Geoscience Reference
In-Depth Information
Abyssal sedimentary habitats are not immune to
the potential effects of ocean acidii cation.
Echinoderms, including a diverse assemblage of
ophiuroids, echinoids, and holothurians, commonly
form a dominant guild of abyssal benthic inverte-
brates, along with decapod crustaceans and i shes.
The weakly calcii ed tests of deep-sea urchins sug-
gest that calcii cation is either unimportant as a pro-
tection against predators, or is energetically costly,
or both. Some taxa, such as
Tromikosoma
sp. in the
North Pacii c, have little or no carbonate in their
test, which is proteinaceous. As anthropogenic CO
2
penetrates to the abyss in the future, seawater will
become corrosive to aragonite and calcite, presum-
ably making it even more difi cult for many echi-
noids and other carbonate-bearing taxa to form
their skeletons. The absence of echinoderms from
areas in the Okinawa Trough exposed continuously
to high-CO
2
vent l uids (A. Boetius, pers. comm.),
suggests that ocean acidii cation could act selec-
tively against this often dominant abyssal phylum.
Weaker calcii cation under more acidic conditions
could affect the survival of a variety of taxa. Mussels
(
Bathymodiolus brevior
) inhabiting low-pH hydro-
thermal vent systems in the western Pacii c survive
and grow, but have poorly calcii ed shells, making
them more vulnerable to predation by decapod
crabs (
Paralomus
sp.) than are conspecii cs with
thickly calcii ed shells that inhabit less corrosive
sites (Tunnicliffe
et al.
2009 ).
The strong link between communities at the sur-
face and in the deep sea suggests that changes in
biodiversity in the upper ocean due to ocean acidi-
i cation could initiate shifts in biodiversity and eco-
system function in the deep sea. Changes in the
export of organic debris from surface waters due to
ocean acidii cation, perhaps in combination with
other environmental changes, could affect bathype-
lagic, abyssal, and benthic ecosystems in the deep
sea. Recycling of organic material in the upper
water column may increase due to increased disso-
lution of coccolithophores and foraminifera, lead-
ing to a reduction in carbonate ballast within organic
aggregates and reduced export of organic carbon to
deep waters. These potential effects of ocean acidii -
cation on the rate of carbonate rain and the biologi-
cal pump are not yet well understood (see Chapter
6). For the food-limited deep sea, however, changes
in sinking organic l ux, in addition to altered pH
and carbonate saturation, may drive important
changes in ecosystem function.
10.6.3 Coastal ecosystems
Coastal ecosystems, including coastal upwelling
zones, coral reefs, mangroves, kelp forests, seagrass
beds, estuaries, and other nearshore systems, are by
far the most important ecosystems that humans
depend upon for i ni sh and shelli sh i sheries and
aquaculture, as well as recreation, and thus are criti-
cally important with respect to future impacts from
ocean acidii cation and other environmental
changes (Cooley
et al.
2009). Coastal systems span a
wide range of physical and oceanographic regimes
from high to low latitudes, upwelling systems to
western boundary currents, and both benthic and
pelagic assemblages. The seawater chemistry and
biological processes in these disparate environ-
ments vary greatly, and thus their sensitivity to
ocean acidii cation is also expected to vary.
Anthropogenic changes in oceanographic and eco-
logical processes in coastal systems related to fossil
fuel emissions and other human activities (e.g.
coastal nutrient loading) are not fully understood
(e.g. Feely
et al.
2010), but will probably also differ
among ecosystems. Considering the diversity of
coastal ecosystems, it is beyond the scope of this
chapter to provide a comprehensive treatment of
their vulnerability to ocean acidii cation. Instead,
we touch on several features of ocean acidii cation
in coastal systems, using upwelling systems and
coral reefs as examples.
10.6.3.1 Upwelling zones
Upwelling zones off the western US coast, along
most eastern boundary currents, and several other
regions worldwide typically have a wider range in
oxygen, pH, and other carbonate system parame-
ters than most open-ocean systems, due largely to
the boom and bust productivity of surface waters
and remineralization of organic material at depth.
Surface pH and carbonate saturation vary through
these cycles, and waters in the oxygen minimum
zone (OMZ) several hundred metres below the sur-
face can be suboxic and corrosive to CaCO
3
.
Estuaries can also have quite strong gradients in
