between reef accretion and reef destruction. In fact, they
state (p. 389) “Because rates of carbonate production and
bioerosion are similar, even modest increases in nutrient
availability can shift a reef community from net production
to net erosion.” Their treatment of the role of nutrients
emphasizes light penetration, and its subsequent diminution
by phytoplankton blooms that are driven by nutrient
increases. With increasing input from runoff, they envisage
a “halo” of nutrients that precede the (usually siliciclastic)
sediment input that impacts reef biota, so that by the time
the reef is buried by sediment it is already dead or dying.
It is this unholy duo of reef stress, the inextricable
linking of sediment input and nutrient increase, that has
made the field such a difficult area of research.
That is not to deny the profound, fundamental effect
sediment supply and type has on reef development.
Roberts and Murray (1983) describe extensive carbonate
bank development off the coastline of Nicaragua, an area,
which receives copious amounts of rainfall and terrigenous sediment: 3 times more fresh water and 15 times
more sediment are introduced per unit length of coastline
than on the east coast of the USA. There is an inner
smooth-bottomed ramp of siliciclastic sediments parallel
to the coast: transition to Halimeda-rich bottom sediments
is abrupt, and takes place approximately 20 km off the
coast. In general, the frequency and proliferation of reefs
increase away from the turbid coastal boundary layer
and toward the cooler and saltier water that upwells at
the shelf margin. But again, nutrients cannot be excluded
from the picture. The lack of reefs along the Nicaraguan
Rise is at least partly attributed to nutrient increase driven
by topographic upwelling (Hallock, 2001).
Acevedo et al. (1989), working in Puerto Rico, reported
that increasing coastal development near Ponce, on the
south coast, had caused an increase in turbidity,
manifesting itself as a plume of sediment-laden water that
drifted to the west over a series of reefs. Their survey work
showed that total coral cover and diversity was reduced
near the sediment source. The surviving corals were
sediment-resistant. The effects of the sediment included
partial or total burial of corals, bleaching, and colonization
of the surface by filamentous cyanobacteria. Reduced
light levels resulted in domination of the remaining coral
community by species normally found at greater depthsfindings similar to those of Cortes and Risk (1985).
Descriptions from the Indo-Pacific of the effects of sediments on modern reefs are equally abundant, and equally
well established (Marshall and Orr, 1931). There are
descriptions of the effects of mining (Chansang et al.,
1981), as well as outlines of the general impact of sediments from activities such as dredging and increased runoff (Endean, 1976; Johannes, 1975; Fabricius, 2005).
Demonstrated effects of sediments on reefs
Decrease in coral growth rates
There have been many studies linking decreases in coral
growth rates with increased sediment stress, usually from
disturbances such as dredging and deforestation. Bak
(1978) found calcification rates decreased by 33%; Dodge
and Brass (1984) reported that corals from sites on reefs of
St. Croix, US Virgin Islands that had been disturbed by
dredging had lower extension and calcification rates than
corals from unaffected reefs. Dodge and Vaisnys (1977)
found that the growth rate of Montastrea annularis in
Jamaica decreased as a function of sediment resuspension
rates; interestingly, Cortes and Risk (1985) studied the
same species in Costa Rica. Their reported resuspension
rates were much higher than Dodge and Vaisnys (1977),
but produced the same linear relationship. This suggests
that coral species have the same responses to sedimentation in different locations.
Tomascik and Sander (1985) investigated the effects of
water quality on M. annularis growth rates off Barbados
over a gradient of increasing eutrophication (primary sewage, industrial effluent). They found that mean concentration of suspended particulate matter (mg I
À1 ) was the
strongest single estimator of growth rates, with the most
rapid coral growth occurring at the site farthest from the
primary pollution source.
On the other hand, there is differential species response
to similar sediment loadings. Large colonies, and/or species with thick tissues, are often relatively resistant,
whereas smaller colonies, and species with thin tissue
layers may be greatly affected (Rogers, 1990; StaffordSmith, 1993; Fabricius, 2005).
Care must be taken when interpreting growth rates
reported in the literature, especially in older articles. Coral
“growth rate” is not simply linear extension, but should be
seen as calcification rate, the rate at which corals can make
skeleton. This is the product of both extension rate and
density, and it must be noted here that Brown et al.
(1990), saw no detectable decrease in coral calcification
as a result of dredging.
Because sediment input and nutrient concentration frequently (usually) covary, corals are affected by two
conflicting factors: increased sediment input slows
growth, while at the same time individual corals may grow
faster under conditions of elevated nutrients. These same
corals will then face intense competition from algae and
non-calcified benthos, and bioerosion soon outstrips
accretion rates. This bifold effect of nutrients on coral
reefs has been called the “Janus Effect,” after the twofaced god of Roman mythology so familiar to theatregoers (Edinger and Risk, 1994).
