A Study of Phosphorus and Calcium Dynamics in an Integrated Rainbow Trout and Spinach (Nores variety) Aquaponic System with Different Crop Densities

Abstract

The goal of this study is to quantify both calcium and phosphorus budgets for an integrated rainbow trout – spinach (Nores variety) aquaponic system, where three crops densities were used (BH1 –59 crops/m2, BH2 – 48 crops/m2 and BH3 – 39 crops/m2 and a control variant). Fish were fed with two types of feed (41% and 50% protein), using 3 different feeding regimes. Total calcium and total phosphorus retention rates for each of the three tested spinach biomass densities were individually determined by water chemical and plant biochemical analysis. Also, the concentration of those two macroelements was determined from fish meat and fish faeces. Significant differences (p<0.05) were recorded between fish faeces total phosphorus content and between total calcium and total phosphorus retention rates for each of the three variants of tested crops densities (significant higher at BH1 compared to BH3, p < 0.05). It is recommended that lower densities to be used for a better crop absorption of both calcium and phosphorus or a lower hydraulic flow regime and a better light intensity to be applied in case of the used integrated aquaponic system.

Full text

Petrea S. M. et al./ Scientific Papers: Animal Science and Biotechnologies, 2014, 47 (2)
A Study of Phosphorus and Calcium Dynamics in an
Integrated Rainbow Trout and Spinach (Nores variety)
Aquaponic System with Different Crop Densities
Ştefan Mihai Petrea*, Victor Cristea, Lorena Dediu, Maria Contoman,
Mirela Cretu, Alina Antache, Marian Tiberiu Coadă, Alexandru-Cristian Bandi
Dunărea de Jos” University of Galati, Faculty of Food Science and Engineering-Aquaculture, Environmental
Science and Cadastre Department, 800008-Galati, Domneascăt, 47, Romania
Abstract
The goal of this study is to quantify both calcium and phosphorus budgets for an integrated rainbow trout – spinach
(Nores variety) aquaponic system, where three crops densities were used (BH1–59 crops/m2, BH2–48 crops/m2 and
BH3–39 crops/m2 and a control variant). Fish were fed with two types of feed (41% and 50% protein), using 3
different feeding regimes. Total calcium and total phosphorus retention rates for each of the three tested spinach
biomass densities were individually determined by water chemical and plant biochemical analysis. Also, the
concentration of those two macroelements was determined from fish meat and fish faeces. Significant differences
(p<0.05) were recorded between fish faeces total phosphorus content and between total calcium and total phosphorus
retention rates for each of the three variants of tested crops densities (significant higher at BH1 compared to BH3,
p<0.05). It is recommended that lower densities to be used for a better crop absorption of both calcium and
phosphorus or a lower hydraulic flow regime and a better light intensity to be applied in case of the used integrated
aquaponic system.
Keywords: aquaponic system, calcium, faeces, phosphorus, rainbow trout, spinach
1. Introduction
Most of the recirculating aquaculture systems
replace 5% to 10% of system water daily to
prevent the buildup of toxic levels of ammonia
and other fish by-products and provide makeup
water for evaporation and for backwashing filters
[1]. Related to this, crops can be cultured
hydroponically in recirculating systems to produce
a valuable by-product, while improving the water
quality is highly desirable [2]. The main attraction
for the implementation of those integrated
aquaponic systems has financial considerations
* Corresponding author: Ştefan Mihai Petrea
Tel.+40765217068
Email: petreastefanmihai@yahoo.com
stefan.petrea@ugal.ro
because nutrient recovery from aquaculture
effluents reduces hydroponic chemical costs [3],
but also ecological reason heavily weighs. High
quality water is repeatedly used to support the
growth of both fish and vegetables, further
reducing costs [3]. The system involves no control
of root pathogens, as these are controlled
biologically by the broad spectrum of antagonistic
micro-organisms that develop in the natural
environment [4, 5]. Plants extract nutrients from
wastewater and convert the metabolic products
which could be toxic for fish, fact that makes
some authors [6, 7] to characterize aquaponics as a
friendly method in relation with the environment,
due to the reuse of wastes and nutrients in the
resulting effluents from the fish farming activity.
