137
stressor that is tightly linked to temperature is ocean acidification. If temperature itself will have such extensive effects
on tropical marine ecosystems, then the combination with
ocean acidification will be unfathomable.
A Sour Ocean
Within the literature, ocean acidification has been closely
coupled with the rise in atmospheric and oceanic temperatures, with all three being attributed primarily to rising levels
of CO 2 in the Earth’s atmosphere (Caldeira and Wickett
2003; Hoegh-Guldberg and Bruno 2010). CO 2 is not only a
key player in climate change due to its ability to trap heat,
but also a vital component of biological mechanisms (e.g.,
photosynthesis), which are important in sustaining life. The
oceans play an important role in the global carbon cycle, acting as a ‘carbon sink’ by taking up about one third of CO 2
from the atmosphere and transporting it around the globe (Le
Quéré et al. 2013). The acidification of our oceans alters
ocean chemistry, which poses significant challenges to these
already threatened tropical marine ecosystems (Kroeker
et al. 2010; Gaylord et al. 2015).
Since the industrial revolution our, on average, slightly
alkaline ocean, with a pH of 8.2, has experienced a decrease
in pH of 0.1 units, which represents a 30% increase in acidity. The pH is predicted to drop a further 0.3 units by the end
of the century (IPCC 2013). The trajectory towards an ocean
with a lower pH will have both positive and negative consequences for marine organisms (Garrard et al. 2014). This was
demonstrated by research conducted near oceanic vents,
which emit large quantities of CO 2 and consequently create
areas of seabed with a lower than usual pH (Frankignoulle
and Distèche 1984; Hall-Spencer et al. 2008; Fabricius et al.
2011; Scartazza et al. 2017). However, research on ocean
acidification primarily focuses on the impacts to calcifying
organisms such as corals, molluscs, echinoderms, crustaceans, coccolithophores, foraminifera, pteropods, and some
species of algae. Increasing atmospheric CO 2 alters the dissolved inorganic carbon distribution in seawater, reducing its
pH and with it the availability of carbonate ions (CO 3
2−
)
(Cohen and Holcomb 2009; Findlay et al. 2010). This
impacts the energy-costly process of calcification as the rate
at which calcifying organisms produce calcium carbonate
(CaCO 3 ), is slowed to a point where rates of erosion exceed
those of skeletal accretion (Cohen and Holcomb 2009;
Gerber et al. 2014). In terms of non-calcifying species, acidification is believed to disturb acid–based (metabolic) physiology and impact their survival, growth, and reproduction
(Fabry et al. 2008; Kroeker et al. 2010). Research into the
response of non-calcifying organisms including jellyfish,
fish, fleshy algae, and seagrasses to acidification is becoming
more commonplace, yet, in the case of mangroves, the
impacts remain vastly understudied (Guinotte and Fabry
2008; Kroeker et al. 2010).
Calcifying species appear to be the ‘losers’ in the case of
a more acidic ocean, and exhibit a range of negative
responses, especially when acidification is combined with
other stressors (Hoegh-Guldberg et al. 2007; Fabricius et al.
2011). For coral calcification, studies have indicated that the
extracellular calcifying medium is maintained at a higher pH
than that of the surrounding seawater in order to facilitate
CaCO 3 precipitation (McCulloch et al. 2012). However, how
changes in seawater pH would affect this internal biological
control is currently unknown. More recently, studies have
revealed that instead of decreasing their growth rate, corals
are acclimatizing by decreasing their skeletal density and
increasing their porosity (Tambutté et al. 2015). Although
this morphological plasticity ensures slower, but continuous
growth of the colony, it weakens the overall reef structure,
making it more susceptible to damage resulting from anthropogenic or climatic perturbations (Hoegh-Guldberg et al.
2007). This weakening of skeletons also affects reef-building
gastropods of the family Vermetidae, which provide coastal
protection to neighboring ecosystems such as mangroves and
seagrasses (Milazzo et al. 2014). The decreased reef resilience can be attributed to reduced structural complexity and
coral species diversity (Anthony et al. 2011; Fabricius et al.
2011). A pH of 7.7 has been shown to cease reef development due to a shift in coral species dominance, away from
structural corals (branching, foliose, and tabulate growth
forms) towards massive growth forms such as Porites corals
(Fabricius et al. 2011). These reductions in reef complexity
can in turn impact the biodiversity of reef-associated species
as well as trophic interactions, and other ecosystem processes (Raven et al. 2005; Kleypas et al. 2006).
For non-calcifying marine consumers, elevated oceanic
CO 2 and the accompanying change in pH will have negative
effects as it will require additional energy for metabolic
acid–based regulation (Pörtner 2008). Ocean acidification
slows larval development in fishes, molluscs and echinoderms (Kurihara 2008; Miller et al. 2009; Dupont et al. 2010;
Talmage and Gobler 2010; Dineshram et al. 2013; Gazeau
et al. 2013). The early life stages of fish are impacted by a
reduction of their oxygen consumption capacity and hence
their activity, along with olfactory cues for predation,
settlement, and reproduction (Munday et al. 2009). However,
these highly mobile organisms have developed intra- or
extracellular pH regulatory mechanisms that may aid them to
be more resilient to ocean acidification (Kroeker et al. 2010).
An additional option for fishes to escape a more acidic environment is finding refuge in seagrass meadows (Hendriks
et al. 2014), and potentially mangrove roots (Chakraborty
et al. 2013).
