Carbon and Nitrogen Balances in Ocean Tank
at the New Jersey State Aquarium
Régulation des flux de carbone et d'azote dans le bassin
«Océan» de l'Aquarium du New Jersey
Gordon GRGURIC1, Christopher J. SONDEY1 and Brian M. DUVALL2
1
Marine Science Program, The Richard Stockton College of New Jersey,
Pomona, NJ 08240, USA
2
New Jersey State Aquariums, 1 Riverside Drive, Camden, NJ 08103, USA
ABSTRACT
A variety of empirical and calculated data from the largest tank at the New Jersey
State Aquarium were used to quantify the fluxes of carbon and nitrogen before and after
the installation of denitrification in this facility. Before denitrification, the stock of
dissolved inorganic carbon (DIC) in Ocean Tank exhibited a decrease of 6.9 kg C/month
and sodium bicarbonate had to be added to maintain DIC in steady state. Nitrogen budget
in Ocean Tank before denitrification is in contrast to that of carbon, and it shows an
increase of 4.8 kg N/month in the form of nitrate. The use of methanol for denitrification
has resulted in a flux of 26.3 kg C/month into the aquarium and, as predicted, an increase
in Ocean Tank DIC stock has been observed without any additions of sodium
bicarbonate.
RESUME
Une multitude de données empiriques et calculées, issues de la plus grande cuve de
l'Aquarium du New Jersey, ont été utilisées pour quantifier le flux de carbone et d'azote
avant et après l'installation d'un système de dénitrification sur ce bac. Avant l'installation
du système de dénitrification, la réserve de carbone inorganique dissous (DIC) présentait
une décroissance moyenne de 6.9 kg/mois et du bicarbonate de soude devait être ajouté
pour maintenir le DIC à un niveau constant. Le bilan d'azote de la cuve Océan avant
l'existence d'un système de dénitrification était inversé par rapport au carbone, avec une
augmentation de près de 48 kg N/mois sous la forme de nitrate. L'utilisation de méthanol
comme source de carbone pour la dénitrification a généré un flux de carbone de 26.3 kg
C/mois dans l'aquarium et un accroissement du DIC a été observé sans ajout de
bicarbonate de sodium.
Bulletin de l’Institut océanographique, Monaco, n° spécial 20, fascicule 1 (2001)
INTRODUCTION
Ocean Tank at the New Jersey State Aquarium has a volume of 2.87 million
liters, which makes it one of the largest aquarium tanks on the East Coast of the
United States. The environment on display mimics the continental shelf adjacent
to the New Jersey coast, including the Hudson River Canyon. The tank houses
about 40 different fish species, ranging from blueback herrings and mullets to sand
tiger sharks and rough-tail stingrays. The medium, in which these species live, is
artificial seawater of salinity 28-30 g/kg and temperature of 20C. Flow dynamics
through Ocean Tank involve surface skimming to produce a total flow rate of
16,000 L/min that is delivered to twelve parallel sand filters and treated afterward
in a bio-filter. A small fraction (less than 10%) of the total flow is ozonated in a
separate line. The filtered and disinfected water is reintroduced to the tank through
a silica quartz gravel bed on the aquarium floor. The volume of the entire system
re-circulates in about 3 h.
We used a variety of empirical data, collected since Ocean Tank opened, to
determine, directly or indirectly, the magnitudes of carbon and nitrogen fluxes in
this system. We compare the steady state of carbon to the non-steady state of
nitrogen and examine how the installation of a biological denitrification system
has affected the balances of these two elements in the aquarium.