The process of by-products (wastes) revalorization
from one species in a second crop generated by
the co-cultured species enhances the profitability
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due to lower refreshment rate and water
consumption or to the crop itself which represent
another source of revenue for the farmer [8, 9].
After nitrogen, phosphorus is the second most
frequently limiting macronutrient for plant
growth, making up about 0.2% of a plant's dry
weight [10]. Plants deficient in phosphorus are
stunted in growth and often have an abnormal
dark-green color [11]. Also, phosphorus is an
important mineral in nucleic acids and cellular
membranes, the main representative of the
structural components of the skeletal tissues, and
it is directly involved in energy processes [12].
Fish can absorb this mineral from the water, but
due to the low waterborne P concentration, dietary
supplementation is necessary [13]. Phosphorus
retention is also directly affected by fish growth
rate and higher values were obtained when growth
performances were good [14]. Nowadays, the
evaluation of micronutrients and essential trace
elements levels of fruits and vegetables is a
growing trend in nutritional studies throughout the
world [15].
Therefore, the present study aims to quantify both
calcium and phosphorus budgets for a floating
rafts integrated rainbow trout–spinach (Nores
variety) aquaponic system, where three crops
densities were used.
2. Materials and methods
Integrated aquaponic system description
The present experiment took place between
February 20th and April 4th, 2013 at the
recirculating system pilot station of Aquaculture,
Environmental Science and Cadastral
Measurements Department of Food Science and
Engineering Faculty–“Dunarea de Jos’’
University of Galati. Figure 1 describes for the
first time, the new emplacement and configuration
of this second recirculating pilot station as
follows: 12 rectangular shape rearing units with a
volume of 0.15 m3/unit–No. 1; 2 rectangular sump
units with a volume of 0.29m3/unit–No. 2; 1
mechanical-quartz sand water conditioning unit
with backwash–No. 4; 1 biological trickling
filtration unit–No. 5; one sterilization UV filter
(TETRA POND, Type UV-C 35000 and 36
Watt)–No.7; 3 recirculating pumps–No. 3,
oxygenation unit (compressor Resun Air- Pump,
Model: ACO-018 A with a flow of 260 l/min) and
also water quality control sensors–No. 6.
The aquaponic modules (No. 8) consist in 4
rectangular glass made units (900x600x200mm),
placed above the recirculating system, on a metal
support (Figure1). A lighting system made of 3
fluorescent lamps, with reddish wavelength and a
luminous power of 1080 lm was placed above the
hydroponic units (Figure 1–No. 9).
Figure 1. The configuration and emplacement of second pilot recirculating system station
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Regarding the water cycle inside the integrated
system, it can be seen from the direction indicated
by the arrows in figure 1, that water flows out
from the rearing units, pass through the
mechanical filter first and after that, by using a
recirculating pump, it goes through the biological
filtration unit and then to the aquaponic modules,
that output the treated water back to rearing units,
as described in previous studies [16, 17]. The total
volume of water from the integrated system is
around the value of 2.5-2.7 m3. An equal water
flow of 6 L/minute was set for the inlet of all 4
hydroponic units. The support media of spinach
consisted of polystyrene plates with holes for
plastic special supports (Figure 2).
Figure 2. Aquaponic modules: AA’. Longitudinal section view; BB’. Crossing section view
Plants were placed in plastic supports and then,
these were filled with a few hydroton balls to
ensure their stability–figure 2 [16, 17]. The
distance between plants was 15cm length and
widthwise and the maximum capacity of an
aquaponic unit was 32 plants (Figure 2) [16, 17].
