Geoscience Reference
In-Depth Information
populations and species that may include reduced
abundance, productivity, and resilience to distur-
bance, as well as increased likelihood of extinction.
For taxa benei ting either directly or indirectly from
high CO
2
levels, the opposite may be true. It is also
important to consider the cumulative effects of
environmental stressors on the demography and
productivity of populations. Effects on different life
stages can sum to signii cant impacts on population
success. For example, during periods of low sea-
surface temperature (<13.1°C), exposure to low-pH
waters reduces the survival of early life stage barna-
cles along the coast of the south-west United
Kingdom by 25%, potentially leading to reduced
local population abundance (Findlay
et al.
2010 ).
Sensitivity to ocean acidii cation is expected to be
coupled primarily to fundamental physiological
adaptations linked closely to phylogeny. Marine
organisms with a natural capacity for gas exchange
(i.e. organisms with well-developed respiratory and
circulatory systems, as well as respiratory proteins
allowing high O
2
and CO
2
l uxes) that support high
metabolic rates and high aerobic scope (e.g. i shes,
decapod crustaceans, and cephalopods) are pre-
adapted for many of the stresses related to ocean
acidii cation (Melzner
et al.
2009b ; see Chapter 8 ).
This is due in part to the overlapping physiological
challenges posed by metabolic CO
2
generation dur-
ing intense aerobic activity (e.g. coping with inter-
nal acid-base disruption) and the effects of ocean
acidii cation. Many taxa in habitats with variable or
low pH (e.g. vesicomyid clams, vestimentiferan
tubeworms, mussels in vent or seep environments)
also have adaptations that allow them to thrive in
naturally hypoxic and low-pH waters (Goffredi and
Barry 2002 ; Tunnicliffe
et al.
2009 ). Mobile crusta-
ceans and i shes may benei t somewhat in a high-
CO
2
ocean, based on their generally higher rates of
growth and calcii cation in low-pH waters (Ries
et al.
2009 ; Kroeker
et al.
2010 ). However, even taxa
with the capacity to cope with activity-related
hypercapnia can experience impaired physiological
performance in high-CO
2
waters. Rosa and Seibel
(2008) found that activity levels in jumbo squid
declined by 45% under a 0.3 unit reduction in pH.
In contrast, cod exposed to a large pH perturbation
(-1 pH unit) for several months displayed no evi-
dence of impaired maximal swimming speed
( Melzner
et al.
2009a ).
Taxa with weaker control over internal l uid
chemistry may be at greater risk from ocean acidii -
cation. For example, echinoderms, brachiopods, and
lower invertebrates (e.g. sponges, cnidarians, and
ctenophores) lack respiratory organs and exchange
gases with seawater by molecular diffusion across
various body tissues. Although physiological toler-
ance to ocean acidii cation has not been examined
closely in most of these groups (other than rates of
calcii cation, see below), their postulated weak con-
trol of internal l uid chemistry (e.g. sea urchins,
Miles
et al.
2007) is expected to increase their sensi-
tivity to changing ocean chemistry. Echinoderms
appear less tolerant of low-pH waters than many
groups, as indicated by their conspicuous absence
from habitats with naturally high CO
2
levels such as
hydrothermal vents (Grassle 1986) and low-pH
areas near shallow CO
2
vents off Italy (Hall-Spencer
et al.
2008). Notably, various taxa with limited physi-
ological capabilities (many cnidarians and sponges)
appear to tolerate low or variable pH, due to their
occurrence in low-pH habitats such as hydrothermal
vents and other natural CO
2
venting sites. Moreover,
generalities based on short-term studies of organism
physiology or survival, as are most common in the
literature, may differ from the eventual long-term
consequences of ocean acidii cation.
For calcifying taxa, the type of carbonate miner-
als formed can inl uence their vulnerability to ocean
Normal
Stressed
R
R
M
G
M
G
Figure 10.2
Hypothetical energy budget for normal and stressed
organisms. Under normal conditions, the energetic cost of maintenance
(M) is a signii cant portion of the total energy budget. If ocean acidii cation
or other environmental changes are stressful, maintenance costs (e.g. ion
regulation) can increase, leaving less energy available for growth (G) or
reproduction (R). In addition, if metabolic depression is induced by ocean
acidii cation, the total energy budget may decrease, hence the smaller pie
size for the energy budget of the stressed organism.
