Impacts in the Next Few Decades and Coming Centuries - Climate Stabilization Targets - page 210

states ( W > 3) for healthy growth (Langdon and Atkinson, 2005; Kleypas and
Yates, 2009). Decreased calcification is observed for corals with symbiotic
zooxanthella (photosynthetic algae living within coral animals), and CO 2
fertilization of zooxanthella does not alleviate acidification effects. Studies
of net community calcification rates for coral reef ecosystems indicate that
overall net calcification also decreases with rising CO 2 (Silverman et al.,
2007), and model studies suggest a threshold of about 500-550 ppm CO 2
where coral reefs would begin to erode rather than grow, negatively im-
pacting the diverse reef-dependent taxa (Silverman et al., 2009). Observed
physiological responses for mollusks, such as pteropods, oysters, clams,
and mussels, include reduced calcification, increased juvenile mortality
and reduced larval settlement, and smaller, thinner, and malformed shells
(Orr et al., 2005; Green et al., 2009; Miller et al., 2009). Crustaceans also
utilize calcium carbonate in their shells, but the response to elevated CO 2
is less well-understood with studies reporting both increased and decreased
calcification rates (Fabry et al., 2008). Decreased calcification rates with
rising CO 2 are observed as well for key planktononic calcifiers including
foramaniferia and most strains or coccolithophores.
Some organisms may benefit in a high-CO 2 world, in particular pho-
tosynthetic organisms that are currently limited by the amount of dissolved
CO 2 . In laboratory experiments with elevated CO 2 , higher photosynthesis
rates are found for certain phytoplankton species, seagrasses, and macroal-
gae, and enhanced nitrogen-fixation rates are found for some cyanobacteria
(Hutchins et al., 2009). Indirect impacts of ocean acidification on non-
calcifying organisms and marine ecosystems as a whole are possible but
more difficult to characterize from present understanding. A limited number
of field studies that have been carried out in mostly benthic systems with
naturally elevated CO 2 are broadly consistent with the laboratory studies in
terms of predicted changes in community structure (e.g., decrease in calci-
fiers; increase in non-calcifying algae) (Hall-Spenser et al., 2008; Wootton
et al., 2008). Polar ecosystems also may be particularly susceptible when
surface waters become undersaturated for aragonite, the mineral form used
by many mollusks including pteropods, which are an important prey species
for some fish. Socioeconomic impacts from degraded fisheries and other
marine resources are possible but poorly known at this point (Cooley and
Doney, 2009).
Based on historical survey data, the geographic range of many marine
species has shifted poleward and into deeper waters due to ocean warm-
ing (Perry et al., 2005; Nye et al., 2009). Model projections indicate that
poleward expansion and equatorial contraction of geographical ranges