Experimental design
Before starting the experiment, the activation of
the biological trickling filtration unit was made as
described in previous studies [8]. Daily ammonia,
nitrite and nitrate levels were monitored to
determine the degree of ammonia oxidation to
nitrate and therefore to observe when a stable state
of bacterial biomass is obtained [16, 17]. For the
44 days experiment, a total number of 228
rainbow trout (Oncorhynchus mykiss), with an
average initial weight of 111.77 grams, was used
in parallel with spinach (Spinacia oleracea),
Nores variety, at an age of 25 days, as described in
previous studies [16, 17]. Total fish biomass from
the recirculating aquaculture system, at the
beginning of the experiment, was 25.51 kg [16,
17]. Fish were divided in six groups, in duplicate,
as described in previous studies [16, 17]. Three of
them (G1) were fed with Clasic Extra 1 P–41%
brute protein; 0.9% phosphorus and the other three
(G2) with Nutra PRO-MP-T–50% brute protein;
1.3% phosphorus; 1% calcium, as in the protocol
described in previous studies [18]. A total amount
of 12 363.32 g of Clasic Extra 1 P feed and 11
579.54 g Nutra PRO-MP-T was administrated
during all 44 experimental days [16, 17]. Nores
variety spinach was placed in the hydroponic units
with the following stocking densities: (BH1–
59crops/m2, BH2–48crops/m2 and BH3–
39crops/m2). The seedlings were obtained at the
Natural Sciences Museum Complex Galaţi. A
daily percentage of 10% water exchange was
applied. The technological water was analyzed in
terms of phosphorus and calcium concentration by
using Spectroquant Nova 400 spectrophotometer,
with Merck compatible kits. Samples of water
were taken from the outlet of biological filter
(inlet of hydroponic units) and outlet of each
hydroponic unit. The luminous intensity was
measured with TESTO 545 light meter. Both
hydraulic loading rate (HLR)=flow rate (Q)/total
surface area of hydroponic module (m/day) and
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hydraulic retention time (HRT)=surface area x
water depth x porosity of hydroponic module/flow
rate (min), were calculate according to previous
studies [19]. The values obtained were 16m/day
for hydraulic loading rate and 0.48 min for
hydraulic retention time. The phosphorus and
calcium removal rates in hydroponic units were
calculated with the following formula [8]:
phosphorus/calcium retained (g/m2/day)=
((Q/V*(Cin-Cout)–dCout/dt)*d, where, Q=the flow
rate (m3/day), V=system volume (m3),
C=concentration of TAN (g/m3), d=depth (m),
t=time (d). The obtained results were then
expressed in m2/day. The fish faeces collection
was made with a special EHEIM water vacuum
cleaner provided with a mesh compartment for
solids retention. Chemical analyses concerning
both phosphorus and calcium levels were carried
out over a number of 5 samples. For determining
the phosphorus pentoxide, SR ISO 2294:2009
reference method was used. Also, for determining
the total calcium content, the organic substance is
oxidized by oxygen through calcination at a
constant temperature of 450oC±25oC and the
obtained extract is solubilized with 0.5N HCl,
neutralized with NaOH -1N until a 12-13 pH
range. Finally, the complexometric method is
used, with murexide as indicator.
Statistical methods
Statistical analysis was performed using the IBM
SPSS Statistics 20 for Windows. Statistical
differences between treatments were tested using
T test (α=0.05) after a normality test
(Kolmogorov-Smirnov). Comparisons between
variants were assessed using post-hoc Duncan test
for multiple comparisons (ANOVA). Also,
bivariate correlations were made for obtaining
Pearson coefficient.
3. Results and discussion
Phosphorus and calcium in fish meat
A total quantity of 12 363.33 g of Clasic Extra 1 P
and 11579.54g of Nutra PRO-MP-T were
distributed in the integrated system throughout the
44 experimental days. The fodder biochemical
analysis confirmed a content of 0.9% phosphorus
for Clasic Extra 1 P and 1.3% phosphorus for
Nutra PRO-MP-T. The total phosphorus input into
the recirculating integrated system, through
administrated feed quantity, is presented in figure
3. Also, Nutra PRO-MP-T feed contains 1%
calcium, fact that makes a total calcium input of
around 116 g throughout the experimental period.
