130
processes and diversity (Fig. 3). Furthermore, a recent study
found that the prevalence of coral disease was 50% less on
reefs with adjacent seagrass meadows, compared to reefs
without (Lamb et al. 2017). Coral tissue mortality caused by
bleaching and sedimentation was also significantly less on
reefs with neighboring seagrass beds (Lamb et al. 2017).
This is due to the role seagrasses play in filtering our pathogenic bacteria, and thus, in the face of continued sewage outflow in some regions, we can expect elevated bacterial and
disease prevalence on neighboring coral reefs (Lamb et al.
2017) (Fig. 3). However, in other regions, seagrass meadow
functions are not as vulnerable or important to ecosystem
connectivity. In Brazil, evidence shows that macroalgal beds
serve as a better nursery area for juvenile fish than adjacent
seagrass beds, and that these seagrass beds had much lower
juvenile fish abundances compared similar beds in the IndoPacific or Caribbean (Eggertsen et al. 2017). That being said,
the negative effects of eutrophication across the rest of the
world far outweigh this sole positive scenario.
Eutrophication of coral reefs environments, can promote
phytoplankton blooms and thereby increase developmental
success of the coral-consuming crown-of-thorns starfish
(Acanthaster planci) larvae, which feed on phytoplankton
(Fabricius et al. 2010; De’ath et al. 2012). Nutrient enrichment can also, as previously mentioned, enhance the growth
and productivity of macroalgae (Schaffelke and Klumpp
1998). Once established on a coral reef, macroalgae may
continue to proliferate if nutrients are available and herbivory is limited. Ultimately, this can result in a phase-shift
(Lapointe 1997; McCook 1999) (Fig. 3). In the case of some
South Pacific islands, the range and abundance of two native
macroalgal species, Turbinaria ornata and Sargassum pacificum, have increased noticeably throughout the reefs since
the 1980’s (Payri and Naim 1982; Stewart 2008). These
algae are primarily found on dead patches of corals on top of
Porites heads, where they form dense aggregations (Stewart
2006). High swells frequently remove these macroalgae
from their substrate, resulting in masses of floating algae
which aggregate in currents to form large compact seaweed
wracks (Zubia et al. 2015). These wracks are also seen as
communities of drift algae such as Sargassum fluitans and
Sargassum natans in the Caribbean. However, when exceptionally large blooms of these wracks reach nearby ecosystems such as seagrasses, they eutrophicate them through
decomposition, reduction of light availability, increased
hypoxia/anoxia, and reduction of pH (van Tussenbroek et al.
2017) (Fig. 3). We can therefore expect that these wracks
stimulate algal growth inside the meadows, leading to seagrass loss, and inhibiting seagrass recovery (van Tussenbroek
et al. 2017).
To summarize the effects of eutrophication on ecosystems
within the tropical seascape, the effects on mangroves are not
well documented, probably because they have a large capacity to absorb nutrients and are not dependent on water clarity
to survive. Seagrass meadows can also buffer nutrient enrichment, but may be more vulnerable than mangroves as they are
completely submerged and their survival depends on light
availability. The greatest impacts of eutrophication are seen
on coral reefs, which are the most vulnerable to excess nutrients, but may not be as exposed when seagrass meadows and
mangrove forest are in the vicinity. With a loss of ecosystem
functions in these ecosystems as a result of nutrient enrichment, we can expect that they will no longer be able to provide important services for each other, such as; nursery
grounds, habitats, feeding grounds for mobile fauna, nutrient
and sediment retention, and export of biomass (Fig. 3).
Nutrients and herbivory are two well-connected concepts
in marine ecosystems, and in most cases, phase-shifts are not
attributed to one or the other but rather a combination of the
two (Adam et al. 2015). That is, eutrophication by itself is
rarely the only reason why a system experiences algal
blooms, as it is also highly dependent on grazing pressure
from consumers (Hughes 1994). The health of these ecosystems is therefore not only dependent on what we add to them,
but also what we remove, through the harvesting mangroves,
corals, fish, and invertebrates.
An Empty Ocean
Human existence is directly and indirectly dependent on
marine ecosystems (Halpern et al. 2008). In the tropics, millions of people rely directly on marine ecosystems to harvest
food (e.g., fishes, clams, crabs) and raw materials (e.g., timber, curio artefacts, medicinal products), either for subsistence purposes, or for their livelihood (Hoegh-Guldberg
2014). As a consequence, these ecosystems are experiencing
accelerating losses of biodiversity with largely unknown
consequences (Worm et al. 2006). With over 1.3 billion people residing along tropical coastlines, primarily in developing countries (Sale et al. 2014), it is important to understand
the impacts of harvesting activities within the tropical seascape, and its subsequent consequences for ecosystem services upon which so many people rely. This section will
investigate the impact of mangrove harvesting for raw materials and the impact of fishing on the functioning and connectivity amongst tropical marine ecosystems.
Mangrove use by humans has a long history, extending
back over 7000 years (Spalding et al. 2010; Tomlinson
2016), as a diverse array of goods can be harvested from
them, including; tannins, honey, medicinal products, thatch,
timber, and firewood (Hamilton and Snedaker 1984; Ellison
1994; Kathiresan and Bingham 2001; Spalding et al. 2010).
