128
contemporary ecosystems are failing to regenerate following
disturbances (Bellwood et al. 2004). The global loss and fragmentation of mangrove forests equates to a loss of ecosystem
services worth US$ 7.2 trillion year
−1
(Costanza et al. 2014).
Other studies have noted significant global declines in seagrass areas, at rates of 110 km
2
year
−1
since 1980, meaning at
least 29% of their known areal extent has been lost (Green
and Short 2003; Waycott et al. 2009; Short et al. 2014). In
terms of coral reefs, estimates show ~19% of the world’s
reefs have been lost (Wilkinson 2008), with 75% of present
day reefs considered threatened when climatic and anthropogenic threats are combined (Burke et al. 2011), and 20% of
these are expected to disappear within 20 years (Wilkinson
2008). Furthermore, 55% of coral reef fisheries in 49 island
countries are considered as unsustainable (Newton et al.
2007). Overfishing threatens reef health by causing trophic
cascades that may induce phase-shifts to macroalgal dominated environments, which subsequently impacts adjacent
ecosystems and cross-ecosystem interactions (Jackson et al.
2001). Modelling studies compliment this research, revealing
that impacts on one ecosystem can have profound impacts on
neighboring ecosystems, and in turn, the ecosystem services
they provide (Saunders et al. 2014).
Despite the overwhelming evidence that loss and degradation of these vital marine ecosystems will have far reaching
ecological and economic impacts, significant gaps in our
knowledge regarding the interconnectivity between these ecosystems remain (Duarte et al. 2008). Appealing to scientific
research efforts, this review provides an overview of the
impacts of augmenting anthropogenic activities, and human
induced climate on the known interconnectivity pathways
amongst tropical marine ecosystems, as opposed to each ecosystem in isolation. Sections “A Nutritious Ocean” and “An
Empty Ocean” explore the response of cross-ecosystem interactions and ecosystems services to anthropogenic activities in
the form of eutrophication and exploitation successively,
whilst sections “A Warmer Ocean” and “A Sour Ocean” investigate their responses to ocean warming and acidification consecutively. Understanding the threats facing interdependencies
between these ecosystems is suggested to be an opportunity
for science to prevent large-scale losses of these critical environments in the face of disturbances in the years to come.
A Nutritious Ocean
Mangroves, seagrasses, and coral reefs are located either on,
or near land masses (Spalding et al. 2001; Green and Short
2003), exposing them to local anthropogenic threats including periodic fertilizer runoff and sewage discharge, which
are delivered to coastal waters (Fabricius 2005; Burke et al.
2011). This process, known as eutrophication, can stimulate
phytoplankton blooms and algal growth in coastal ecosystems (McGlathery et al. 2007), which can lead to anoxia and
toxic sulphide production due to increased microbial activity
degrading this additional biomass (Flindt et al. 1999;
Herbeck et al. 2014). These periodic enrichment events have
become more prevalent within the last five decades, as annual
global usage of nitrogen fertilizers has increased 14-fold,
and is expected to increase even further (Matson et al. 1997;
FAO 2016a).
Eutrophication can impact coastal ecosystems either
directly, by affecting the fitness of organisms, or indirectly,
by affecting processes within the ecosystem or altering the
connectivity between ecosystems. In tropical regions, such
as the Great Barrier Reef, seagrasses and mangroves are
nutrient limited (Schaffelke et al. 2005). Therefore, the most
common direct effect of nutrient enrichment is an increase in
productivity and growth of these marine plants (Schaffelke
et al. 2005), which alone is a positive effect. These ecosystems can therefore buffer eutrophication to a certain extent,
and protect the oligotrophic waters of their vulnerable neighbors, coral reefs, from nutrient enrichment (Kitheka 1997;
McGlathery et al. 2007). Indirect effects of eutrophication on
marine plant communities are more commonplace, as excess
nutrients also increase the productivity of other competing
autotrophs, namely algae (Schaffelke and Klumpp 1998;
McGlathery et al. 2007). In mangroves, there is little evidence of the direct effects of excess nutrients, however indirect links to mangrove dieback and damage do exist. The
dieback of Avicennia marina in southern Australia was indirectly linked to eutrophication through the increased proliferation of the green macroalgae, Ulva sp., which smothered
and killed the aerial roots of established mangroves, as well
as smothering and inhibiting the growth of mangrove seedlings (Fig. 3) (Schaffelke et al. 2005). As mangroves have a
high nutrient uptake capacity, they are also at risk of taking
up herbicides and heavy metals which run-off agricultural
land together with the nutrients. The build-up of these toxic
substance has also been linked to mangrove dieback and
damage in downstream estuarine habitats (Schaffelke et al.
2005).
Algal blooms may be even more detrimental for seagrasses
as they are completely submerged in water and are highly
dependent on light availability and water quality (McGlathery
2001). Light availability can be reduced by both biotic and
abiotic factors. Biotic factors are primarily based on the abundance of phytoplankton, epiphytic algae, and seaweed wracks
(Fig. 3) (McGlathery 2001; Herbeck et al. 2014; van
Tussenbroek et al. 2017), whilst abiotic factors include
increased particle loads from sewage effluent which settle on
seagrass leaves or attenuate light within the water column
(Herbeck et al. 2014). The increase in algal biomass will be
followed by faster decomposition rates (Flindt et al. 1999),
and therefore increased sulphide production in the sediment
(Herbeck et al. 2014) (Fig. 3). Sulphide is toxic for seagrasses
and leads to a decrease in shoot density, rhizome extension,
and growth (Díaz-Almela et al. 2008; Herbeck et al. 2014;
H. S. Earp et al.
contemporary ecosystems are failing to regenerate following
disturbances (Bellwood et al. 2004). The global loss and fragmentation of mangrove forests equates to a loss of ecosystem
services worth US$ 7.2 trillion year
−1
(Costanza et al. 2014).
