Geoscience Reference
In-Depth Information
will become subject to net dissolution at an approxi-
mate seawater p CO 2 close to 1000 μatm. This value
is surprisingly close to the daily average seawater
p CO 2 observed in replicated l ow-through meso-
cosms containing subtropical calcifying communi-
ties that were marginally subject to net dissolution
( Andersson et al . 2009). Regardless of the timing of
this transition in nature, it represents a reversal of a
major geochemical process that has been in effect
for thousands of years. It is also important to make
the distinction and to realize that coral reef ecosys-
tems will become subject to a condition of net dis-
solution well before individual live coral colonies
are likely to experience this condition.
Manzello et al . ( 2008 ) proposed that the coral reef
ecosystems of the eastern tropical Pacii c, which
naturally experience high CO 2 , low pH, and low Ω a
as a result of upwelling, could serve as an example
of how coral reefs will be affected in a high-CO 2
world. As previously discussed, Manzello et al .
(2008) showed that reef cementation is limited in
this region and the reported rates of bioerosion are
high. Furthermore, in general, CaCO 3 accumula-
tions of the reefs in this region are thin deposits
relative to those of Indo-Pacii c and Caribbean reefs,
small in areal extent and limited to shallow depths.
According to the current experimental results using
individual organisms exposed to elevated CO 2 condi-
tions (Table 7.1), it appears likely that benthic marine
ecosystems will undergo a gradual transition in the
relative composition (and success) of calcifying and
non-calcifying organisms, favouring the latter group
of organisms. This transition may be specii cally
noticeable on coral reefs, where a combination of
changes in community structure with decreasing,
and perhaps even negative, reef accretion could result
in a drastically different ecosystem (e.g. Hoegh-
Guldberg et al . 2007). Similar changes may occur else-
where because calcifying organisms are ubiquitous
components of almost all marine ecosystems (Chave
1967). Observations from a region in the Mediterranean
Sea subject to a natural gradient in seawater pH and
CO 2 as a result of volcanic vent activity show con-
vincing evidence of a gradual decrease in the abun-
dance of marine calcii ers including sea urchins,
coralline algae, gastropods, limpets, and barnacles as
a function of decreasing seawater pH (e.g. Hall-
Spencer et al . 2008 ; Martin et al . 2008 ). Some gastro-
pod shells observed near the vent site even showed
direct evidence of dissolution. In contrast, non-calci-
fying algae and seagrasses thrived and l ourished
under the high-CO 2 , low-pH conditions observed
near the vent site (see also Chapter 10).
Feely et al . ( 2009) recently described how upwelling
along the California Shelf occasionally exposes the
benthic systems on the continental shelf to under-
saturated and corrosive seawater with respect to
aragonite. As the upwelling phenomenon is not a
new occurrence, the benthic ecosystems on the
California Shelf may provide important information
regarding an ecosystem subject to high CO 2 and low
pH. However, the frequency, extent, and duration of
these upwelling events are currently poorly known.
Smith ( 1971 ) carefully quantii ed the carbonate
budget on the southern California continental shelf.
Despite high abundance and production of CaCO 3
by marine benthic calcii ers present on hard-bottom
portions of the shelf as well as encrusting kelps and
other plants, negligible quantities of carbonate mate-
rial were found to accumulate within this region.
Smith (1971) suggested that the material was trans-
ported to adjacent basins and subsequently dis-
solved. Upwelling of corrosive seawater would
greatly accelerate this process and may provide an
explanation (in addition to export) of why only neg-
ligible amounts of CaCO 3 relative to its production
are found on the California Shelf.
7.4
Conclusions and i nal remarks
As the carbonate chemistry of the oceans changes as
a result of anthropogenic CO 2 emissions, benthic
processes, organisms, communities, and ecosystems
will most certainly undergo changes rel ecting this
major change in ocean chemistry. Many benthic eco-
systems already experience high seawater CO 2 , low
pH, and a low saturation state with respect to CaCO 3
minerals and may serve as real-world examples of
how other systems will be affected in a high-CO 2
world. Based on observations and trends from these
systems combined with current experimental results,
it is likely that marine benthic ecosystems will change
in a predictable sense, although the rate and extent of
change are more difi cult to predict. Certainly, there
could also be unexpected consequences that our cur-
rent knowledge fails to recognize. Nevertheless,
 
 
 
Search WWH ::




Custom Search