appear to reflect adaptations to sediment stress (Bak and
Elgershuizen, 1976). Logan (1988) suggested that the frequently observed orientation of the large calyx in Scolymia
cubensis at a steep angle from the horizontal may be an
adaptation, which facilitates sediment removal. Under high
sedimentation conditions, massive coral colonies often take
on a multilobate morphology, which allows the coral colony
to continue extending upward in some portions, while other
portions of the colony grow slowly or die altogether.
Reduced species richness and diversity
A reduction in the number of species of coral, coral cover,
and the diversity of the coral assemblage seems to be virtually universal results of sediment stress. Such results
have been reported by many authors, and only a few
examples can be given here: Caribbean-Costa Rica,
Cortes and Risk (1985); Puerto Rico, Acevedo and
Morelock (1988); Virgin Islands, Rogers (1983); Bermuda, Dryer and Logan (1978); Indian Ocean-Kenya,
Obura et al. (2000); Arabian Gulf, Sheppard et al.
(2000); Madagascar, Gabrié et al. (2000).
It is perhaps in Southeast Asia, where major rivers,
abundant reefs, impoverished coastal populations, and
rampant development all converge, that perhaps the most
depressing examples of sediment-driven reef degradation
may be found: Philippines, Hodgson and Dixon (1988);
Singapore, Chou (1997), Dikou and van Woesik (2006);
Thailand, Chansang et al. (1981).
Indonesia has more coral reefs than any other nation,
and also some of the most badly damaged reefs. Edinger
et al. (1998) described the trajectories of decline of reefs
from several areas near Java, Sulawesi, and Ambon
(Moluccas), and outline the impacts on diversity from sediments, nutrients, and direct human impacts such as anchor
damage. Combined sedimentation and eutrophication
caused greater change in species composition than either
factor alone, or than mechanically induced stresses such
as blast fishing or anchor damage (Edinger et al., 2000a).
Science writing is supposed to be dry and objective, yet it
is hard to read the description, by Tomascik et al. (1994),
of the decline of the reefs in Jakarta Bay without feeling it
is a modern tragedy. They quote Umbgrove (1939), writing
about the reefs of Nyamuk Besar (“Big Mosquito” in
Bahasa Indonesia; Leiden, to the Dutch residents): “The
unrivaled splendor and wealth of forms and the delicate tints
of the coral structures, the brilliant colors of fishes, clams,
sea anemones, worms, crabs, star fishes and the whole rest
of the reef animals are so attractive and interesting that it
seems impossible to give an adequate description of such
a profusion of serene and fascinating beauty.”
Those reefs have now disappeared. Tomascik et al.
describe the disappearance of entire reef tracts in Jakarta
Bay and the adjacent ocean waters, driven by the
onslaught of sediments and nutrients from the city. They
go on to say (p. 309): “Jakarta Bay is not an isolated case
of reef degradation, which is now a worldwide phenomenon. . .monitoring. . .will not prevent the demise of coastal
reef ecosystems, but ecologically sound management
practices and coastal development based on sustainable
principles just might.”
Reduced larval settlement
As might be expected, the early stages of coral development, the processes of larval settlement and development,
are strongly affected by sediment in the water column.
Tomascik and Sander (1987) suggested that lower light
levels may inhibit development of coral larvae by reducing the amount of energy available to maturing ova or
embryos. They found reduced numbers of larvae from colonies of P. porites growing on reefs polluted by nutrients
and suspended particulate matter with increasing levels
of sedimentation and eutrophication.
Sedimentation mortality thresholds for coral recruits
are an order of magnitude lower than those for larger colonies (Fabricius et al., 2003). Few coral larvae settle on
sediment-covered surfaces, and survival on such surfaces
is minimal. At moderate to high rates of sedimentation,
successful larval settlement is restricted to downwardfacing surfaces where growth and survival are negatively
affected by low light.
In short, there is a long list of perceived impacts on
corals that have been attributed to the effects of sediments.
At present, however, these are impacts in search of mechanisms. The field requires more research on the specific
modes of action, by which sediment stress achieves the
observed results.
Mode of action of sediments
There are basically three avenues that have been suggested
and by which sediments can stress corals:
Decreasing the available light
The logic behind this hypothesis is that decreased light
levels would reduce the amount of energy translocated
from the zooxanthellae to the host, resulting in slower
growth. Rogers (1979) shaded corals in the field, and
observed partial mortality. Several other authors have also
observed the decrease in coral vigor, which may be produced by decreased light levels (Telesnicki and Goldberg,
1995; Anthony and Hoegh-Guldberg, 2004). Furthermore, fine-grained siliciclastic sediments reduce light penetration more than equivalent concentrations of carbonate
sediments (Te, 1997).
Metabolic drain
When sediments land on corals, the corals usually try to
clean themselves by ciliary action, tentacle waving, or
mucus sheet secretion. These responses on the part of
the coral suggest that there is a metabolic cost to the
corals involved in sediment deposition (Telesnicki and
Goldberg, 1995). Corals have a variety of mechanisms for
coping with sediments including use of their tentacles and
cilia, stomodeal distension through uptake of water, and
entanglement of particles in mucus, which later sloughs
578
IMPACTS OF SEDIMENT ON CORAL REEFS
Elgershuizen, 1976). Logan (1988) suggested that the frequently observed orientation of the large calyx in Scolymia
cubensis at a steep angle from the horizontal may be an
adaptation, which facilitates sediment removal. Under high
sedimentation conditions, massive coral colonies often take
on a multilobate morphology, which allows the coral colony
to continue extending upward in some portions, while other
portions of the colony grow slowly or die altogether.
