Geoscience Reference
In-Depth Information
could lead to a restructuring of phytoplankton
assemblages with consequences that would rever-
berate throughout marine communities. For exam-
ple, the composition of phytoplankton assemblages
in the north-east Atlantic changed as waters
warmed during the latter half of the last century,
with a poleward expansion of warm-temperate
plankton and recession of colder forms (Hays
et al.
2005). In the English Channel, coccolithophores
and dinol agellates have become more abundant
and dominant over the past ~20 yr, while diatoms
and
Phaeocystis
have decreased (Widdicombe
et al.
2010). The expansion of dinol agellates may be
expected based on the emergence of these groups
during warmer, high-CO
2
periods in earth's history
(Beardall and Raven 2004). Cyanobacteria, which
thrived under high CO
2
levels earlier during earth's
history, are also expected to benei t from ocean
acidii cation. Photosynthetic rates of two dominant
oceanic cyanobacterial genera (
Synechococcus
and
Trichodesmium
) were observed to increase markedly
under expected future climate conditions, so long
as nutrient levels were sufi ciently high, while
other closely related taxa (
Prochlorococcous
and
Nodularia)
showed little change or even lower rates
( Fu
et al.
2007). An increase in the diversity, abun-
dance, or productivity of cyanobacteria could
increase rates of global nitrogen i xation (Hutchins
et al.
2009), which would be likely to drive responses
in primary production by other groups. High CO
2
levels have also been shown to cause enhanced
production of dimethyl sulphide by natural phyto-
plankton assemblages, which could promote cli-
mate homeostasis by stimulating cloud formation
(see Chapter 11 ).
Elevated
p
CO
2
can also affect phytoplankton
communities and the entire water column commu-
nity through changes in elemental uptake or calcii -
cation by major groups. Increased C:N and C:P
ratios have been measured in mixed phytoplankton
assemblages in response to high CO
2
levels
( Riebesell
et al.
2007 ; see Chapter 6 ), potentially
changing their nutritional value to consumers and
leading to changes in growth and reproduction of
zooplankton. Reduced rates of calcii cation for coc-
colithophores and foraminifera, two major plank-
tonic calcifying groups, may also affect the l ux of
organic debris to deeper waters, due to changes in
ballasting of organic aggregates (Fabry
et al.
2008 ;
Ridgwell
et al.
2009 ).
Holoplankton can also be affected by changing
ocean chemistry, perhaps especially in weakly sat-
urated waters of high-latitude systems. Euphausiids
and thecosomatous pteropods, two key planktonic
groups thought to be critical linkages in global food
webs, may be affected differently by ocean acidii -
cation. Pteropods (e.g.
Limacina
sp. and
Clio
sp.) are
important planktonic calcii ers in open-ocean food
webs and represent major prey taxa for higher
predators (e.g. many salmon species), and thus are
a key link in open-ocean food webs and energy
l ow (Fabry
et al.
2009). Immersion of shelled ptero-
pods in high-CO
2
, low-CO
3
2-
waters is known to
weaken their aragonitic shells (Orr
et al.
2005 ) and
is expected to reduce their survival and productiv-
ity (Comeau
et al.
2009 , 2010 ). To date, only minor
decadal-scale changes have been observed globally
in these groups. In the California Current ecosys-
tem, pteropod abundance has not declined, and
may have increased over the past 50 yr (Ohman
et al.
2009). In the Southern Ocean, where aragonite
undersaturation is predicted to begin as early at
2030 (McNeil and Matear 2008), ocean acidii cation
may already be affecting pteropod populations.
Roberts
et al.
(2008) reported that the shell weights
of pteropods collected in sediment traps deployed
in sub-Antarctic waters (47°S) have decreased over
the past decade. This decrease was not correlated
with chlorophyll abundance or temperature, but
was consistent with changes in aragonite satura-
tion, and thus the potential inl uence of ocean acid-
ii cation cannot be rejected. The loss of
Limacina
or
other key pteropod taxa in undersaturated waters
due to reduced calcii cation (e.g. Comeau
et al.
2009 , 2010 ) could have signii cant implications for
their predators and energy l ow through open-
ocean food webs.
Euphausiids, copepods, and other planktonic
crustaceans are dominant elements of food webs
worldwide, and are often suitable alternative prey
for many oceanic pteropod predators (Cooley
et al.
2009). In contrast to pteropods, krill and other crus-
taceans may not be strongly affected by ocean acid-
ii cation, and some taxa may even benei t, but few
studies have examined this topic. Kroeker
et al.
(2010) report that the literature available to date
