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sensitivity of various components of the commu-
nity to the CO
2
perturbation, may have produced
some of the variations in response of DMS and
DMSP concentrations that were observed between
the three experiments (Fig. 11.3). The 2003 and 2006
experiments gave similar results, with signii cantly
lower DMS and DMSP under high CO
2
. Avgoustidi
(2007) reported a 59% decrease in DMS concentra-
tions in the high-CO
2
mesocosm M1 relative to the
present-day CO
2
mesocosm M4 (see Fig. 11.3).
Similarly, Hopkins
et al.
( 2010 ) observed a highly
signii cant 46% decrease in mean DMS concentra-
tions under high CO
2
, a result that was strongly rep-
licated in all three high-CO
2
mesocosms (see Fig.
11.3). By contrast, during the 2005 experiment small
increases in production of DMS under high CO
2
were reported, although the differences were not
signii cant (Vogt
et al.
2008 ). However, the temporal
development of DMS did display small but statisti-
cally signii cant differences between CO
2
treat-
ments. Vogt
et al
. (2008) concluded that as there
were no signii cant differences in species composi-
tion or succession between treatments, the small
observed differences in DMS concentrations were
most likely due to differences in bacterial or viral
activity. The observed signii cant differences in
plankton communities seen during the 2003 and
2006 experiments are likely to explain some of the
observed differences in DMS/P.
During the 2006 study, the DMSP concentration
was less signii cantly affected by high CO
2
than the
DMS concentration, suggesting that the impact was
greatest on the conversion of DMSP to DMS rather
than on the initial production of DMSP. This impli-
cates an effect on secondary factors, such as graz-
ing, viral lysis, and bacterial metabolism of DMSP.
In addition, the signii cant decreases in DMS can
also be directly attributable to signii cant changes in
ecosystem composition. For all of the studies, a
direct impact on DMS concentrations may be the
result of bacterial consumption of DMS (Kiene and
Bates 1990), a process that may have been either
enhanced (2003 and 2006) or diminished (2005)
under high CO
2
.
One study has investigated the combined impacts
of rising ocean acidity and increasing temperatures
on DMSP production (Lee
et al.
2009 ). During ship-
board experiments, North Atlantic plankton com-
munities incubated in 'greenhouse' conditions (690
μatm CO
2
, +4°C) exhibited a signii cant increase in
the production of particulate DMSP compared with
control conditions (390 ppmv CO
2
, ambient tem-
perature). However, the dissolved DMSP fraction
decreased under high CO
2
, a result of decreased
grazing pressure.
Despite some conl icting results, assimilation of
the currently available data suggests that a negative
impact of ocean acidii cation on net DMS and/or
DMSP production is likely. Avgoustidi (2007) per-
formed
in vitro
experiments on natural seawater
assemblages and monospecii c cultures of
E. hux-
leyi
, and again observed a reduction in DMS con-
centrations under elevated CO
2
. This supports the
i ndings of the 2003 and 2006 mesocosm experi-
ments ( Avgoustidi 2007 ; Hopkins
et al
. 2010 ), and
suggests that a decrease in net DMS and DMSP pro-
duction is possible in a future high-CO
2
world. To
further elucidate the impacts of ocean acidii cation
on DMS production additional investigation is
required, perhaps in the form of further mesocosm
experiments or incubations of natural assemblages
from various oceanic regions.
11.2.2
Atmospheric and climatic implications
Although clearly not applicable to the entire global
ocean, mesocosm studies are arguably representa-
tive of highly productive regions such as high-
latitude waters, coastal waters, and upwellings,
with such areas expected to be rapidly, and in some
cases dramatically, affected by ocean acidii cation
( Orr
et al.
2005 ; IPCC 2007 ; Feely
et al.
2008 ;
Steinacher
et al.
2009 ; see also Chapter 3 ). Therefore,
it is pertinent to assess the impact that changes in
seawater concentrations of DMS may have on the
climate-regulating properties of DMS and its atmos-
pheric oxidation products. It is important to note
that, as yet, no modelling studies have considered
the impact of ocean acidii cation on marine DMS
emissions (see Chapter 12 ).
By inl uencing both the rel ection and absorption
of solar radiation through a variety of complex radi-
ative and microphysical processes, atmospheric
aerosols are able to directly exert a strong inl uence
on the earth's radiative budget (Andreae and
Crutzen 1997 ; Ramanathan
et al.
2001). Aerosols also