Other studies have noted significant global declines in seagrass areas, at rates of 110 km
2
year
−1
since 1980, meaning at
least 29% of their known areal extent has been lost (Green
and Short 2003; Waycott et al. 2009; Short et al. 2014). In
terms of coral reefs, estimates show ~19% of the world’s
reefs have been lost (Wilkinson 2008), with 75% of present
day reefs considered threatened when climatic and anthropogenic threats are combined (Burke et al. 2011), and 20% of
these are expected to disappear within 20 years (Wilkinson
2008). Furthermore, 55% of coral reef fisheries in 49 island
countries are considered as unsustainable (Newton et al.
2007). Overfishing threatens reef health by causing trophic
cascades that may induce phase-shifts to macroalgal dominated environments, which subsequently impacts adjacent
ecosystems and cross-ecosystem interactions (Jackson et al.
2001). Modelling studies compliment this research, revealing
that impacts on one ecosystem can have profound impacts on
neighboring ecosystems, and in turn, the ecosystem services
they provide (Saunders et al. 2014).
Despite the overwhelming evidence that loss and degradation of these vital marine ecosystems will have far reaching
ecological and economic impacts, significant gaps in our
knowledge regarding the interconnectivity between these ecosystems remain (Duarte et al. 2008). Appealing to scientific
research efforts, this review provides an overview of the
impacts of augmenting anthropogenic activities, and human
induced climate on the known interconnectivity pathways
amongst tropical marine ecosystems, as opposed to each ecosystem in isolation. Sections “A Nutritious Ocean” and “An
Empty Ocean” explore the response of cross-ecosystem interactions and ecosystems services to anthropogenic activities in
the form of eutrophication and exploitation successively,
whilst sections “A Warmer Ocean” and “A Sour Ocean” investigate their responses to ocean warming and acidification consecutively. Understanding the threats facing interdependencies
between these ecosystems is suggested to be an opportunity
for science to prevent large-scale losses of these critical environments in the face of disturbances in the years to come.
A Nutritious Ocean
Mangroves, seagrasses, and coral reefs are located either on,
or near land masses (Spalding et al. 2001; Green and Short
2003), exposing them to local anthropogenic threats including periodic fertilizer runoff and sewage discharge, which
are delivered to coastal waters (Fabricius 2005; Burke et al.
2011). This process, known as eutrophication, can stimulate
phytoplankton blooms and algal growth in coastal ecosystems (McGlathery et al. 2007), which can lead to anoxia and
toxic sulphide production due to increased microbial activity
degrading this additional biomass (Flindt et al. 1999;
Herbeck et al. 2014). These periodic enrichment events have
become more prevalent within the last five decades, as annual
global usage of nitrogen fertilizers has increased 14-fold,
and is expected to increase even further (Matson et al. 1997;
FAO 2016a).
Eutrophication can impact coastal ecosystems either
directly, by affecting the fitness of organisms, or indirectly,
by affecting processes within the ecosystem or altering the
connectivity between ecosystems. In tropical regions, such
as the Great Barrier Reef, seagrasses and mangroves are
nutrient limited (Schaffelke et al. 2005). Therefore, the most
common direct effect of nutrient enrichment is an increase in
productivity and growth of these marine plants (Schaffelke
et al. 2005), which alone is a positive effect. These ecosystems can therefore buffer eutrophication to a certain extent,
and protect the oligotrophic waters of their vulnerable neighbors, coral reefs, from nutrient enrichment (Kitheka 1997;
McGlathery et al. 2007). Indirect effects of eutrophication on
marine plant communities are more commonplace, as excess
nutrients also increase the productivity of other competing
autotrophs, namely algae (Schaffelke and Klumpp 1998;
McGlathery et al. 2007). In mangroves, there is little evidence of the direct effects of excess nutrients, however indirect links to mangrove dieback and damage do exist. The
dieback of Avicennia marina in southern Australia was indirectly linked to eutrophication through the increased proliferation of the green macroalgae, Ulva sp., which smothered
and killed the aerial roots of established mangroves, as well
as smothering and inhibiting the growth of mangrove seedlings (Fig. 3) (Schaffelke et al. 2005). As mangroves have a
high nutrient uptake capacity, they are also at risk of taking
up herbicides and heavy metals which run-off agricultural
land together with the nutrients. The build-up of these toxic
substance has also been linked to mangrove dieback and
damage in downstream estuarine habitats (Schaffelke et al.
2005).
Algal blooms may be even more detrimental for seagrasses
as they are completely submerged in water and are highly
dependent on light availability and water quality (McGlathery
2001). Light availability can be reduced by both biotic and
abiotic factors. Biotic factors are primarily based on the abundance of phytoplankton, epiphytic algae, and seaweed wracks
(Fig. 3) (McGlathery 2001; Herbeck et al. 2014; van
Tussenbroek et al. 2017), whilst abiotic factors include
increased particle loads from sewage effluent which settle on
seagrass leaves or attenuate light within the water column
(Herbeck et al. 2014). The increase in algal biomass will be
followed by faster decomposition rates (Flindt et al. 1999),
and therefore increased sulphide production in the sediment
(Herbeck et al. 2014) (Fig. 3). Sulphide is toxic for seagrasses
and leads to a decrease in shoot density, rhizome extension,
and growth (Díaz-Almela et al. 2008; Herbeck et al. 2014;
H. S. Earp et al.