138
Primary producers including seagrasses and macroalgae
appear to be the winners in the face of elevated oceanic CO 2
concentrations and lower seawater pH. Increased CO 2 concentrations in seawater are a resource for these primary producers (Palacios and Zimmerman 2007; Fabricius et al.
2011; Hepburn et al. 2011) which allow them to enhance
their productivity and growth (Harley et al. 2012; Koch et al.
2013). Seagrasses are known to alter the carbonate chemistry
in the water column, which is of particular importance in
regions where they neighbor coral reef environments
(Dorenbosch et al. 2005; Hendriks et al. 2014). In the
Mediterranean, Posidonia oceanica diurnally modify the
water column pH by as much as 0.2–0.7 units through photosynthesis and respiration (Frankignoulle and Distèche
1984; Invers et al. 1997; Hall-Spencer et al. 2008; Scartazza
et al. 2017), and a similar process is also exhibited by macroalgae dominated reef-tops (Russell et al. 2009).
Additionally, ocean acidification results in decreased carbon
to nitrogen (C:N) ratios in P. oceanica tissues, which
increases shoot density, leaf proteins, and asparagine accumulation in the rhizomes (Scartazza et al. 2017). This in turn
provides a positive contribution to associated food-webs
given the nutritional quality of organic matter available for
herbivores and consequently an increase in the grazing rate is
observed (Kroeker et al. 2010; Arnold et al. 2012; Rossoll
et al. 2012; Scartazza et al. 2017). However, the spatial scale
of these processes, ranging from millimeters to entire water
layers, must be kept in mind when extrapolating their impacts
to an ecosystem extent (Hendriks et al. 2015).
This enhanced productivity of seagrass meadows is likely
to contribute to enhanced productivity in neighboring coral
reef ecosystems on the tropical seascape. Modelling studies
suggests that calcification on coral reefs with seagrass neighbors may be up to 18% greater compared to reefs without
neighboring seagrasses (Unsworth et al. 2012). Their role in
enhancing calcification rates will continue and possibly even
increase (Zimmerman et al. 1997), allowing coral and invertebrate communities to persist (Unsworth et al. 2012; Garrard
et al. 2014). The term connectivity is primarily used in the
context of ocean acidification to discuss the disruption to
organismal reproduction, dispersal and hence, the connectivity of populations in a more acidic ocean (Cowen et al. 2006;
Gerber et al. 2014). Ocean acidification appears to exhibit an
especially strong capacity to drive ecological change and
hence its impacts are not straight forward in the bigger picture (Gaylord et al. 2015). The coupled responses create a
complex interplay among the physiological susceptibility of
organisms to ocean acidification, the availability of resources,
and the intensity of competition (Gaylord et al. 2015).
Models suggest that a decreasing ocean pH will impose additional physiological stresses to the global distribution of species, narrowing the breadth of the thermal performance curve
(Pörtner 2008). Ocean acidification effects would lead to
smaller overall ranges, and ranges for which equatorward
boundaries shift more dramatically towards poleward ones
(Gaylord et al. 2015). How species will respond within the
context of their communities is yet to be investigated.
However, it is almost certain that many of the most striking
consequences of acidification will arise through altered
biotic interactions (Fabricius et al. 2011; Falkenberg et al.
2013; Kroeker et al. 2013; McCormick et al. 2013).
In summary, primary producers like seagrass beds are a
crucial buffer zone of potential stressors for the calcifying
fauna of coral reefs, with which interactions seem to be key
for the resilience of many different species and even ecosystems in the face of environmental perturbations. With this in
mind a more interconnected approach needs to be taken into
consideration for tropical ecology under ocean acidification
(Fabry et al. 2008; Garrard et al. 2014). Similar to Gaylord
et al. (2015) and their argumentation that ocean acidification
needs to be seen not only in the individual but ecosystem
context, we argue that ecosystems need to be investigated in
a connected manner. It is unequivocal that this issue requires
global human action (Kennedy et al. 2013).
Summary
The evolution of mangroves, seagrasses, and coral reefs in
ever-changing environments has allowed them to form
highly-adapted, and for the most part, resilient ecosystems.
This resilience, however, is often facilitated by their connectivity to adjacent ecosystems. But within one generation,
anthropogenic activities and human-induced climate change
have exerted such pressures on these connectivity pathways
that a decline in ecosystem resilience and services has been
observed. Consequently, places on Earth previously considered refugia for a range of species may cease to exist. Perhaps
one of the most significant examples of these combined
stressors on tropical marine ecosystems occurred in the Red
Sea, where in the 1960s 98% of its coasts were considered to
be “in practically virgin condition” (Ormond 1987).
However, rapid development in this area, as a result of expansion in petroleum-based economies, meant the ‘virgin’ status
of many regions was lost (Gladstone 2008). Over 75% of
mangrove forests were degraded by activities including felling, cutting, sewage, root burial or overgrazing by camels
(Gladstone 2008), and coral reefs, especially those by
industrializing areas were impacted by dredging, sewage,
and tourism (Gladstone 2008). Further to these threats,
industrial trawling depleted economically important species
(Gladstone 2008).
The underlying cause of many of these activities, both in
the Red Sea and around the globe are: expanding populations, rapid urbanization, and weak governance, coupled
with a lack of baseline information on tropical marine ecoH. S. Earp et al.
