Geoscience Reference
In-Depth Information
acidii cation. Carbonate skeletal structures of
marine taxa vary considerably both in terms of how
much calcium carbonate (CaCO
3
) is included, from
nearly 100% CaCO
3
to mixtures of chitin and CaCO
3
(e.g. many crustacean shells). The form of CaCO
3
also varies, with most taxa precipitating aragonite
or calcite, the latter which may include some per-
centage of magnesium. Of these, high-Mg calcite is
the most soluble in seawater, and thus most suscep-
tible to dissolution by ocean acidii cation, followed
by aragonite and calcite (see Box 1.1 in Chapter 1).
Although calcii cation rates by organisms are
generally impaired under low-pH conditions, there
is considerable variation among the responses of
major taxonomic groups (Hendriks
et al.
2010 ;
Kroeker
et al.
2010 ). Scleractinian corals (aragonite)
exhibit the largest reduction in calcii cation and
most consistent response to low-pH waters.
Coccolithophores (calcite) and molluscs (mostly
calcitic) had somewhat weaker, variable, and non-
signii cant changes in calcii cation. Individual stud-
ies have reported generally reduced rates of
calcii cation for bivalve and gastropod molluscs
under high-CO
2
conditions (Gazeau
et al.
2007 ;
Doney
et al.
2009 ; Ries
et al.
2009 ; Hendriks
et al.
2010). In contrast, echinoderms (calcite) are highly
variable in response, mainly due to the great varia-
bility in degree of calcii cation within the phylum
(e.g. Wood
et al.
2008 ). Crustaceans (chitin, calcite,
amorphous carbonate) are the single group show-
ing a signii cant increase in calcii cation rate under
high-CO
2
conditions (Kroeker
et al
. 2010 ).
Reef-building corals in particular, due to their
aragonitic skeletons, are perceived to be at high
risk from ocean acidii cation, based on the pro-
jected future reduction in aragonite saturation
throughout the world's oceans (Kleypas
et al.
1999 ).
These projections are consistent with the existing
global distribution of deep-sea aragonitic corals,
which are most abundant in the Atlantic and rela-
tively rare in habitats with low aragonite satura-
tion, such as the Pacii c Basin (Guinotte
et al.
2006 ;
Manzello 2010). Surprisingly, some corals may sur-
vive acidic conditions without carbonate skeletons.
Two Mediterranean species (
Oculina patagonica
and
Madracis pharencis
) survived a 12-month exposure
to acidic (pH
T
= 7.3-7.6) waters but lost their car-
bonate skeletons, which dissolved in the corrosive
waters (Fine and Tchernov 2007). Upon immersion
in ambient pH waters (pH
T
= 8.3) the corals recalci-
i ed. This type of recovery is unlikely for many
other coral taxa with modes of life linked strongly
to their structural framework.
Calcii ed shells and skeletons can play important
roles for organisms coping with environmental var-
iability. In some cases, more robust calcii cation may
increase the survival (and presumably the i tness)
of organisms, which can affect the biodiversity and
function of marine communities. For example, the
intertidal snail
Littorina littorea
thickens its shell
after exposure to chemical cues produced by its
main predator, the green shore crab,
Carcinus mae-
nas
, in effect increasing its defence against preda-
tion (Bibby
et al.
2007). Such shell thickening does
not occur under high-CO
2
conditions, presumably
due to the increased energetic cost of calcii cation at
lower-pH, less saturated conditions, thereby
increasing their risk of predation. Very few studies
have examined the effects of ocean acidii cation on
behavioural responses that mediate interactions
between interacting species and populations.
10.6 Habitats
The risk of changes in the biodiversity and function
of marine ecosystems due to ocean acidii cation is
likely to vary considerably among habitats. Seawater
carbonate chemistry is affected by temperature and
biological processes, leading to signii cant patterns
in carbonate chemistry across zonal and meridional
gradients (Kleypas
et al.
1999 ; Feely
et al.
2004 ), with
depth, and in relation to biological productivity.
Colder high-latitude regions have naturally lower
saturation states for aragonite and calcite due to the
higher solubility of CO
2
at low temperatures, and
will be the i rst surface waters to be persistently
undersaturated with respect to aragonite (Orr
et al.
2005 ; see Chapter 2 ), with potentially signii cant
effects on marine calcii ers. The inl uence of ocean
acidii cation on sediment ecosystems is considered
in Chapter 9 .
10.6.1 Upper ocean
Changes in environmental conditions through this
century, including ocean acidii cation and warming,
