OCEAN ACIDIFICATION
SMERA MARIA POULOSE
SCIENTIST, VSSC, TRIVENDRUM
Email “ p_smeramaria[at]vssc.gov.in
Significance of Ocean Acidification
Bioavailability of carbonate
Many marine organisms make shells or supporting
plates out of calcium carbonate (CaCO
3
) in a
process called calcification. As water becomes
more acidic, the calcification process is inhibited
and the growth and/or survival of certain
organisms could be affected. As many of these
organisms form the primary production of oceans,
any change in their life cycle has the potential to
impact all marine ecosystems.
The Southern Ocean has been identified as being
particularly
vulnerable
to
becoming
under
saturated in calcium carbonate as it already has
very low saturation levels [3]. The saturation level
of the carbonate minerals is not only dependent
on the concentrations of dissolved CO
2
and
carbonate but also with water temperature and
pressure. The solubility of CaCO
3
increases with
decreasing temperature and increasing depth.
However,
with increasing
dissolved
CO
2
concentrations the depth at which the CaCO
3
minerals will become under saturated will rise, i.e.
the depth at which CaCO
3
minerals will begin to
dissolve (particularly the more soluble mineral
form- Aragonite) will become shallower.
This saturation depth is predicted to reach the
surface in some areas of the Southern Ocean if
CO
2
levels rise to twice their current levels, which
could have a significant impact on the marine food
webs on Australiaâ„¢s southern coastline.
Speciation of Nutrients
The speciation, or the ionic form, of compounds is
dependent on a number of factors such as their
concentration, the presence or absence of other
ions and pH. For example, in seawater phosphate
can be present as PO
4
3-
, HP0
4
4
2-
, H
2
PO
4
- and
H
3
3PO
4
depending on what the pH of the solution
is.
Introduction
Acidification is defined as an increase in the
concentration of H + in a solution or a lowering of
a solution's pH. Ocean acidification is therefore
the reduction of the pH of the world's oceans.
This can occur when CO
2
dissolves in water and
there is a reaction between the H
2
O andCO
2
to
form carbonic acid (H
2
CO
3
).
[CO
2
] + [H
2
O] <=> [H
2
CO
3
]
This weak acid readily releases a proton (H
+
) and
a negatively charged inorganic carbon ion.
[H
2
CO
3
] <=> [H
+
] + [HCO
3
-]
The release of the H
+
into the water will make it
more acidic, that is it will drive the pH down. This
increase in H
+
will also react with the carbonate
ion (CO
3
2-
) to form HCO
3
-
[H
+
] + [CO
3
2-
] <=> [HCO
3
-]
The overall effect of CO
2
dissolving into water is
that the concentrations of H
+
, H
2
CO
3
and HCO
3
-
increase and the concentration of CO
3
2-
decreases
and the solution is more acidic (i.e. a decrease in
pH. The world's oceans readily exchange CO
2
with the atmosphere. As the concentration of CO
2
in the Earths atmosphere increases, so to does
the level of CO
2
that the oceans absorb and
therefore increasing the concentrations of H
+
in
the ocean making them more acidic.
What is causing ocean acidification?
Carbon dioxide obeys Henry's Law, which states
that the concentration of a dissolved gas in a
solution is directly proportional to the partial
pressure of that gas above the solution. An
increase in the concentration of CO
2
in the
atmosphere directly leads to an increase in the
amounts of CO
2
absorbed by the oceans. Human
activity, mostly the burning of fossil fuels and the
production of cement, has lead to an increase in
average atmospheric CO
2
levels from pre-
industrial values of 280 parts per million (ppm) to
about 380ppm today.Page 68

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 59
Figure 1. Changes in speciation of phosphate, silicate and ammonia with pH.
The red box shows the range that pH is predicted to change within.
Theoretical speciation diagrams predict that the
speciation of nutrients such as phosphate,
silicate, iron and ammonia would all be impacted
within the range of pH decreases predicted [6].
For example as pH decreases ammonia (NH
3
)
concentrations would be lowered in preference to
the ammonium species (NH
4
+
).
The speciation of ions could affect their
bioavailability. For example the ratio of soluble to
insoluble iron may be increase making iron more
available and reducing the growth limiting effect
that low soluble iron concentrations have in some
areas.
