Geoscience Reference
In-Depth Information
by phytoplankton in the surface oceans (Broadgate
et al.
1997). Upon emission to the atmosphere, iso-
prene is oxidized to produce secondary aerosols that
contribute to particle growth and cloud droplet
number, affect the chemical composition of CCN,
and potentially exert a signii cant impact on climate
feedback mechanisms (Meskhidze and Nenes 2006).
The oceans are also a source of a number of long-
lived, radiatively active and oxidizing gases, includ-
ing methane (CH
4
) and nitrous oxide (N
2
O). CH
4
,
produced by methanogenic bacteria in anoxic sedi-
ments, contributes around 15% to current green-
house forcing ( IPCC 2007 ). N
2
O, a by-product of
microbial denitrii cation and nitrii cation, has 300
times the radiative forcing capacity of CO
2
and par-
ticipates in catalytic ozone removal (Law and Owens
1990). Clearly, changes in the source rate and sea-to-
air l ux of any of the above trace gases could have
signii cant implications for atmospheric chemistry
and climatic processes. The role that ocean acidii ca-
tion may play in such changes now requires further
investigation.
11.4.3
'Natural analogue' sites
Regions that experience naturally lowered seawater
pH may lend themselves as useful sites for study-
ing the long-term effects of ocean acidii cation on a
range of marine organisms and processes. Volcan-
ically acidii ed sites, such as sites around the island
of Ischia, Italy, have received some attention because
the high-CO
2
conditions have prevailed for rela-
tively long time periods (hundreds to thousands of
years), allowing long-term adaptation of marine
communities to the perturbed conditions (Hall-
Spencer
et al.
2008 ; see Chapter 10 ). The Ischia site
has served as a good natural analogue for studying
the effects of ocean acidii cation on the benthic com-
munities that inhabit the area (Hall-Spencer
et al.
2008). However, the site is considered less ideal for
studies of the effects of ocean acidii cation on plank-
tonic communities and their associated processes
due to rapidly l uctuating seawater pH over small
temporal and spatial scales, and rapid overturning
of seawater into and out of the site (Hopkins 2010).
Therefore other natural analogue sites need to be
identii ed, ideally in more open-ocean situations.
Oceanic upwelling regions that experience pro-
longed periods of undersaturation of CaCO
3
and
low seawater pH may have some potential in this
kind of research (Feely
et al.
2008; see Box 1.1 in
Chapter 1 ).
11.4.2 Mesocosm experiments
Mesocosm experiments are currently the best avail-
able tool for assessing the impacts of ocean acidii -
cation on pelagic ecosystems and their associated
processes, in large volumes of water (compared to
laboratory studies) and under quasi-natural mete-
orological and oceanic conditions (Riebesell
et al.
2008). The results of mesocosm studies are most rel-
evant in terms of highly productive regions (high-
latitude waters, coastal waters, bloom events, and
upwelling regions). Such regions are not only
expected to experience the greatest changes as a
result of anthropogenic ocean acidii cation and
other global climatic changes (Orr
et al.
2005 ; IPCC
2007 ; Feely
et al.
2008 ; Steinacher
et al.
2009 ; see also
Chapter 3), but also represent important source
regions of a number of climatically important trace
gases (Class and Ballschmiter 1988; Carpenter and
Liss 2000 ; Quack and Wallace 2003 ; Quack
et al.
2004 , 2007 ; Chuck
et al.
2005). For these reasons, the
continued use of mesocosm experiments is vital for
furthering our understanding of the impacts of
ocean acidii cation on the pelagic community, and
the associated trace gas production.
11.4.4
Regions sensitive to ocean acidii cation
A number of regions of the oceans have been identi-
i ed as being particularly sensitive to future ocean
acidii cation. High-latitude polar seas are particu-
larly vulnerable due to low seawater temperatures
and increased solubility of CO
2
. Such regions may
experience undersaturation with respect to arago-
nite as soon as 2023 (Steinacher
et al.
2009 ; see
Chapter 3). Similarly, upwelling regions are natu-
rally acidii ed by the inl ux of CO
2
-rich water from
depth, although anthropogenic CO
2
is now increas-
ing the extent of such acidii cation (Feely
et al.
2008 ).
Temperate coastal regions are expected to be
substantially impacted by ocean acidii cation,
which is likely to be enhanced by the deposition of
sulphur and nitrogen from human activities (Doney
et al.
2007). Such regions also experience high
