Geoscience Reference
In-Depth Information
they are likely to contain waters that are corrosive
to CaCO
3
(at least along coastal areas with strong
oxygen minimum zones) earlier than many surface
communities (e.g. Feely
et al.
2008 ), with potentially
important impacts for calcii cation and survival.
Other chapters in this volume give a more thorough
discussion of benthic (Chapter 7) and sedimentary
( Chapter 9 ) habitats.
ment of corals can be affected by low-pH waters
(Cohen and Holcomb 2009). Reduced calcii cation
coupled with poor cementation can promote ero-
sion and reef l attening, which severely reduces the
structural heterogeneity of reefs and lowers its
potential to support biodiversity (Alvarez-Filip
et al.
2009). Reefs experiencing a loss of structural
complexity also experience a loss or change in i sh
assemblages, lower densities of commercially
important species, and lower rates of larval i sh
recruitment (Feary
et al.
2007 ). Weaker reef cemen-
tation also increases the potential for reef damage as
storm frequency and intensity increases with con-
tinued global warming, leading to further reef deg-
radation. Eventually, erosion of poorly cemented
(and non-accreting) forereef habitats will open
lagoon and backreef areas to erosion and provide
less effective shoreline protection for coastal
communities.
The effect of ocean acidii cation on the function of
entire communities has been addressed only rarely.
In a mesocosm-based study of a temperate rocky
shore community in the UK, Hale
et al.
(2011)
observed a reduction in the species diversity and
evenness of the faunal assemblage with reduced
pH, with a general ranking of vulnerability from
echinoderms (most sensitive), to molluscs, crusta-
ceans, and polychaetes (least sensitive). Nematode
abundance increased as pH was reduced, most
likely due to a release from biological disturbance
(predation and competition) rather than a direct
benei t from ocean acidii cation. The effects of ele-
vated temperature differed among pH treatments,
highlighting the difi culty of extrapolating from
studies involving single species or single environ-
mental factors, to predict the effects of future envi-
ronmental changes on natural communities.
Natural CO
2
venting sites offer perhaps the
strongest evidence of shifts in the biodiversity of
entire communities in response to high ocean CO
2
levels. Benthic communities along a persistent gra-
dient in pH and carbonate saturation near CO
2
vents in intertidal to shallow subtidal depths adja-
cent to the island of Ischia off the southern coast of
Italy show dramatic faunal and l oral patterns that
are apparently a result of differential tolerance to
ocean acidity (Hall-Spencer
et al.
2008 ). Continuous
seal oor venting of nearly pure CO
2
alters seawater
10.6.3.2 Coral reefs
Coral reefs are perhaps the ecosystems understood
the best in terms of potential impacts of ocean acidi-
i cation for marine biodiversity. Precipitation of
CaCO
3
by corals and calcifying algae forms the
physical structure and much of the habitat complex-
ity of coral reefs upon which additional biodiversity
develops. Tropical reef systems have also been
shown to be 'cradles of evolution', where more spe-
cies have originated during earth's history than any
other region (Kiessling
et al.
2010 ). Likewise, tropi-
cal reefs have been shown to disappear from the
fossil record during several mass extinctions (Veron
2008), indicating that they are also vulnerable to
environmental change (see Chapter 4).
Ocean acidii cation, coupled with ocean warm-
ing, can pose important risks for the biodiversity
and function of coral reef systems in several ways,
ranging from basic changes in the biomineraliza-
tion and erosion of the physical foundation of reefs,
to less understood changes in biological interac-
tions among species. Numerous studies have docu-
mented a reduction in calcii cation by corals and
coralline algae in response to ocean acidii cation
( Doney
et al.
2009 ). De'ath
et al.
( 2009 ) documented
a recent decline in calcii cation rates on the Great
Barrier Reef attributable to warming, ocean acidii -
cation, or both. In addition to reduced calcii cation
rates, the strength of cementation may also be
reduced in waters with a lower pH, promoting
higher rates of physical and bio-erosion (Manzello
et al.
2008). Processes other than calcii cation are
also affected by ocean acidii cation. Competition
between corals and other taxa, particularly non-
calcifying macroalgae, may also be mediated by
reduced rates of calcii cation or other physiological
processes linked to ocean acidii cation, which are
likely to reduce the ability of corals to compete for
space (Kuffner
et al.
2008 ). Reproduction and recruit-
