Geoscience Reference
In-Depth Information
2002; see also Chapter 5), as well as an increase in
carbohydrate to protein ratios of particulate matter
( Tortell
et al
. 2002 ).
There is some evidence for a direct
p
CO
2
effect on
diatom silicii cation and silicon quotas, with higher
quotas under low CO
2
conditions (Milligan
et al.
2004). Indirect effects may also arise from CO
2
-
induced shifts in species composition. Lower Si:N
consumption in response to elevated
p
CO
2
was
observed by Tortell
et al.
(2002) in a natural commu-
nity of the equatorial Pacii c, associated with a shift
in community composition from diatom- to
Phaeocystis
-dominated. Increasing Si:N ratios due to
a shift from lightly to more heavily silicii ed dia-
toms in response to elevated
p
CO
2
was observed by
Tortell
et al.
( 2008 ) and Feng
et al.
( 2008 ). No change
in silicate drawdown was found in a natural com-
munity exposed to 350, 700, and 1050 μatm CO
2
in a
mesocosm experiment (Bellerby
et al.
2008 ). The
reasons for CO
2
-induced changes in phytoplankton
stoichiometry remain poorly understood but are
critical for understanding trophic- and ecosystem-
level consequences of ocean acidii cation. Changes
in the C:N and C:P ratios of primary produced
organic matter alter its nutritional value and may
adversely affect the growth and reproduction of
herbivorous consumers, for example, as seen in
copepods and daphnids (Sterner and Elser 2002). A
change in the stoichiometry of export production is
also a powerful mechanism by which biology can
alter ocean carbon storage (Boyd and Doney 2003;
Riebesell
et al.
2009 ; see also Chapter 12 ).
capacity for nutrient storage in intracellular vacu-
oles (Sarthou
et al.
2005 ). As the phytoplankton
bloom progresses, decreased nutrient availability
and an increased biomass of grazers (e.g. copepods)
lead to a demise of diatom populations, shifting
phytoplankton assemblages towards a greater frac-
tion of smaller cells with high surface area to vol-
ume ratios and, correspondingly, more efi cient
nutrient uptake systems. Some large dinol agellates
can also thrive under these conditions through mix-
otrophic metabolism (i.e. facultative photoautotro-
phy supplemented by grazing) and by their ability
to migrate into nutrient-rich deeper waters.
In the context of ocean acidii cation research, the
critical question is how decreasing pH (and the
associated increase in
p
CO
2
) might alter ecological
dynamics in planktonic ecosystems. Early work by
Hinga (1992) demonstrated changes in phytoplank-
ton succession in mesocosms induced by pH manip-
ulations. Chen and Durbin (1994) demonstrated
large differences in the composition of phytoplank-
ton assemblages across a broad range of natural pH
variation. Results from the latter study are difi cult
to interpret, since natural pH gradients across sys-
tems are typically associated with changes in multi-
ple variables (e.g. macronutrient concentrations
and mixing depth) that are also likely to inl uence
phytoplankton productivity and species composi-
tion. More recent studies have focused on control-
led
p
CO
2
manipulations in various oceanic regimes.
A number of authors have reported an increase in
the relative abundance of diatoms under elevated
CO
2
(decreased pH) at the expense of haptophytes
and other nanol agellates (Tortell
et al.
2002 , 2008 ),
and a shift within diatom assemblages to large cen-
tric species ( Tortell
et al.
2008 ; Feng
et al.
2010 ).
However, other studies have not observed the
same CO
2
-dependent increases in relative diatom
biomass (Hare
et al.
2007 ; Feng
et al.
2009 ). This
discrepancy may result in part from differences
in experimental methodology (chemostats versus
semi-continuous cultures) and/or from region-
specii c differences attributable to background
oceanographic conditions.
To understand, mechanistically, how elevated
p
CO
2
and ocean acidii cation could inl uence phyto-
plankton species succession, fundamental informa-
tion is needed on the taxonomic diversity of carbon
6.3.6
Species composition and succession
The process of phytoplankton species succession is
relatively well understood for classic food webs, for
example in the North Atlantic bloom. In such sys-
tems, there is a recurrent and predictable transition
from diatoms to coccolithophores, dinol agellates,
and cyanobacteria as nutrient concentrations and
mixing depths decrease from early spring to sum-
mer. This ecological succession was explained by
Margalef (1958) as a result of changing energy input
into the upper mixed layer. Early successional spe-
cies such as diatoms thrive under high-nutrient
conditions (associated with signii cant vertical mix-
ing) by virtue of their rapid growth rates and large
