Environmental Engineering Reference
In-Depth Information
Ocean acidifi cation is a consequence of the incorporation of high
amounts of atmospheric CO
2
to the seawater through physical processes
such as waves and rainfall. Once the CO
2
is absorbed by the seawater, it
becomes carbonic acid (HCO
3
H), a weak acid which results in an increase
of H
+
and HCO
3
- and an increase of dissolved CO
2
, leading to a reduction
of carbonate ions (CO
3
2-
) and a consequent change in pH. The ocean surface
is slightly alkaline with pH values of approximately 8.2, however estimates
indicate that if CO
2
emissions to the atmosphere continue towards the end
of this century, the pH of seawater is expected to drop others from 0.3 to 0.4
units, resulting in an increase of 150% in the H
+
ions and a corresponding
increase in the availability of CO
2
from 300% to 400% (Arnold et al. 2012).
The pH is a critical variable in marine systems. It is known that small
changes in pH can have a deep impact on the dynamics of chemical
compounds, nutrients and trace metals in oceans. Because the incorporation
of high levels of CO
2
in the oceans directly affects the pH of the seawater,
the fi nal impact of acidifi cation on marine biological systems is extremely
high. Many organisms are sensitive to changes in carbonate chemistry and
their responses to these changes can lead to profound ecological changes in
marine ecosystems (Doney et al. 2009, Rilov and Trebes 2010).
Seaweed responses to ocean acidifi cation
Seaweeds play a fundamental role in carbon cycle of coastal ecosystems
(Reiskind et al. 1989). Their response to changes in seawater CO
2
can be very
different for each species; this is because different species have different
strategies to obtain the carbon needed for photosynthesis (Kroeker et al.
2010, Zou and Gao 2010). Several studies on the potential effect of seawater
acidifi cation on seaweeds have shown highly variable responses, including
positive, negative or neutral responses (Hall-Spencer et al. 2008, Wu et al.
2008, Roleda and Hurd 2012). Understanding the sensitivity of seaweed
to ocean acidifi cation is important to understand the impacts that this
phenomenon may have on coastal ecosystems.
The consequences of ocean acidifi cation on seaweed-based coastal
ecosystems range from organism to community levels. On one hand, this
process may be able to infl uence physiological processes of seaweeds. On
the other hand, ocean acidifi cation may infl uence the process of tissue
calcifi cation in those species that use calcium carbonate to synthesize
structures. Changes in any of these metabolic processes result in a direct
impact on seaweed growth.
Photosynthesis and growth in the context of seawater acidifi cation
CO
2
is the substrate for the process of photosynthesis, thus it is expected
that an increase in its availability as a result of ocean acidifi cation would