Reduced species richness and diversity
A reduction in the number of species of coral, coral cover,
and the diversity of the coral assemblage seems to be virtually universal results of sediment stress. Such results
have been reported by many authors, and only a few
examples can be given here: Caribbean-Costa Rica,
Cortes and Risk (1985); Puerto Rico, Acevedo and
Morelock (1988); Virgin Islands, Rogers (1983); Bermuda, Dryer and Logan (1978); Indian Ocean-Kenya,
Obura et al. (2000); Arabian Gulf, Sheppard et al.
(2000); Madagascar, Gabrié et al. (2000).
It is perhaps in Southeast Asia, where major rivers,
abundant reefs, impoverished coastal populations, and
rampant development all converge, that perhaps the most
depressing examples of sediment-driven reef degradation
may be found: Philippines, Hodgson and Dixon (1988);
Singapore, Chou (1997), Dikou and van Woesik (2006);
Thailand, Chansang et al. (1981).
Indonesia has more coral reefs than any other nation,
and also some of the most badly damaged reefs. Edinger
et al. (1998) described the trajectories of decline of reefs
from several areas near Java, Sulawesi, and Ambon
(Moluccas), and outline the impacts on diversity from sediments, nutrients, and direct human impacts such as anchor
damage. Combined sedimentation and eutrophication
caused greater change in species composition than either
factor alone, or than mechanically induced stresses such
as blast fishing or anchor damage (Edinger et al., 2000a).
Science writing is supposed to be dry and objective, yet it
is hard to read the description, by Tomascik et al. (1994),
of the decline of the reefs in Jakarta Bay without feeling it
is a modern tragedy. They quote Umbgrove (1939), writing
about the reefs of Nyamuk Besar (“Big Mosquito” in
Bahasa Indonesia; Leiden, to the Dutch residents): “The
unrivaled splendor and wealth of forms and the delicate tints
of the coral structures, the brilliant colors of fishes, clams,
sea anemones, worms, crabs, star fishes and the whole rest
of the reef animals are so attractive and interesting that it
seems impossible to give an adequate description of such
a profusion of serene and fascinating beauty.”
Those reefs have now disappeared. Tomascik et al.
describe the disappearance of entire reef tracts in Jakarta
Bay and the adjacent ocean waters, driven by the
onslaught of sediments and nutrients from the city. They
go on to say (p. 309): “Jakarta Bay is not an isolated case
of reef degradation, which is now a worldwide phenomenon. . .monitoring. . .will not prevent the demise of coastal
reef ecosystems, but ecologically sound management
practices and coastal development based on sustainable
principles just might.”
Reduced larval settlement
As might be expected, the early stages of coral development, the processes of larval settlement and development,
are strongly affected by sediment in the water column.
Tomascik and Sander (1987) suggested that lower light
levels may inhibit development of coral larvae by reducing the amount of energy available to maturing ova or
embryos. They found reduced numbers of larvae from colonies of P. porites growing on reefs polluted by nutrients
and suspended particulate matter with increasing levels
of sedimentation and eutrophication.
Sedimentation mortality thresholds for coral recruits
are an order of magnitude lower than those for larger colonies (Fabricius et al., 2003). Few coral larvae settle on
sediment-covered surfaces, and survival on such surfaces
is minimal. At moderate to high rates of sedimentation,
successful larval settlement is restricted to downwardfacing surfaces where growth and survival are negatively
affected by low light.
In short, there is a long list of perceived impacts on
corals that have been attributed to the effects of sediments.
At present, however, these are impacts in search of mechanisms. The field requires more research on the specific
modes of action, by which sediment stress achieves the
observed results.
Mode of action of sediments
There are basically three avenues that have been suggested
and by which sediments can stress corals:
Decreasing the available light
The logic behind this hypothesis is that decreased light
levels would reduce the amount of energy translocated
from the zooxanthellae to the host, resulting in slower
growth. Rogers (1979) shaded corals in the field, and
observed partial mortality. Several other authors have also
observed the decrease in coral vigor, which may be produced by decreased light levels (Telesnicki and Goldberg,
1995; Anthony and Hoegh-Guldberg, 2004). Furthermore, fine-grained siliciclastic sediments reduce light penetration more than equivalent concentrations of carbonate
sediments (Te, 1997).
Metabolic drain
When sediments land on corals, the corals usually try to
clean themselves by ciliary action, tentacle waving, or
mucus sheet secretion. These responses on the part of
the coral suggest that there is a metabolic cost to the
corals involved in sediment deposition (Telesnicki and
Goldberg, 1995). Corals have a variety of mechanisms for
coping with sediments including use of their tentacles and
cilia, stomodeal distension through uptake of water, and
entanglement of particles in mucus, which later sloughs
578
IMPACTS OF SEDIMENT ON CORAL REEFS