Geoscience Reference
In-Depth Information
oxygen, pH, and carbonate saturation (Miller
et al.
2009). Along the Californian coast (and eastern Pacii c
margin) the concentration of oxygen can approach
anoxia (less than 5% saturation, ~10 μmol kg
-1
) in the
core of the OMZ near a depth of 700 m. Wind-driven
coastal upwelling and other processes can transport
hypoxic, low-pH waters from the upper OMZ toward
the surface, creating conditions of undersaturation
with respect to CaCO
3
for surface organisms and
coastal benthic communities (Feely
et al.
2008 , 2010 ).
The highly variable conditions found in coastal
upwelling zones act as a physiological i lter, allowing
species to thrive only if all of their life stages can toler-
ate the variable local conditions. Natural environ-
mental variability has been shown to have large
effects on the structure and function of the California
Current ecosystem (e.g. Chavez
et al.
2003 ), which
may mask or interact with the effects of ocean acidii -
cation or other anthropogenic changes. Increasing
acidii cation of coastal environments through this
century due to elevated CO
2
emissions or other fac-
tors is likely to pose new physiological challenges for
coastal taxa.
The response of coastal species to future ocean
chemistry has been examined in various recent
studies indicating mixed, but generally negative,
responses (Kroeker
et al.
2010 ). As noted above,
some photosynthetic taxa may benei t from ocean
acidii cation (e.g. some seagrasses; Hall-Spencer
et al.
2008 ; Hendriks
et al.
2010 ). However, even
though they thrive in highly variable environments,
coastal heterotrophs have generally responded to
low-pH or low-carbonate-saturation waters with
impaired performance (e.g. acid-base balance, cal-
cii cation, growth, or survival), including adults
(e.g. barnacles, Findlay
et al.
2010 ; bivalves, Gazeau
et al.
2007 ; ophiuroids, Wood
et al.
2008 ; urchins,
Miles
et al.
2007) and early life-history phases (bar-
nacles, Findlay
et al.
2010 ; echinoderms, Dupont
et al.
2010 ; molluscs, Kurihara 2008 ). Not all coastal
fauna respond strongly or negatively to ocean acidi-
i cation. Adults of some higher taxa have shown lit-
tle sensitivity to acidii cation (e.g. Dungeness crab,
Pane and Barry 2007; cod, Melzner
et al.
2009a ), and
Ries
et al.
( 2009 ) observed mixed responses to ocean
acidii cation for juveniles of various taxa, including
enhanced calcii cation and growth for a lobster and
a shrimp.
Changes in the biodiversity and function of
coastal ecosystems due to ocean acidii cation are
likely to be linked to the vulnerability of key inter-
mediate prey taxa, with broader effects driven indi-
rectly through trophic dependences. Top predators
in these systems, including i shes, cephalopods,
birds, and mammals, are expected to have greater
physiological capacities (at least as adults) to cope
with elevated CO
2
levels, than many of their prey.
Large, active cephalopods, especially those inhabit-
ing suboxic habitats, may experience respiratory
problems with future acidii cation (e.g. Rosa and
Seibel 2008), but most vertebrates, especially air-
breathing birds and mammals, are not expected to
be affected directly by ocean acidii cation. However,
the indirect effects of ocean acidii cation for these
higher predators could be substantial. Impacts on
highly sensitive taxa (e.g. thecosomatous ptero-
pods) could cascade through food webs, as has been
observed in relation to other environmental changes
(warming). For example, the breeding success of
planktivorous and piscivorous seabirds along the
central California coast varies among warm and
cold periods, but is linked directly to the availabil-
ity of key prey taxa rather than specii c physical
parameters (Sydeman
et al.
2001 ). It remains
unknown whether the potentially negative effects
of ocean acidii cation on some groups (e.g. ptero-
pods) will be balanced by positive responses in
other groups with similar functional roles (e.g.
krill). In coastal systems with generally reduced
functional redundancy among species (Micheli and
Halpern 2005), the likelihood of indirect food web
effects due to ocean acidii cation and other environ-
mental changes may be high.
Benthic communities in coastal systems and sea-
bed habitats throughout the world's oceans are cen-
tres of biodiversity where much of the species
richness of the oceans is found. These communities
include a wide range of organisms that play key
roles as ecosystem engineers, providing habitat for
other taxa (e.g. corals, kelp, seagrasses, burrowing
taxa, and oyster beds), and play important roles in
elemental l uxes of carbon and nitrogen through
bioturbation and other effects of burrowing
(Widdicombe and Spicer 2008). Benthic systems in
coastal zones may be more vulnerable to ocean
acidii cation than some more offshore sites because