RESULT AND DISCUSSION
The largest carbon reservoir in Ocean Tank seawater is dissolved inorganic
carbon (DIC), which is defined as the sum of CO2 (aq), HCO3- and CO32- species
(Libes, 1992) and serves as a buffering system for seawater pH. In ambient ocean,
the size of this reservoir is 30 times greater than that of dissolved and particulate
organic carbon (Millero, 1996). Occasional measurements of total organic carbon
in Ocean Tank show a concentration of less than 60 μM. This value is essentially
the concentration of dissolved organic carbon (DOC), because recirculation and
filtering quickly remove most particulates. DIC in Ocean Tank can be determined
from pH and total alkalinity measurements, where the average values of those
quantities in Ocean Tank are 7.65 and 2.46 meq/L, respectively. Carbonic acid
equilibria in seawater as given in Millero (1996) can be used to show that these
values translate to a DIC concentration that is at least 40 times greater than the
DOC concentration in the system. Therefore, the DOC reservoir was considered
insignificant relative to the other carbon reservoirs and fluxes in this study.
The main source of carbon to Ocean Tank is food given to the fish in the tank.
This food consists of raw seafood as well as processed fishmeal. The feeding
records show that, on average, 26.1 kg of raw seafood and 1.3 kg of fishmeal are
given each day. The elemental composition of nitrogen and phosphorus in the
fishmeal can be used to compute the weight fraction of carbon, using Redfield
ratio (Redfield et al., 1963). For raw seafood, the amount of carbon can be
Bulletin de l’Institut océanographique, Monaco, n° spécial 20, fascicule 1 (2001)
determined from the ratio of particulate organic matter to particulate organic
carbon (Pilson, 1998) for dry, salt-free organic matter. The amount of carbon thus
calculated is 9.3 kg/day from raw seafood, and 0.6 kg/day from fishmeal. These
values can be combined to yield a flux of 296 kg C/month from food sources.
Continuous formation of white precipitates has been observed on artificial
rocks in Ocean Tank. These precipitates were found to be carbonaceous in nature,
with Mg and Ca as the principal cations. The carbon flux into these precipitates
can be calculated if their formation rate is known. This rate can be determined
from the magnesium budget in Ocean Tank, which exhibits non-conservative
behavior, as evidenced by the significantly lower Mg/Cl ratio in Ocean Tank
relative to the ratio in freshly prepared artificial seawater (Figure 1). Precipitation
of these cations as carbonate was computed to remove 4.2 kg C/month from
Ocean Tank seawater.
Figure 1. Three-year averages and standard deviations for Mg/Cl ratios in
Ocean Tank and in freshly prepared artificial seawater in Salt Water Tank
There are several other carbon fluxes in Ocean Tank that were not yet
discussed. All of them are net carbon losses and they are specifically: (1)
outgassing of carbon dioxide, (2) excretion and removal of fecal pellets, and (3)
retention of organic carbon for the growth of aquarium organisms. These fluxes
were not individually quantified due to the difficulty in obtaining accurate
measurements and/or interpreting them. However, their net magnitude can be
calculated from the known fluxes, if carbon in the system is in steady state. Steady
state carbon conditions in the aquarium are achieved through occasional additions
of sodium bicarbonate to the tank. The magnitude of this flux can be determined
from sodium bicarbonate addition records to Ocean Tank, given in Table 1. The
difference between DIC concentrations calculated from initial and final pH and
Bulletin de l’Institut océanographique, Monaco, n° spécial 20, fascicule 1 (2001)
alkalinity values in Table 1 is 1.25%, so the sodium bicarbonate addition record
for this period can be interpreted as that needed to maintain carbon in steady state.
From the total mass of sodium bicarbonate added and the time elapsed, this flux is
calculated as 8.6 kg C/month.
______________________________________________________________________________
Table 1: Record of pH, total alkalinity and sodium bicarbonate
additions to Ocean Tank over a three month period
______________________________________________________________________________
Date
pH
Total Alkalinity
(meq/L)
December 18, 1997
7.75
2.50
December 23, 1997
7.78
2.51
January 6, 1998
7.81
2.39
January 20, 1998
7.77
2.29
January 21, 1998
7.76
2.25
January 22, 1998
7.74
2.24
February 5, 1998
7.62
2.08
February 19, 1998
7.61
2.00
March 4, 1998
7.57
1.84
March 11, 1998
45.4 kg
NaHCO3 added
March 12, 1998
45.4 kg
"
"
March 13, 1998
45.4 kg
"
"
March 14, 1998
45.4 kg
"
"
March 17, 1998
7.72
2.46
______________________________________________________________________________
To understand why additions of sodium bicarbonate are necessary in a closed
system, heterotrophic respiration in seawater needs to be considered, and the
subsequent decrease in pH due to equilibration of the released carbon dioxide (eq.