Figure 3. The dynamics of phosphorus input into
the recirculating integrated system
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Regarding rainbow trout meat phosphorus and
calcium content, before the start of the
experiment, a mean value of 252.66±7.25 mg
P2O5/100 g phosphorus and 31.11±0.96 mg/100g
total calcium were registered. At the end of the
experiment, the following mean values were
obtained: 248.7±12.5 mg P2O5/100g and
31.97±2.11 mg/100g total calcium at G1 fish
group; 249.9±13.62 mg P2O5/100g and
37.84±1.96 mg/100g total calcium at G2 fish
group. The results were lower compare to
previous studies [20], where two types of feed
were administrated, one with 1.8% phosphorus
and the other with 0.99% and that registered a
rainbow trout meat phosphorus content of 370 mg
P2O5/100 g, respectively 430 mg P2O5/100 g,
given the fact that at the beginning of the
experiment, the rainbow trout mean phosphorus
content was 500 mg P2O5/100g. The maximum
phosphorus absorption in rainbow trout is 520 mg
P2O5/100g and higher dietary levels only increase
the amount of excreted phosphorus [21]. It has
been mentioned that rainbow trout fed fish meal
based-diets showed higher phosphorus retention
than fish fed soy protein concentrate [22]. Also,
diets with high lipid levels and lower phosphorus
content improved phosphorus retention [23].
Values between 42-58 mg/100g total calcium in
trout meat, higher than our current values, were
registered in previous studies [24] and also it was
mentioned that an increase of 10 g in body weight
is accompanied by a storing of 0.04 g of total
calcium. According to other studies [25], the
accumulation of dietary calcium in the body of
rainbow trout was higher in a diet containing no
phosphorus than in a diet whose Ca/P ratio was l.
Regarding the data series distribution of final total
calcium meat content (Figure 4), by analyzing
skewness and kurtosis, it can be said that the
mesokurtic distribution has a little platikurtic
tendency, a bit flatter than a normal distribution,
with the tendency of values scattering over a
longer interval around the mean, a little tilted to
the right, with more extreme values to the left, fact
that goes also for G1 variant–meat phosphorus
content. The values indicated by the median were
close to average values. By using two multiple
comparisons test (Tukey and Duncan–ANOVA),
Figure 4: Phosphorus (P2O5) and total calcium content in rainbow trout meat
it was observed that differences between the initial
meat phosphorus values and final G1 and G2
values are not significant (p>0.05)–1 data subset:
Initial+G1+G2. Also, the differences between the
initial meat calcium values and final G1 values are
significant, comparing with final G2 values
(p<0.05)–2 data subset: Initial+G1; G2. For
measuring the correlation intensity between
phosphorus and calcium meat accumulation,
Pearson correlation was used. The value of
Pearson correlation coefficient was-0.329, stating
that there is not an indirect strong correlation.
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Phosphorus and calcium in spinach variants
Regarding spinach final phosphorus and calcium
content, for each one of the three experimental
variants and for market spinach, the following
mean values were obtained: 21.68±0.39 mg
P2O5/100 g fresh weight (FW) and 17.88±1.39 mg
total Ca/100g FW at BH1; 26.88±0.63 mg
P2O5/100 g FW and 24.17±1.62 mg total Ca/100g
FW at BH2; 32.23±1.32 mg P2O5/100 g FW and
30.87±0.97 mg total Ca/100g FW at BH3;
31.83±1.79 mg P2O5/100g FW and 65.64±1.37 mg
total Ca/100g FW for market spinach. The current
values were near the ones from previous studies
[26], where a phosphorus content between 29-37
mg P2O5/100 g FW in spinach leaves was
reported. It must be pointed out that although
other studies [27] regarding phosphorus level of
green lettuce, cultured in an integrated aquaponic
floating rafts system together with pikeperch,
found higher phosphorus levels of aquaponic
green lettuce (34.81–37.33 mg P2O5/100g Fw),
comparing with normal cultured one (20.13mg
P2O5/100 g FW) on soil, in case of the current
experiment, only the BH3 crops phosphorus level
registered this tendency (32.23 mg P2O5/100g FW,
comparing with 31.83 mg P2O5/100 g FW at
market spinach). Regarding total calcium content,
significant lower levels were registered for
spinach grown in aquaponic conditions,
comparing with market spinach, cultured in soil.