For a World Without Boundaries: Connectivity Between Marine Tropical Ecosystems in Times of Change
stressor that is tightly linked to temperature is ocean acidification. If temperature itself will have such extensive effects
on tropical marine ecosystems, then the combination with
ocean acidification will be unfathomable.
A Sour Ocean
Within the literature, ocean acidification has been closely
coupled with the rise in atmospheric and oceanic temperatures, with all three being attributed primarily to rising levels
of CO 2 in the Earth’s atmosphere (Caldeira and Wickett
2003; Hoegh-Guldberg and Bruno 2010). CO 2 is not only a
key player in climate change due to its ability to trap heat,
but also a vital component of biological mechanisms (e.g.,
photosynthesis), which are important in sustaining life. The
oceans play an important role in the global carbon cycle, acting as a ‘carbon sink’ by taking up about one third of CO 2
from the atmosphere and transporting it around the globe (Le
Quéré et al. 2013). The acidification of our oceans alters
ocean chemistry, which poses significant challenges to these
already threatened tropical marine ecosystems (Kroeker
et al. 2010; Gaylord et al. 2015).
Since the industrial revolution our, on average, slightly
alkaline ocean, with a pH of 8.2, has experienced a decrease
in pH of 0.1 units, which represents a 30% increase in acidity. The pH is predicted to drop a further 0.3 units by the end
of the century (IPCC 2013). The trajectory towards an ocean
with a lower pH will have both positive and negative consequences for marine organisms (Garrard et al. 2014). This was
demonstrated by research conducted near oceanic vents,
which emit large quantities of CO 2 and consequently create
areas of seabed with a lower than usual pH (Frankignoulle
and Distèche 1984; Hall-Spencer et al. 2008; Fabricius et al.
2011; Scartazza et al. 2017). However, research on ocean
acidification primarily focuses on the impacts to calcifying
organisms such as corals, molluscs, echinoderms, crustaceans, coccolithophores, foraminifera, pteropods, and some
species of algae. Increasing atmospheric CO 2 alters the dissolved inorganic carbon distribution in seawater, reducing its
pH and with it the availability of carbonate ions (CO 3
2−
)
(Cohen and Holcomb 2009; Findlay et al. 2010). This
impacts the energy-costly process of calcification as the rate
at which calcifying organisms produce calcium carbonate
(CaCO 3 ), is slowed to a point where rates of erosion exceed
those of skeletal accretion (Cohen and Holcomb 2009;
Gerber et al. 2014). In terms of non-calcifying species, acidification is believed to disturb acid–based (metabolic) physiology and impact their survival, growth, and reproduction
(Fabry et al. 2008; Kroeker et al. 2010). Research into the
response of non-calcifying organisms including jellyfish,
fish, fleshy algae, and seagrasses to acidification is becoming
more commonplace, yet, in the case of mangroves, the
impacts remain vastly understudied (Guinotte and Fabry
2008; Kroeker et al. 2010).
Calcifying species appear to be the ‘losers’ in the case of
a more acidic ocean, and exhibit a range of negative
responses, especially when acidification is combined with
other stressors (Hoegh-Guldberg et al. 2007; Fabricius et al.
2011). For coral calcification, studies have indicated that the
extracellular calcifying medium is maintained at a higher pH
than that of the surrounding seawater in order to facilitate
CaCO 3 precipitation (McCulloch et al. 2012). However, how
changes in seawater pH would affect this internal biological
control is currently unknown. More recently, studies have
revealed that instead of decreasing their growth rate, corals
are acclimatizing by decreasing their skeletal density and
increasing their porosity (Tambutté et al. 2015). Although
this morphological plasticity ensures slower, but continuous
growth of the colony, it weakens the overall reef structure,
making it more susceptible to damage resulting from anthropogenic or climatic perturbations (Hoegh-Guldberg et al.
2007). This weakening of skeletons also affects reef-building
gastropods of the family Vermetidae, which provide coastal
protection to neighboring ecosystems such as mangroves and
seagrasses (Milazzo et al. 2014). The decreased reef resilience can be attributed to reduced structural complexity and
coral species diversity (Anthony et al. 2011; Fabricius et al.
2011). A pH of 7.7 has been shown to cease reef development due to a shift in coral species dominance, away from
structural corals (branching, foliose, and tabulate growth
forms) towards massive growth forms such as Porites corals
(Fabricius et al. 2011). These reductions in reef complexity
can in turn impact the biodiversity of reef-associated species
as well as trophic interactions, and other ecosystem processes (Raven et al. 2005; Kleypas et al. 2006).
For non-calcifying marine consumers, elevated oceanic
CO 2 and the accompanying change in pH will have negative
effects as it will require additional energy for metabolic
acid–based regulation (Pörtner 2008). Ocean acidification
slows larval development in fishes, molluscs and echinoderms (Kurihara 2008; Miller et al. 2009; Dupont et al. 2010;
Talmage and Gobler 2010; Dineshram et al. 2013; Gazeau
et al. 2013). The early life stages of fish are impacted by a
reduction of their oxygen consumption capacity and hence
their activity, along with olfactory cues for predation,
settlement, and reproduction (Munday et al. 2009). However,
these highly mobile organisms have developed intra- or
extracellular pH regulatory mechanisms that may aid them to
be more resilient to ocean acidification (Kroeker et al. 2010).
An additional option for fishes to escape a more acidic environment is finding refuge in seagrass meadows (Hendriks
et al. 2014), and potentially mangrove roots (Chakraborty
et al. 2013).
For a World Without Boundaries: Connectivity Between Marine Tropical Ecosystems in Times of Change