1 and 2).
CO2 + H2O <═══════> H+ + HCO3-
(1)
HCO3- <═══════> H+ + CO32(2)
Figure 2 shows pH and alkalinity in Ocean Tank during a three-month period,
when no sodium bicarbonate was added. The continuous decrease in both
parameters is due to acidity produced during bacterial nitrification reactions in
aquarium bio-filters. The data shown in Figure 2 are in contrast to ambient marine
observations (Park, 1969; Goyet and Brewer, 1993), and a carbon loss of 6.9
kg/month has been computed. This result is in reasonable agreement with the flux
of carbon through additions of sodium bicarbonate needed to maintain DIC in
steady state (previously shown as 8.6 kg C/month).
Bulletin de l’Institut océanographique, Monaco, n° spécial 20, fascicule 1 (2001)
Figure 2. Changes in pH (circles) and alkalinity (squares) over a three month
period when no sodium bicarbonate was added to Ocean Tank
For the first several years of its operation, Ocean Tank seawater exhibited a
continuous increase in its nitrate concentration. The rate of nitrate concentration
increase translated to a nitrogen addition flux of 4.8 kg N/month. Concerns about
nitrate toxicity in Ocean Tank prompted the installation of a biological
denitrification system in 1998. Biological denitrification under controlled
conditions has been employed to decrease nitrate concentrations in other large
seawater aquaria (Grguric and Coston, 1998). Denitrification in Ocean Tank is
separate from the main water circulation and treatment loop, and the system hosts
facultative anaerobic bacteria, which use nitrate as an oxidizing agent for their
metabolic processes. Methanol is used as a source of organic carbon for the
bacteria and the net denitrification reaction (Jeris and Owens, 1975) is given in
equation (3):
6 NO3- + 5 CH3OH ────────> 3 N2 + 5 CO2 + 7 H2O + 6 OH-
(3)
Since 1998, nitrogen in Ocean Tank has a sink as well as a continued source in
animal feed, and its concentration should eventually reach a steady state. The
effect of denitrification on carbon fluxes in Ocean Tank is particularly interesting,
and can be seen from the right side of equation (3). The production of hydroxide
ions leads to an increase in pH and therefore, equilibration of the released carbon
dioxide means that a relatively small fraction will be lost through outgassing, and a
much larger fraction will remain in solution (largely as HCO3- ions).
Bulletin de l’Institut océanographique, Monaco, n° spécial 20, fascicule 1 (2001)
Denitrification is thus expected to serve as a net source of carbon to the aquarium,
and the magnitude of this flux has been determined from DIC concentrations in
denitrification influent vs. effluent. The resulting flux of 26.3 kg C/month is
greater than either the observed carbon loss in the aquarium (6.9 kg/month) or the
amount of carbon needed to maintain steady state before denitrification (8.6
kg/month). The carbon stock in Ocean Tank is therefore expected to increase over
time, and during denitrification, no sodium bicarbonate has to be added to the
aquarium seawater. Thus, both carbon and nitrogen balances in Ocean Tank have
been fundamentally affected by the presence of a biological denitrification
environment.
ACKNOWLEDGEMENTS
The authors wish to thank Mark Kind and Frank Steslow of the New Jersey
State Aquarium, as well as David Vieira of Richard Stockton College for
discussions and valuable assistance during this study.
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Bulletin de l’Institut océanographique, Monaco, n° spécial 20, fascicule 1 (2001)
Bulletin de l’Institut océanographique, Monaco, n° spécial 20, fascicule 1 (2001)