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In-Depth Information
organic matter (DOM) and hypoiodous acid/molecu-
lar iodine (Martino
et al.
2009 ).
The production of organohalogens in surface sea-
water is balanced by a number of loss processes,
including hydrolysis and nucleophilic attack
( Moelwyn-Hughes 1938 ; Elliott and Rowland 1993 ),
photochemical loss (Jones and Carpenter 2005;
Martino
et al.
2005), and uptake by bacteria (Goodwin
et al.
1997 , 1998 ; King and Saltzman 1997 ). The
remainder of the volatile organohalogen pool under-
goes exchange to the atmosphere. Here it is subject to
photolysis and oxidation to produce highly reactive
halogen radicals and aerosols, which are involved in
a number of atmospheric and climatic processes (Fig.
11.2). Similarly to DMS-derived aerosols, iodine
oxide (IO) radicals are aerosols which can indirectly
inl uence climate through involvement with CCN
and cloud formation, and thereby impact on the
rel ection of solar radiation (Andreae and Crutzen
1997 ). Furthermore, organohalogen-derived free
radicals (I, IO, Br, and BrO) act as effective catalytic
ozone-depleting species (Chameides and Davis 1980;
Solomon
et al.
1994 ; Davis
et al.
1996 ), with a poten-
tially signii cant inl uence on the atmosphere's oxi-
dative capacity, global radiative forcing, and air
quality. Finally, the sea-to-air l ux of iodine repre-
sents a crucial step in its biogeochemical cycling—an
element essential to the health of terrestrial organ-
isms, including humans (Fuge and Johnson 1986).
were suspended in the fjord from a moored l oating
raft (volume in 2003 and 2005: 20-25m
3
; 11m
3
in
2006). Each enclosure was i lled with nutrient-poor
uni ltered fjord water, and the tops of the meso-
cosms were covered with tetral uoroethylene i lms
in order to form a tent covering more than 90% of
the surface area of the mesocosm. Seawater
p
CO
2
and pH were initially set to target values by aerat-
ing the water with CO
2
/air mixtures. Nutrients
were added in order to stimulate phytoplankton
blooms, and the bloom was monitored over the
course of 20-23 days. In 2003, three treatments were
used: 180 μatm (glacial), 370 μatm (present), and
700 μatm, and the 2005 experiment again compared
three
p
CO
2
levels: 375 μatm (present), 750 μatm
(future), and 1150 μatm (far future). In 2006, two
CO
2
treatments were used: 380 μatm (present) and
750 μatm (future). In order to encourage diatom
blooms, the 2003 experiment received an addition
of silicate, as well as nitrate and phosphate, on day
0 and again on day 7, whereas the 2005 and 2006
experiments received only nitrate and phosphate
on day 0. The concentration of chlorophyll
a
varied
between experiments; relatively low concentrations
were experienced in 2003 with a maximum of 4.6
mg m
-3
, whilst a maximum of 15.0 mg m
-3
was
recorded in 2005. There were no signii cant differ-
ences in chlorophyll
a
between these treatments in
either experiment. By contrast, chlorophyll
a
con-
centrations during the 2006 study showed a signii -
cant 40% decrease under high CO
2
during the bloom
phase, with maximum concentrations of 6 mg m
-3
and 10.3 mg m
-3
under high CO
2
and present-day
CO
2
, respectively.
The response of the plankton communities to the
CO
2
perturbation varied between experiments. As
coccolithophores are considered to be prolii c pro-
ducers of DMSP it is pertinent to i rstly consider
their response to high-CO
2
conditions. The abun-
dance of the coccolithophore
Emiliania huxleyi
showed some variation between experiments. In
the 2003 experiment a strong coccolithophore bloom
occurred, with maximum abundances of 56 × 10
6
cells ml
-1
. There was a small reduction in
E. huxleyi
numbers under high CO
2
, but the differences
between treatments were not signii cant (Engel
et al.
2008 ).
Emiliania huxleyi
was notably less prolii c in
the 2005 experiment, with maximum abundances of
11.2 Effects of ocean acidii cation on
DMS production and its impact on
climate
11.2.1
Summary of experimental evidence
To date, DMS and DMSP concentrations have been
measured during three CO
2
perturbation experi-
ments, all performed at the large-scale mesocosm
facilities at the Marine Biological Field Station,
Raunefjorden, Norway (60.3°N, 5.2°E): 2003 (Pelagic
Ecosystem Enrichment Experiment; PeECE II;
Avgoustidi 2007), 2005 (PeECE III; Wingenter
et al.
2007 ; Vogt
et al.
2008), and 2006 (UK NERC Microbial
Metagenomics experiment; Hopkins
et al.
2010 ). The
data are summarized in Fig. 11.3 and Table 11.1.
The concept and design of each of the experi-
ments were very similar. Polyethylene enclosures
