Geoscience Reference
In-Depth Information
exists in very low concentrations in seawater (~10
to 15 μmol kg
-1
at current typical surface-seawater
conditions), marine organisms have mechanisms to
catalyse the uptake of CO
2
from this relatively scarce
source of carbon, including enzymatic activity (e.g.
carbonate anhydrase) and the ability to utilize the
bicarbonate ion (HCO
3
-
) as a source of carbon for
photosynthesis ( Raven 1997 ). HCO
3
-
is approxi-
mately two orders of magnitude more abundant
than CO
2
(aq) (CO
2
(aq) includes dissolved CO
2
and
H
2
CO
3
in the approximate ratio of 400:1), but dehy-
drating HCO
3
algae and corals show complex and species-specii c
responses with variable results. Anthony
et al
. ( 2008 )
reported a decrease in net productivity of a coral-
line alga as a function of increasing CO
2
and decreas-
ing pH. A number of experiments conducted with
corals have reported increased zooxanthellae den-
sity and/or chlorophyll
a
content with increasing
CO
2
(e.g. Reynaud
et al
. 2003 ; Crawley
et al
. 2009 ),
but both gross photosynthesis and respiration
showed variable responses with no clear trend in
terms of the resulting net production. Schneider
and Erez (2006) observed no effect on the net pro-
ductivity in the coral
Acropora eurystoma
as a func-
tion of CO
2
.
Increased benthic primary production could act as
a small negative feedback to rising atmospheric CO
2
and ocean acidii cation if the organic material pro-
duced were exported to the deep sea or permanently
buried within the sediments. However, this assumes
that decomposition and remineralization of organic
material remain unchanged or that any increase in
these processes is smaller relative to the amount of
carbon buried in the sediments; that is, gross produc-
tivity exceeds gross respiration (net ecosystem pro-
duction is positive). The magnitude of this negative
feedback in the early 21st century is of the order of
0.1 Gt C yr
-1
( Mackenzie and Lerman 2006 ).
-
to CO
2
has a high energetic cost.
Regardless of the source of carbon for photosynthe-
sis, ocean acidii cation results in increasing concen-
trations of both of these dissolved carbon species,
although the relative increase in CO
2
is much greater
than the relative increase in HCO
3
-
(see Chapter 1 ).
However, other constituents such as nitrogen, phos-
phorus, and iron may limit photosynthesis, so
increasing CO
2
does not automatically result in
increased production of organic matter. Although
studies are limited, some experimental results sug-
gest that photosynthesis as well as net primary pro-
duction (= gross primary production - respiration)
for certain benthic autotrophs will increase in a
high-CO
2
world. For example, seagrasses, which
appear limited by the availability of CO
2
, respond
positively to conditions of increasing seawater CO
2
( Table 7.1 ; Palacios and Zimmerman 2007 ; Hall-
Spencer
et al
. 2008). Seagrasses might also benei t
from a reduction in calcareous epibiont organisms
that currently foul their blades and decrease their
photosynthetic area. On the other hand, calcii ca-
tion by the same epibionts could benei t seagrasses
by supplying CO
2
that could be used for photosyn-
thesis (Barrón
et al
. 2006 ). Similar to seagrasses,
experiments conducted with non-calcifying algae
have shown increased production and growth in
response to elevated CO
2
conditions. For example,
primary production was observed to increase in an
Arctic specimen of the brown alga
Laminaria saccha-
rina
exposed to elevated CO
2
(S. Martin, pers.
comm.), and the relative growth rate of the red sea-
weed
Lomentaria articulata
was observed to increase
in response to the CO
2
levels anticipated by the end
of this century (Kübler
et al
. 1999 ). Experiments
investigating the effect of elevated CO
2
on photo-
synthesis and/or carbon production of calcifying
7.2.2
Oxidation of organic matter
Living organisms break down organic material in
order to extract energy, typically using oxygen as a
terminal electron acceptor and releasing CO
2
as a
waste product. Under suboxic or anoxic conditions,
many microbes can use alternative electron accep-
tors such as NO
3
-
, SO
4
2-
, CH
4
, and CO
2
(see Chapter
9). The decomposition of organic material causes
acidii cation of seawater. This natural acidii cation
is evident from the general trends of increasing dis-
solved inorganic carbon and decreasing pH as a
function of depth in the oceans (see Chapter 3). The
same trends are also observed as a function of age
of the water mass, i.e. the time since the water mass
was last in contact with the atmosphere. For exam-
ple, bottom waters in the Pacii c are more acidic
than Atlantic bottom waters, which have been in
contact with the atmosphere more recently than
waters in the Pacii c. The acidii cation of seawater