From figure 5 it can be concluded that the data
series distribution of final phosphorus spinach
content, by analyzing skewness and kurtosis, have
a mesokurtic distribution with a little platikurtic
tendency, a bit flatter than a normal distribution,
with the tendency of values scattering over a
longer interval around the mean, a little tilted to
the right, with more extreme values to the left and
the values indicated by the median close to
average values. Also, regarding total calcium
spinach content, the distribution is almost the
same, except that here the histogram is a little
tilted to the left, with more extreme values to the
right (Figure 5) with the values indicated by the
median close to average values. By using two
multiple comparisons test (Tukey and Duncan–
ANOVA), it was observed that differences
between the experimental variants (BH1, BH2,
BH3) in term of phosphorus spinach content, are
significant (p<0.05) and the ones between BH3
and market spinach in term of phosphorus content
are not significant (p>0.05)-3 data subset: BH1;
BH2; BH3+Market spinach.
Figure 5. Phosphorus (P2O5) and total calcium spinach leaves content
Also, the differences between the experimental
variants (BH1, BH2, BH3, market spinach) in
term of total calcium spinach content, are
significant (p<0.05)-4 data subset: BH1; BH2;
BH3; Market spinach. For measuring the
correlation intensity between phosphorus and
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calcium spinach leaves content, Pearson
correlation was used. The value of Pearson
correlation coefficient was 0.675, stating that there
is direct strong correlation between them.
Phosphorus in rainbow trout faeces
Fish faeces tend to be highly variable in their
chemical content, which is also the case with other
animal faeces [28- 30]. According to other studies
[28], it was found difficult to compare faeces
phosphorus (P2O5) values with those from other
studies because of the differences in conditions
under which the solids were produced, separated,
stored, and collected. The mean faeces phosphorus
(P2O5) values, registered in the current research
were 2.95±0.48% P2O5/faeces DW for G1 fish
group, where Clasic Extra 1P–0.9% phosphorus
feed was administrated and 3.63±0.47%
P2O5/faeces DW where Nutra Pro MP-T–1.3%
phosphorus feed was used. The differences
between G1 and G2 phosphorus faeces content
were statistical significant (p<0.05). In Figure 6 it
can be observed that the faeces phosphorus
content has a decreasing trend throughout the
experimental period, most probably because of a
progressive positive tendency manifested toward
fish phosphorus retention.
Figure 6. The evolution of faeces phosphorus content
In previous studies [28], a phosphorus level for
rainbow trout faeces of 2.54% P2O5 was
registered. Also, similar values were reported in
other studies [31]-2.22% P2O5 and [30] 3.51%
P2O5. Lower values were reported in [29]-0.35%
P2O5-1.85% P2O5; [32]-1.79% P2O5, 1.49% P2O5
and [33] 0.94% P2O5, all for rainbow trout faeces.
Also, other studies [34], registered different
concentration of phosphorus in rainbow trout
faeces, by administrating different feed types:
2.51±0.13% P2O5 in faeces for 1.12±0.07%
phosphorus feed type; 3.86±0.13% P2O5 for
1.2±0.10% phosphorus in feed; 2.25±0.07% P2O5
for 0.90±0.02% phosphorus in feed and
2.87±0.86% P2O5 for 1.08±0.16 phosphorus in
feed. According to other studies [20], it has been
concluded that if fish are fed available phosphorus
above their requirement, an increment of the
nonfecal phosphorus excretion at some level can
be expected in large-sized fish due to a reduction
of the retention efficiency. Also, it is known that
while the efficiency of phosphorus retention into
the carcass is decreased, more is discharged as
soluble (presumably urine) and insoluble
(presumably faeces) waste [35].
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Phosphorus and calcium in technological water
The mean water phosphorus and calcium
concentrations at the inlet of aquaponic modules
and also at the outlet of each one of them,
registered the following values: 9.95±3.47 mg
P2O5/L and 94.48±4.12 mg Ca2+/L at the
aquaponic modules inlet and 8.11±3.18 mg P2O5/L
and 92.6±4.76 mg Ca2+/L at BH1 outlet;
8.49±3.21 mg P2O5/L and 93.18±4.71 mg Ca2+/L
at BH2 outlet; 8.9±3.2mg P2O5/L and 93.54±4.65
mg Ca2+/L at BH3 outlet; 9.91±3.46 mg P2O5/L
and 94.4±4.14 mg Ca2+/L at control variant outlet.