The low availability of soluble iron has been
shown to limit the growth of phytoplankton in the
Southern Ocean and by artificially increasing
soluble iron concentrations there is an increase in
photosynthetic
activity
and
phytoplankton
biomass.
Changes to biodiversity
Some species will be better suited to higher
CO
2
and lower pH and a shift from one set of
dominant species to another could have a huge
impact on the entire ecosystems. For example
animals such as deep sea fish and cephalopods
are particularly sensitive to external CO
2
. Squid
are seen as sensitive due to their energy
intensive form of movement. This extremely
active use of muscles requires a large supply of
oxygen. However, the capacity of blood to carry
oxygen can be reduced by high CO
2
levels. At
this stage it is unclear what impacts the levels
of CO
2
predicted for the next 100 years would
have on the life cycles of multicellular
organisms.Page 69

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 60
Measurement and Interpretation
Predicting Atmospheric CO
2
While the absorption of CO
2
by seawater is
defined by Henry's Law, there is no such certainty
about the future levels of CO
2
in the atmosphere.
The projections of CO
2
emissions are based on a
number of scenarios which outline future patterns
of economic growth, fossil fuel dependence and
technology development. These projections
forecast CO
2
reaching levels of between 550 ppm
to 850-970 ppm by the year 2100. The lower
value assumes a strong uptake of clean and
efficient technology across the globe with the
higher value resulting from a continued and
increased worldwide dependence on fossil fuels.
The
rate
at
which
atmospheric
CO
2
concentrations increase will dependent on
economic development, technology advancement
and societal pressures along with several
environmental processes.
An example of an environmental process in the
carbon cycle is calcification, which releases CO
2
.
2[HCO
3
3
-
] + Ca
2+
<=> [CaCO
3
] + [H
2
O] + [CO
2
]
It is unclear how acidification, and any associated
restriction of calcification, might be offset by the
decrease in CO
2
released by calcification.
Zondervan et al. [10] estimate that with increasing
atmospheric CO
2
, CO
2
emitted by the oceans
could increase from 0.63 Gt of carbon per year in
1850 to 0.85 Gt per year by 2150. However, they
assumed a constant rate of calcification which
would not be the case if calcification was inhibited
by increasing acidity. Also with decrease in the
amount of
CO
2
released during carbonate
production, the oceans would have lower
dissolved CO
2
concentrations and be able to
absorb more CO
2
from the atmosphere.
Carbonate Buffering
As CO
2
levels in the atmosphere increase and
there is an increase in H
+
and a decrease in CO
3
-2
the depth below the surface of the ocean at which
CaCO
3
becomes supersaturated (below which
CaCO
3
will become under saturated and therefore
dissolves) will become shallower. This would
result in some currently mineralized CaCO
3
deposits dissolving and releasing carbonate
ions.
[CaCO
3
] + [H
2
CO
3
] <=> [Ca
2+
] + 2[HCO
3
-
]
Note that this reaction would utilise the carbonic
acid resulting from increased CO
2
absorption
and decrease the level of acidification
produced. This
natural
process,
called
buffering, acts to continuously stabilise the pH
of seawater. The work that has been done to
model the extent to which ocean acidification
will affect the carbonate saturation depth, and
how changes to that depth will modify the
capacity of seawater to buffer any increases in
H
+
concentrations, suggests that that the
process will be very slow and not fast enough to
offset the rapid acidification from CO
2
absorption.
Temperature and Circulation Effects
Temperature and pressure (water depth) both
have an impact on the solubility of CO
2
and
CaCO
3
in seawater. Sea surface temperatures
(SST) also impact on ocean circulation and
mixing which will affect the rate and extent to
which any changes to seawater pH will occur. It
is unclear at this time how any increases in SST
and or changes to oceanic circulation
associated with climate change might enhance
or ease the overall ocean acidification effect of
increased CO
2
in the earth's atmosphere.
Reference:
1. Caldeira, K.; Wickett, M.E. (2003).
"Anthropogenic carbon and ocean pH
2. Raven, J. A. et al. (2005). Ocean
acidification due to increasing atmospheric
carbon dioxide. Royal Society, London, UK.
3. Raven, J.A.; Falkowski, P.G. (1999).
"Oceanic sinks for atmospheric C