Effects of ocean acidification on pelagic organisms and ecosystems - Ocean Acidification - page 110

that produce CaCO 3 shells. These organisms clearly
show decreased calcii cation under more acidic
conditions (see above), but the implications of this
for growth, reproduction, and grazing rates remain
unclear. Less environmentally relevant information
is available for non-calcifying zooplankton species.
Much of the work that has been done with copep-
ods, for example, has utilized extreme CO 2 levels
(e.g. >10 000 μatm) that induce juvenile mortality in
a number of species (see Fabry et al. 2008 for a recent
and comprehensive review). In a mesocosm CO 2
perturbation study with p CO 2 levels of 350, 700, and
1050 μatm no effect of ocean acidii cation was
observed on microzooplankton grazing (Suffrian et
al. 2008) and copepod feeding and egg production
( Carotenuto et al. 2007 ), but differences between
CO 2 treatments were observed with respect to cope-
pod nauplii recruitment (Carotenuto et al. 2007 ;
Riebesell et al. 2008b ). Experimental data are
urgently needed on the potential effects of environ-
mentally relevant CO 2 /pH levels on feeding rates,
growth, and assimilation efi ciencies of both micro-
and mesozooplankton. In a recent i eld experiment,
Rose et al. (2009) reported no consistent CO 2 or tem-
perature effects on microzooplankton biomass dur-
ing a late North Atlantic spring bloom. Such effects,
if present, will result from a combination of direct
physiological inl uences on grazers, and indirect
ecological effects associated with altered phyto-
plankton assemblage composition.

increasing temperature. However, each individual
species has a temperature optimum and its growth
is inhibited above a certain threshold value. As a
result, observed temperature responses of mixed
phytoplankton assemblages rel ect the aggregate
response of many interacting species. Recent exper-
iments have examined the inl uence of temperature
on phytoplankton productivity in bottle experi-
ments, demonstrating signii cantly increased rates
of carbon i xation under elevated temperature (Hare
et al. 2007 ; Feng et al. 2009 ). This temperature stimu-
lation occurred irrespective of CO 2 /pH levels, but
the extent of any temperature response should
depend on other limiting factors (e.g. light, nutri-
ents, trace metals; see below). Moreover, the meta-
bolic rates of grazers also increase with temperature
(e.g. Rose et al. 2009 ), potentially reducing the
impacts of increased phytoplankton productivity.

6.4.2 Shoaling of the mixed-layer depth

While the direct ecological impacts of increased sea-
surface temperature remain unclear, the effects of
warming on physical circulation have been exam-
ined by a number of authors. As surface waters
warm, mixed-layer stratii cation (i.e. vertical den-
sity gradients) increases and this effect is enhanced
in some regions by increased sea-ice melt (Boyd and
Doney 2003 ; Sarmiento et al. 2004 ). Increased strati-
i cation has several important implications for bio-
logical processes, which could offset or exacerbate
the effects of elevated p CO 2 alone. First, increased
stratii cation reduces vertical water mass exchange
and hence the l ux of nutrients from the subsurface
into the mixed layer. This, in turn, decreases the
capacity of an ecosystem for new production.
Second, enhanced stratii cation increases the mean
depth-integrated irradiance and decreases the range
(i.e. variability) of irradiance levels experienced by
phytoplankton mixed to a shallower depth. The
potential decrease in nutrient concentrations and
increased irradiance levels could exert opposing
effects on primary productivity. As CO 2 increases
(pH decreases), phytoplankton may have lower
nitrogen requirements associated with reduced cel-
lular RubisCO content, as well as lower energy
requirements associated with down-regulated car-
bon concentration (Raven and Johnston 1991).

6.4 Synergistic effects of ocean
acidii cation with other environmental
changes
6.4.1 Ocean warming

Temperature exerts a fundamental control on the
metabolic rates of planktonic organisms, and warm-
ing of surface-ocean waters is thus expected to
inl uence marine metabolic cycles. The classic tem-
perature-dependent growth characteristics of phy-
toplankton were described by Eppley (1972), and
have been incorporated in many ecosystem models
and productivity algorithms (e.g. Sarmiento et al.
2004 ; Schmittner et al. 2008 ). In general, growth
rates of individual phytoplankton species have
been observed to increase exponentially with