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obtained for N:P ratios, which show large differ-
ences within and between phytoplankton functional
groups. While single-species laboratory experi-
ments also revealed mixed responses in C:N ratio
with rising p CO 2 , a more consistent pattern is
observed in natural communities in i eld assays and
mesocosm experiments, generally showing stable
or higher C:N (and higher C:P) at elevated p CO 2
(Table 6.5). An increase in the community C:N ratio
would increase the carbon drawdown for a given
amount of nutrient supply to the sunlit surface
layer, with the potential of accelerating the biologi-
cal pump. This would provide a negative feedback
to rising atmospheric p CO 2 ( Riebesell et al. 2007 ),
but could also greatly expand the volume of oxy-
gen-depleted waters (Oschlies et al. 2008 ). In view
of the scarcity of information, global extrapolations
of the observed responses should be viewed merely
as sensitivity tests. Open questions concern the lon-
gevity of the observed changes in elemental ratios,
possible dampening effects from adaptation and
shifts in species composition and the interacting
effects from changes in temperature, light, and
nutrient availability.
depth, it is uncertain to what extent the additional
bioavailable nitrogen would lead to enhanced car-
bon sequestration or would remain suspended in
the surface layer. In the latter scenario, an accumu-
lation of biologically available nitrogen in the sur-
face ocean would eventually eliminate the ecological
niche for nitrogen-i xing cyanobacteria, in a self-
limiting process. As CO 2 -sensitivity of nitrogen i xa-
tion has so far been reported for only two species, it
is too early to speculate about the sensitivity and
longevity of this response. Indeed, one still lacks
fundamental information on the identity and meta-
bolic characteristics of key nitrogen i xers in the
oceans, and signii cant certainty still exists on the
rate of global nitrogen i xation (Gruber and
Galloway 2008). Future experiments should con-
sider the potential phylogenetic and metabolic
diversity of marine nitrogen i xers, while modelling
studies should take into account the selective pres-
sures favouring (or limiting) the abundance of these
organisms.
6.5.3 Changes in the composition
of phytoplankton assemblages
The qualitative nature of phytoplankton and zoo-
plankton assemblages exerts a strong mitigating
inl uence on the biogeochemical impacts of primary
productivity. This was demonstrated by Boyd and
Newton (1995) in a study of the 1989-1990 North
Atlantic spring bloom. Despite similar rates of total
primary productivity and nutrient drawdown in
both years, very different rates of vertical carbon
export were observed, in conjunction with signii -
cant differences in the species composition of phy-
toplankton and zooplankton. Higher carbon l uxes
were observed when phytoplankton assemblages
were dominated by larger diatom species. Such a
shift has been observed to occur under high-CO 2
conditions in several experiments (Tortell et al. 2002 ,
2008). In contrast, other studies have observed
reductions in diatom biomass under high-CO 2
conditions ( Hare et al. 2007). In general, a CO 2
(pH)-dependent shift in phytoplankton species
composition can inl uence biogeochemical cycles in
several ways. First, larger cells (e.g. diatoms) should
have intrinsically higher sinking rates and thus pro-
mote greater carbon l uxes out of the surface layer.
6.5.2
Changes in biological nitrogen cycling
Nitrogen i xation, carried out by a few specialized
microorganisms, represents a major input of 'new'
nitrogen to oligotrophic marine ecosystems and is
key in controlling primary production in large
regions of the world's oceans. Nitrogen i xation is
an 'expensive' biochemical process that requires
synthesis of a complex iron-rich enzyme and uses
large amounts of energy. This process is thus only
ecologically viable under conditions of nitrogen
dei ciency in sunlit surface waters. pH-dependent
changes in the availability of iron and, possibly,
other nutrients may thus change the rate of nitrogen
i xation in the oceans. Enhanced nitrogen i xation at
elevated p CO 2 , reported for the diazotrophic cyano-
bacterium Trichodesmium ( Table 6.3 ) has the poten-
tial to increase the reservoir of bioavailable nitrogen
in the surface layer which, in a predominantly
nitrogen-limited ocean, would increase primary
production and carbon i xation (negative feedback).
However, since the biomass produced by cyanobac-
teria is generally thought not to be exported to great
 
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