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concentrations of substrates and products in living
systems (including those of oxygen and CO 2 ) are
actively maintained at disequilibrium to ensure a
constant Gibb's free energy change, and thus ther-
modynamic drive, for forward l ux through the
pathways of energy metabolism (cf. Seibel et al .
2009). These considerations preclude the use of RI
as a simple indicator of limiting conditions in
metazoan habitats.
Individual species have evolved unique toler-
ances to their habitats. Comparative physiology has
dei ned critical gas tensions, which rel ect species-
specii c adaptations and limitations in oxygen
uptake and acid-base regulation that have evolved
for each species within their specii c habitat.
However, such limits specii c for one factor benei t
from the integration of limits to other factors to take
the full range of environmental stressors into
account (Pörtner 2010). With respect to the specii c
interactions between hypoxia and hypercapnia, in
addition to the evaluation of critical oxygen partial
pressures ( P c O 2 ) or CO 2 tensions ( P c CO 2 ) as tradition-
ally done by comparative and environmental physi-
ologists, an experimentally determined, redei ned
RI, which integrates the whole-organism limits to
both gases, may be useful as a proxy to qualify
resistance to the synergistic effects of hypoxia and
hypercapnia. Such a proxy would need to be tested
with respect to whether the sensitivity threshold to
hypoxia ( P c O 2 ) is inl uenced by hypercapnia, and
vice versa. Only on the basis of such empirical data
would it be suitable to project species-specii c criti-
cal levels of climate-induced hypoxia and ocean
acidii cation or critical conditions of combined
hypoxia and hypercapnia in OMZs. Any predictive
use of such an indicator requires additional mecha-
nistic understanding of physiological tolerance and
its ecological implications.
As a general conclusion, data on the effects of
hypercapnia are scarce in nektonic species as in
other groups. This is especially true for the long-
term effects on processes such as investments in
reproduction, development, growth, foraging capac-
ity, behaviour(s), and resistance to disease which are
crucial in setting i tness levels and, as a result, com-
petitiveness with other species. Multigeneration
studies or studies of species populations in different
climate zones (according to differences in ambient
p CO 2 , temperature, and associated CO 2 solubilities)
are needed to address the evolutionary consequences
of ocean acidii cation.
Increased mechanistic knowledge for specii c
groups of organisms according to climate and habi-
tat characteristics can help in the development of
long-term projections of ecosystem change.
Identifying the physiological background of rela-
tive changes in performance and competitiveness is
important for identifying winners and losers at eco-
system levels (Pörtner and Farrell 2008). For a com-
prehensive understanding of the effects of ocean
acidii cation in the context of further environmental
challenges, the concept of OCLTT provides a matrix
suitable for the integration of various stressors
(Pörtner 2010). The data currently available in
marine invertebrates and i shes support the follow-
ing framework: the relatively low sensitivity of
nektonic, and thus active, organisms to ocean acidi-
i cation is related to their high capacity for (extracel-
lular) acid-base regulation. Efi cient acid-base
regulation results in an accumulation of base in the
extracellular and intracellular l uids, which facili-
tates calcii cation from ions in 'internal' l uids. In
'good' acid-base regulators such internally control-
led calcii cation then operates at a higher saturation
of calcium carbonates (see Box 1.1, Chapter 1).
However, elevated P CO 2 values in body l uids
might at the same time reduce relevant perform-
ance capacities, for example by causing unfavoura-
ble shifts in cellular, tissue, and organismal energy
budgets and depressing growth and reproduction,
or by 'dampening' organism behaviour(s) through
neurobiological effects (e.g. by triggering the accu-
mulation of adenosine). Transient exposure to ele-
vated p CO 2 levels combined with hypoxia as in the
OMZ or in intertidal sediments may even be benei -
cial (Reipschläger et al. 1997), for example by sup-
porting strategies for saving energy. A transient
reduction in spontaneous activity may result, which,
if it does not interfere with foraging success or other
crucial behaviours, can support an increase in
growth efi ciency and at the same time a reduced
oxygen demand, in similar ways as adaptation to
progressively colder temperatures along a latitudi-
nal cline enhances growth efi ciency (Heilmayer
et al. 2004). Depending on environmental scenarios,
on the level of energy turnover and associated mode
 
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