Environmental Engineering Reference
In-Depth Information
landing stations in the river mouth or estuaries,
port/harbour structures and even on the trunk of
mangrove trees (Fig. 6.3 ).
Oysters have the potential of sequestering
carbon in their shells, which are made of calcium
carbonate. However, through the process of shell
generation, the oyster also produces some carbon
dioxide.
The potential for oysters to sequester carbon is
regulated by the process through which the hard
calcareous shell is formed. The chemical process
through which this shell is formed is called cal-
ci
2 Gt of carbon per year (Stewart 2005 ). As
atmospheric carbon dioxide production and
concentration increase, the amount of carbon
dioxide absorbed by the ocean also increases,
thus elevating the ocean
s carbon dioxide con-
tent. However, all the carbon dioxide does not
remain in this form in water. As the concentra-
tion of carbon dioxide in water increases, a series
of reactions occur in order to allow for more
carbon dioxide absorption and to balance the
different forms of carbon in the water. One of
these equilibrium reactions proceeds as follows:
'
cation. The process involves the formation of
calcium carbonate as per the equation:
CO 2 þ CO 3 2 þ H 2 O ! 2HCO 3
This shows how carbon dioxide combines
with carbonate and water to form two bicarbon-
ates after entering the water. Therefore, increased
amounts of carbon dioxide entering the ocean
due to increase in atmospheric carbon dioxide
would also cause an increase in bicarbonate
concentrations (Gattuso 2009 ). Using these two
chemical reactions, it is possible to evaluate the
role oyster shell formation that plays in the car-
bon cycle. Based on these equations, Fig. 6.5
depicts this process. The
Ca 2 þ þ 2HCO 3 ! CaCO 3 þ CO 2 þ H 2 O
The chemical reaction indicates how the
oyster converts a calcium ion and two bicar-
bonate molecules into one calcium carbonate,
one carbon dioxide molecule, and water. It may
seem that since there is a release of carbon
dioxide from this reaction, these bivalves are
actually producing carbon dioxide instead of
reducing the amount in the water column.
However, this is not the complete chemical
process. The source of the reactant carbon and
fate of the carbon products need to be investi-
gated in order to fully understand the overall
effect on the cycle.
The calcium carbonate deposited is accounted
for in the shell mass, and the carbon in the shell
may be locked up for millennia. Carbon makes
up 12 % of calcium carbonate by mass. Oysters
offer a medium to capture and store carbon for a
much longer time than agricultural products or
even forest. Thus, the main question in the pro-
cess is the source of bicarbonate and the fate of
the carbon dioxide. The simple answer to this
question is the ambient aquatic medium, but this
does not resolve the question of the oyster
gure illustrates how
the oyster uses calcium (Ca) and carbonate (CO 3 )
to form its shell and the rest of the reactants and
products are recycled. This illustration enables
the equation to be simpli
ed and written in
Fig. 6.5 .
It is now clear that increase in carbon dioxide
in the atmosphere leads to an increase in oceanic
carbon dioxide absorption. This has profound
effects on the seawater carbonate system.
It
lowers the aquatic pH (ocean acidi
cation), and
more importantly, it decreases the availability of
carbonate ions. Increases in carbon dioxide push
this equation to the right and use up carbonate in
the process, which reduces the amount that will
be available for calci
s role
in carbon sequestration. To get a complete
answer of the mechanism, one has to critically
analyse the carbon cycle within the oceanic
compartment.
The main source for oceanic carbon input is
from the atmosphere through absorption of car-
bon dioxide; this process currently absorbs about
'
cation. Therefore,
the
process of calci
cation actually promotes ocean
acidi
potential for
carbon uptake by locking up carbonate. How-
ever, few studies (Gazeau et al. 2007 ) have
shown that oysters are quite resilient in spite of
the changes in pH. It is also known that marine
animals are, in general, intolerant to changes in
cation and reduces the oceans
'
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