The physical properties of mangroves differ amongst species; Rhizophora is most widely harvested due to its hard,
dense, easily-splitting wood which makes it an ideal material
H. S. Earp et al.
processes and diversity (Fig. 3). Furthermore, a recent study
found that the prevalence of coral disease was 50% less on
reefs with adjacent seagrass meadows, compared to reefs
without (Lamb et al. 2017). Coral tissue mortality caused by
bleaching and sedimentation was also significantly less on
reefs with neighboring seagrass beds (Lamb et al. 2017).
This is due to the role seagrasses play in filtering our pathogenic bacteria, and thus, in the face of continued sewage outflow in some regions, we can expect elevated bacterial and
disease prevalence on neighboring coral reefs (Lamb et al.
2017) (Fig. 3). However, in other regions, seagrass meadow
functions are not as vulnerable or important to ecosystem
connectivity. In Brazil, evidence shows that macroalgal beds
serve as a better nursery area for juvenile fish than adjacent
seagrass beds, and that these seagrass beds had much lower
juvenile fish abundances compared similar beds in the IndoPacific or Caribbean (Eggertsen et al. 2017). That being said,
the negative effects of eutrophication across the rest of the
world far outweigh this sole positive scenario.
Eutrophication of coral reefs environments, can promote
phytoplankton blooms and thereby increase developmental
success of the coral-consuming crown-of-thorns starfish
(Acanthaster planci) larvae, which feed on phytoplankton
(Fabricius et al. 2010; De’ath et al. 2012). Nutrient enrichment can also, as previously mentioned, enhance the growth
and productivity of macroalgae (Schaffelke and Klumpp
1998). Once established on a coral reef, macroalgae may
continue to proliferate if nutrients are available and herbivory is limited. Ultimately, this can result in a phase-shift
(Lapointe 1997; McCook 1999) (Fig. 3). In the case of some
South Pacific islands, the range and abundance of two native
macroalgal species, Turbinaria ornata and Sargassum pacificum, have increased noticeably throughout the reefs since
the 1980’s (Payri and Naim 1982; Stewart 2008). These
algae are primarily found on dead patches of corals on top of
Porites heads, where they form dense aggregations (Stewart
2006). High swells frequently remove these macroalgae
from their substrate, resulting in masses of floating algae
which aggregate in currents to form large compact seaweed
wracks (Zubia et al. 2015). These wracks are also seen as
communities of drift algae such as Sargassum fluitans and
Sargassum natans in the Caribbean. However, when exceptionally large blooms of these wracks reach nearby ecosystems such as seagrasses, they eutrophicate them through
decomposition, reduction of light availability, increased
hypoxia/anoxia, and reduction of pH (van Tussenbroek et al.
2017) (Fig. 3). We can therefore expect that these wracks
stimulate algal growth inside the meadows, leading to seagrass loss, and inhibiting seagrass recovery (van Tussenbroek
et al. 2017).
To summarize the effects of eutrophication on ecosystems
within the tropical seascape, the effects on mangroves are not
well documented, probably because they have a large capacity to absorb nutrients and are not dependent on water clarity
to survive. Seagrass meadows can also buffer nutrient enrichment, but may be more vulnerable than mangroves as they are
completely submerged and their survival depends on light
availability. The greatest impacts of eutrophication are seen
on coral reefs, which are the most vulnerable to excess nutrients, but may not be as exposed when seagrass meadows and
mangrove forest are in the vicinity. With a loss of ecosystem
functions in these ecosystems as a result of nutrient enrichment, we can expect that they will no longer be able to provide important services for each other, such as; nursery
grounds, habitats, feeding grounds for mobile fauna, nutrient
and sediment retention, and export of biomass (Fig. 3).
Nutrients and herbivory are two well-connected concepts
in marine ecosystems, and in most cases, phase-shifts are not
attributed to one or the other but rather a combination of the
two (Adam et al. 2015). That is, eutrophication by itself is
rarely the only reason why a system experiences algal
blooms, as it is also highly dependent on grazing pressure
from consumers (Hughes 1994). The health of these ecosystems is therefore not only dependent on what we add to them,
but also what we remove, through the harvesting mangroves,
corals, fish, and invertebrates.
An Empty Ocean
Human existence is directly and indirectly dependent on
marine ecosystems (Halpern et al. 2008). In the tropics, millions of people rely directly on marine ecosystems to harvest
food (e.g., fishes, clams, crabs) and raw materials (e.g., timber, curio artefacts, medicinal products), either for subsistence purposes, or for their livelihood (Hoegh-Guldberg
2014). As a consequence, these ecosystems are experiencing
accelerating losses of biodiversity with largely unknown
consequences (Worm et al. 2006). With over 1.3 billion people residing along tropical coastlines, primarily in developing countries (Sale et al. 2014), it is important to understand
the impacts of harvesting activities within the tropical seascape, and its subsequent consequences for ecosystem services upon which so many people rely. This section will
investigate the impact of mangrove harvesting for raw materials and the impact of fishing on the functioning and connectivity amongst tropical marine ecosystems.
Mangrove use by humans has a long history, extending
back over 7000 years (Spalding et al. 2010; Tomlinson
2016), as a diverse array of goods can be harvested from
them, including; tannins, honey, medicinal products, thatch,
timber, and firewood (Hamilton and Snedaker 1984; Ellison
1994; Kathiresan and Bingham 2001; Spalding et al. 2010).
The physical properties of mangroves differ amongst species; Rhizophora is most widely harvested due to its hard,
dense, easily-splitting wood which makes it an ideal material
H. S. Earp et al.