Primary producers including seagrasses and macroalgae
appear to be the winners in the face of elevated oceanic CO 2
concentrations and lower seawater pH. Increased CO 2 concentrations in seawater are a resource for these primary producers (Palacios and Zimmerman 2007; Fabricius et al.
2011; Hepburn et al. 2011) which allow them to enhance
their productivity and growth (Harley et al. 2012; Koch et al.
2013). Seagrasses are known to alter the carbonate chemistry
in the water column, which is of particular importance in
regions where they neighbor coral reef environments
(Dorenbosch et al. 2005; Hendriks et al. 2014). In the
Mediterranean, Posidonia oceanica diurnally modify the
water column pH by as much as 0.2–0.7 units through photosynthesis and respiration (Frankignoulle and Distèche
1984; Invers et al. 1997; Hall-Spencer et al. 2008; Scartazza
et al. 2017), and a similar process is also exhibited by macroalgae dominated reef-tops (Russell et al. 2009).
Additionally, ocean acidification results in decreased carbon
to nitrogen (C:N) ratios in P. oceanica tissues, which
increases shoot density, leaf proteins, and asparagine accumulation in the rhizomes (Scartazza et al. 2017). This in turn
provides a positive contribution to associated food-webs
given the nutritional quality of organic matter available for
herbivores and consequently an increase in the grazing rate is
observed (Kroeker et al. 2010; Arnold et al. 2012; Rossoll
et al. 2012; Scartazza et al. 2017). However, the spatial scale
of these processes, ranging from millimeters to entire water
layers, must be kept in mind when extrapolating their impacts
to an ecosystem extent (Hendriks et al. 2015).
This enhanced productivity of seagrass meadows is likely
to contribute to enhanced productivity in neighboring coral
reef ecosystems on the tropical seascape. Modelling studies
suggests that calcification on coral reefs with seagrass neighbors may be up to 18% greater compared to reefs without
neighboring seagrasses (Unsworth et al. 2012). Their role in
enhancing calcification rates will continue and possibly even
increase (Zimmerman et al. 1997), allowing coral and invertebrate communities to persist (Unsworth et al. 2012; Garrard
et al. 2014). The term connectivity is primarily used in the
context of ocean acidification to discuss the disruption to
organismal reproduction, dispersal and hence, the connectivity of populations in a more acidic ocean (Cowen et al. 2006;
Gerber et al. 2014). Ocean acidification appears to exhibit an
especially strong capacity to drive ecological change and
hence its impacts are not straight forward in the bigger picture (Gaylord et al. 2015). The coupled responses create a
complex interplay among the physiological susceptibility of
organisms to ocean acidification, the availability of resources,
and the intensity of competition (Gaylord et al. 2015).
Models suggest that a decreasing ocean pH will impose additional physiological stresses to the global distribution of species, narrowing the breadth of the thermal performance curve
(Pörtner 2008). Ocean acidification effects would lead to
smaller overall ranges, and ranges for which equatorward
boundaries shift more dramatically towards poleward ones
(Gaylord et al. 2015). How species will respond within the
context of their communities is yet to be investigated.
However, it is almost certain that many of the most striking
consequences of acidification will arise through altered
biotic interactions (Fabricius et al. 2011; Falkenberg et al.
2013; Kroeker et al. 2013; McCormick et al. 2013).
In summary, primary producers like seagrass beds are a
crucial buffer zone of potential stressors for the calcifying
fauna of coral reefs, with which interactions seem to be key
for the resilience of many different species and even ecosystems in the face of environmental perturbations. With this in
mind a more interconnected approach needs to be taken into
consideration for tropical ecology under ocean acidification
(Fabry et al. 2008; Garrard et al. 2014). Similar to Gaylord
et al. (2015) and their argumentation that ocean acidification
needs to be seen not only in the individual but ecosystem
context, we argue that ecosystems need to be investigated in
a connected manner. It is unequivocal that this issue requires
global human action (Kennedy et al. 2013).
Summary
The evolution of mangroves, seagrasses, and coral reefs in
ever-changing environments has allowed them to form
highly-adapted, and for the most part, resilient ecosystems.
This resilience, however, is often facilitated by their connectivity to adjacent ecosystems. But within one generation,
anthropogenic activities and human-induced climate change
have exerted such pressures on these connectivity pathways
that a decline in ecosystem resilience and services has been
observed. Consequently, places on Earth previously considered refugia for a range of species may cease to exist. Perhaps
one of the most significant examples of these combined
stressors on tropical marine ecosystems occurred in the Red
Sea, where in the 1960s 98% of its coasts were considered to
be “in practically virgin condition” (Ormond 1987).
However, rapid development in this area, as a result of expansion in petroleum-based economies, meant the ‘virgin’ status
of many regions was lost (Gladstone 2008). Over 75% of
mangrove forests were degraded by activities including felling, cutting, sewage, root burial or overgrazing by camels
(Gladstone 2008), and coral reefs, especially those by
industrializing areas were impacted by dredging, sewage,
and tourism (Gladstone 2008). Further to these threats,
industrial trawling depleted economically important species
(Gladstone 2008).
The underlying cause of many of these activities, both in
the Red Sea and around the globe are: expanding populations, rapid urbanization, and weak governance, coupled
with a lack of baseline information on tropical marine ecoH. S. Earp et al.