Geoscience Reference
In-Depth Information
Similarly, phytoplankton may have higher light-use
efi ciencies under elevated CO
2
as a result of the
lowered metabolic costs of inorganic carbon assimi-
lation. Such a response has been observed in culture
studies with freshwater dinol agellates (Berman-
Frank
et al.
1998). Future experiments should thus
carefully consider how best to manipulate tempera-
ture, light, and nutrients in conjunction with
p
CO
2
.
With respect to potential changes in mixed-layer
depth, it will be important to simulate not only
changes in mean irradiance levels, but also
decreased variability in the light i eld. This latter
effect could have important implications for the
ability of phytoplankton to photo-acclimate, and
could thus impose a selective pressure on phyto-
plankton species.
changes in speciation will also increase the thermo-
dynamic and kinetic activity of the metals. Because
most organic particles in seawater are negatively
charged, the surface sites will become less available
to adsorb metals as pH decreases. These changes
are expected to alter the availability and toxicity of
metals for marine organisms.
Contrary to expectations, reduced uptake of iron
in response to seawater acidii cation was observed
in monospecii c cultures of diatoms and coccolitho-
phores by Shi
et al.
(2010) and was attributed to
decreased bioavailability of dissolved iron under
ocean acidii cation. The iron requirement of the phy-
toplankton remained unchanged with increasing
CO
2
. Decreased iron and zinc requirements and
increased growth and carbon i xation rates at ele-
vated CO
2
were observed in a centric diatom by
King
et al
. (pers. comm.). In a mesocosm CO
2
enrich-
ment study Breitbarth
et al.
( 2010 ) found signii cantly
higher concentrations of dissolved iron and higher
Fe(II) turnover rates in high- compared with the
mid- and low-CO
2
treatments, suggesting enhanced
iron bioavailability in response to ocean acidii ca-
tion. This agrees with the expectation of Millero
et al. (2009), who predicted that declining pH will
increase the half-life of Fe(II) in seawater, making it
more available for biological consumption. Clearly,
more work is needed to entangle the effects of ocean
acidii cation on trace metal bioavailability and the
synergistic effects with changing chemistry of other
essential nutrients to assess their impacts on biogeo-
chemical cycling. Coastal upwelling and oxygen
minimum zones, which already have low pH, would
be useful areas to study these processes in the mod-
ern ocean (Millero
et al
. 2009 ).
6.4.3
Nutrient speciation and availability
In different oceanic regions, primary production is
limited by the availability of various key nutrients,
most commonly nitrogen, phosphorus, or iron. The
acquisition of such nutrients depends on their
chemical form(s)—the chemical 'species'—present
in the water. For example, free phosphate in seawa-
ter (HPO
4
2-
) can be readily taken up by phytoplank-
ton but, in most cases, phosphate bound in organic
compounds must i rst be cleaved enzymatically
from the organic moiety before being utilized. In
the same way, nitrate (NO
3
-
) and ammonium (NH
4
+
)
are readily bioavailable, while organic nitrogen
compounds, including urea, amino acids, or amines
can only be utilized through specialized enzymatic
processes. Changing the pH of seawater is expected
to affect the efi ciency of the enzymatic processes
involved in acquiring organic forms of nitrogen and
phosphorus (Millero
et al
. 2009 ). A similar situation
applies to essential trace metals such as iron (Fe),
which are readily taken up when present as free
ions or ions bound to chloride, hydroxide, or other
inorganic ligands, but require specialized uptake
machinery when bound to organic complexing
agents (Maldonado and Price 2001). As ocean acidi-
i cation decreases the concentrations of hydroxyl
(OH
-
) and carbonate (CO
3
2-
) ions, which both form
strong complexes with divalent and trivalent met-
als, these metals will have a higher fraction in their
free forms at lower pH (Millero
et al
. 2009 ). These
6.5 Ecological processes
Responses of pelagic ecosystems to ocean change
have the potential to affect marine biogeochemical
cycles and carbon sequestration, leading to a series
of climate feedback mechanisms ( Fig. 6.1 , Table 6.6 ).
These potential biogeochemical feedbacks, i.e.
changes in the ocean's elemental cycling with reper-
cussions on the climate system, are discussed in
detail by Riebesell
et al.
( 2009 ) and in Chapter 12 .
The discussion below aims at highlighting the