Figure 7. The evolution of P2O5 and Ca2+ concentration in technological water
The evolution of P2O5 and Ca2+ concentration in
technological water is presented in figure above. It
can be observed that Ca2+ concentrations have a
downward trend, while P2O5 concentrations had
some fluctuations at the beginning of the
experimental period, but immediately after, had a
constantly evolving tendency (Figure 7).
Water treatment capacity
Regarding phosphorus and calcium removal rates,
the following mean values were obtained:
3.83±1.15 mg/day P2O5 and 3.9±1.39 mg/day Ca2+
at BH1; 3.04±0.91 mg/day P2O5 and 2.69±1.27
mg/day Ca2+ at BH2; 2.18±0.54 mg/day P2O5 and
1.95±1.12 mg/day Ca2+ at BH3. The evolution of
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phosphorus and calcium removal rates is
presented in Figure 8. It can be observed that
phosphorus removal rate has some fluctuations
given by the variations of the crops nutritional
demand for phosphorus, but the general
impressions one of upward tendency. The
evolution of calcium removal rate, comparing with
the one of phosphorus, presents a less abrupt
upward tendency, revealing a constant increase of
crops calcium absorption rate.
Figure 8. The evolution of P-PO43- and Ca2+ removal rates
By using two multiple comparisons test (Tukey
and Duncan–ANOVA), it was observed that
differences between the experimental variants and
the control variant in terms of phosphorus
retention rate are significant (p<0.05). Also
significant differences are revealed between BH1
and BH3 experimental variants (p<0.05)-3 data
subset: BH1+BH2; BH2+BH3; Control. The
differences between the experimental variants and
the control variant in terms of calcium retention
rate are significant (p<0.05). Also significant
differences are revealed between BH1, BH2 and 3
experimental variants (p<0.05)-4 data subset:
BH1; BH2; BH3; Control. Pearson correlation was
used. The value of Pearson correlation coefficient
was 0.901, stating that there is a direct strong
correlation between phosphorus and calcium
removal rates.
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4. Conclusions
As a main conclusion to this study it can be stated
that plant density applied in BH1 case is the best
of all three tested densities in terms of water
chemical treatment, given the highest values of
phosphorus (P2O5) and calcium (Ca2+) removal
rates. By analyzing the phosphorus (P2O5) and
calcium (Ca2+) removal rates, it must be pointed
out that plants have different evolution periods in
their lifetime and therefore different nutrient
absorption rates, more constant in case of calcium.
Also, the balance between plants absorption rates
and administrated feed quantity in terms of
calcium and phosphorus budgets is found to be
important. Feed phosphorus and calcium
concentration was found to be less important in
relation with rainbow trout meat phosphorus
(P2O5) and total calcium content. From the faeces
phosphorus (P2O5) content evolution, we can
conclude that fish has the upward tendency to
retain phosphorus, manifested especially after the
first 20 days of the experimental period.
Regarding the spinach phosphorus (P2O5)
concentration, in BH3 experimental variant there
have been registered values similar to market
spinach, in comparison with the other two
experimental variants where phosphorus
deficiency was observed. Also, total calcium
deficiencies were observed in spinach from all
three experimental variants, compared with
marketable spinach. Visual signs of these
deficiencies were manifested by necrotic leaf
margins on young leaves or curling of the leaves,
especially in case of BH1 and BH2 experimental
variants. It is recommended that others feeding
regimes or other feed types to be used, by
respecting the plants stocking density applied in
case of BH3 variant, to achieve an equilibrium
between plants calcium absorption rates and Ca2+
concentration in water.
Acknowledgements
The work has been funded by the Sectoral Operational
Programme Human Resources Development 2007-
2013 of the Ministry of European Funds through the
Financial Agreement POSDRU/159/1.5/S/132397 -
ExcelDOC. The autors thanks to the management staff
of the project for their support.
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