one of these macro or micronutrients may have acted as a limiting factor,leading to lower phosphate removal by plants than would otherwise have occurred ifthe micronutrient had not been in limited supply.

Dissolved oxygen maintenance may also be affected by the hydroponic sub-sys-tem employed. Goto et al. (1996) found that the D.O. concentration for vigorouslettuce growth needed to be greater than 2.1 mg l)1. For nitrifying bacteria (i.e. theaerobic bacteria living in biofilters), D.O. levels above 2.0 mg l)1 are required forefficiency (Masser et al. 1999; Alleman and Preston 2002) and for warm water fishspecies (like Murray Cod), D.O. concentrations of 5.0 mg l)1 are recommended(Masser et al. 1999). From this, it may be concluded that D.O. concentration min-imums in aquaponic systems should be set to the fish D.O. requirements. Alltreatment replicates in the present study easily exceeded this minimum, fish basedrequirement (Table 2).

In this recirculation system, bicarbonate was added (Fig. 2) to modify pH and sothe two are integrally linked. In terms of bicarbonate addition, all test treatmentsonly exhibited a significant difference in bicarbonate addition when compared withcontrols, but not when compared with each other, which follows a trend establishedin earlier reported experiments (Lennard and Leonard 2004). Treatments containingplants counteracted acidification caused by bacterial mediated nitrification, as plants

are known to release either hydroxyl (OH

)

) or bicarbonate (HCO3)

) ions when theyare actively assimilating nitrate (NO3)) ions across the root wall (Salsac et al 1987;Graves 1993; Imsande and Touraine 1994).

It has been suggested (Rakocy and Hargreaves 1993) that potassium and calcium-based buffers are more suited to aquaponic applications, as plants have a require-ment for these ions, whereas they have little requirement for sodium. The presentstudy was part of a series, and therefore, a sodium-based buffer was used so thatresults could be compared to the previous experiments. Sodium-based buffers wouldbe detrimental in a longer-term study or in a commercial situation, as sodium build-up would most likely eventually lead to sodium toxicity problems for the plants

(Rakocy and Hargreaves 1993).Conductivity followed trends expected for fish-only culture (Table 2), withControl treatment replicates exhibiting a steady increase in mean conductivity overthe course of the experiment (Table 2). With conventional recirculation systemsoperated in the absence of plants, as in the fish-only controls used here, there isno way for these nutrient accumulations to be counteracted, except via water

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exchanges, and so these nutrient and salt accumulations contribute to increasingconductivity in closed systems (Rakocy and Hargreaves 1993). Treatments con-taining plants (Gravel bed, Floating and NFT) exhibited significantly (P < 0.05,n=54) lower accumulations in mean conductivity (Table 2) than did the control

treatment. This is due to the fact that plants actively uptake and remove nutrients(NO3

2) and PO43)) from the system, as well as other ions and salts.

One of the advantages of Aquaponic systems is that water replacement can betheoretically lowered, since nutrient build-ups are not as great, because plants up-take the nutrients (Rakocy and Hargreaves 1993). Alternatively, fish only recircu-lating systems may have daily water replacement rates as high as 15% (Singh et al.1996) from filter back-washes or dilution in order to control nutrient and wastebuild-ups. In Aquaponic systems, water is replaced (not exchanged) only to accountfor the water lost through plant-mediated evapotranspiration. In the present study,water was lost from the experimental systems through one of two mechanisms, the

transpiration mediated by plants (test treatments) or evaporation from the surface ofthe hydroponic component (Control and Gravel treatments). With no significantdifferences between Control and test treatments, it appears that evaporative losseswere the main mechanism for the loss of water in these experiments, and anytranspiration losses by plants in the test treatments were not sufficient to produce asignificant difference from the Controls, where plants were not present.

In summary, aside from the MSc thesis of Wren (1984), little information isavailable on the relative efficiencies of different hydroponic plant growth compo-nents in an Aquaponic context. The present study, comparing Gravel, Floating andNFT hydroponic sub-systems for lettuce yield, nitrate and phosphate removal, wateruse and buffering capacity, showed that the NFT hydroponic subsystem was sig-nificantly less efficient than either Gravel bed or Floating raft hydroponic culturetechnologies. The poorer result for NFT is probably due to low levels of root contactwith water, compared with Gravel bed and Floating raft hydroponic systems (100%)(Graves 1993). Nonetheless, growth of Murray Cod over the time frame of theexperiment (21 days) was good, and not affected by treatment. NFT may still be anappropriate technology for aquaponics for other reasons, such as capital cost andease of use, but system designers will need to take account of the lower nutrientremoval capacity of the NFT methodology.

Acknowledgements This research was partially funded by the Australian Federal GovernmentsRural Industry Research and Development Corporation (RIRDC). The authors also wish to thankBoomaroo Nurseries (Lara, Victoria, Australia) for the provision of the many lettuce seedlings usedto complete this study and Dr. Brett Ingram of the Victorian Institute of Marine and FreshwaterResearch for his invaluable knowledge of Murray Cod culture.

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