Edinger et al. (2000b), working in Indonesia, were able
to tease apart the conflicting influences of nutrients and
sediment stress. Porites lobata coral extension rates
increased from oligotrophic to mesotrophic conditions,
then decreased again toward eutrophic and hypertrophic
conditions as turbidity from sediments and phytoplankton
reduced light penetration to corals (Figure 1). High variance in extension rates was observed on polluted Java Sea
reefs. They found that extension rates on polluted nearshore
reefs were high, and could not be statistically distinguished
from extension rates from cleaner offshore reefs, not
576
IMPACTS OF SEDIMENT ON CORAL REEFS
state (p. 389) “Because rates of carbonate production and
bioerosion are similar, even modest increases in nutrient
availability can shift a reef community from net production
to net erosion.” Their treatment of the role of nutrients
emphasizes light penetration, and its subsequent diminution
by phytoplankton blooms that are driven by nutrient
increases. With increasing input from runoff, they envisage
a “halo” of nutrients that precede the (usually siliciclastic)
sediment input that impacts reef biota, so that by the time
the reef is buried by sediment it is already dead or dying.
It is this unholy duo of reef stress, the inextricable
linking of sediment input and nutrient increase, that has
made the field such a difficult area of research.
That is not to deny the profound, fundamental effect
sediment supply and type has on reef development.
Roberts and Murray (1983) describe extensive carbonate
bank development off the coastline of Nicaragua, an area,
which receives copious amounts of rainfall and terrigenous sediment: 3 times more fresh water and 15 times
more sediment are introduced per unit length of coastline
than on the east coast of the USA. There is an inner
smooth-bottomed ramp of siliciclastic sediments parallel
to the coast: transition to Halimeda-rich bottom sediments
is abrupt, and takes place approximately 20 km off the
coast. In general, the frequency and proliferation of reefs
increase away from the turbid coastal boundary layer
and toward the cooler and saltier water that upwells at
the shelf margin. But again, nutrients cannot be excluded
from the picture. The lack of reefs along the Nicaraguan
Rise is at least partly attributed to nutrient increase driven
by topographic upwelling (Hallock, 2001).
Acevedo et al. (1989), working in Puerto Rico, reported
that increasing coastal development near Ponce, on the
south coast, had caused an increase in turbidity,
manifesting itself as a plume of sediment-laden water that
drifted to the west over a series of reefs. Their survey work
showed that total coral cover and diversity was reduced
near the sediment source. The surviving corals were
sediment-resistant. The effects of the sediment included
partial or total burial of corals, bleaching, and colonization
of the surface by filamentous cyanobacteria. Reduced
light levels resulted in domination of the remaining coral
community by species normally found at greater depthsfindings similar to those of Cortes and Risk (1985).
Descriptions from the Indo-Pacific of the effects of sediments on modern reefs are equally abundant, and equally
well established (Marshall and Orr, 1931). There are
descriptions of the effects of mining (Chansang et al.,
1981), as well as outlines of the general impact of sediments from activities such as dredging and increased runoff (Endean, 1976; Johannes, 1975; Fabricius, 2005).
Demonstrated effects of sediments on reefs
Decrease in coral growth rates
There have been many studies linking decreases in coral
growth rates with increased sediment stress, usually from
disturbances such as dredging and deforestation. Bak
(1978) found calcification rates decreased by 33%; Dodge
and Brass (1984) reported that corals from sites on reefs of
St. Croix, US Virgin Islands that had been disturbed by
dredging had lower extension and calcification rates than
corals from unaffected reefs. Dodge and Vaisnys (1977)
found that the growth rate of Montastrea annularis in
Jamaica decreased as a function of sediment resuspension
rates; interestingly, Cortes and Risk (1985) studied the
same species in Costa Rica. Their reported resuspension
rates were much higher than Dodge and Vaisnys (1977),
but produced the same linear relationship. This suggests
that coral species have the same responses to sedimentation in different locations.
Tomascik and Sander (1985) investigated the effects of
water quality on M. annularis growth rates off Barbados
over a gradient of increasing eutrophication (primary sewage, industrial effluent). They found that mean concentration of suspended particulate matter (mg I
À1 ) was the
strongest single estimator of growth rates, with the most
rapid coral growth occurring at the site farthest from the
primary pollution source.
On the other hand, there is differential species response
to similar sediment loadings. Large colonies, and/or species with thick tissues, are often relatively resistant,
whereas smaller colonies, and species with thin tissue
layers may be greatly affected (Rogers, 1990; StaffordSmith, 1993; Fabricius, 2005).
Care must be taken when interpreting growth rates
reported in the literature, especially in older articles. Coral
“growth rate” is not simply linear extension, but should be
seen as calcification rate, the rate at which corals can make
skeleton. This is the product of both extension rate and
density, and it must be noted here that Brown et al.
(1990), saw no detectable decrease in coral calcification
as a result of dredging.
Because sediment input and nutrient concentration frequently (usually) covary, corals are affected by two
conflicting factors: increased sediment input slows
growth, while at the same time individual corals may grow
faster under conditions of elevated nutrients. These same
corals will then face intense competition from algae and
non-calcified benthos, and bioerosion soon outstrips
accretion rates. This bifold effect of nutrients on coral
reefs has been called the “Janus Effect,” after the twofaced god of Roman mythology so familiar to theatregoers (Edinger and Risk, 1994).
Edinger et al. (2000b), working in Indonesia, were able
to tease apart the conflicting influences of nutrients and
sediment stress. Porites lobata coral extension rates
increased from oligotrophic to mesotrophic conditions,
then decreased again toward eutrophic and hypertrophic
conditions as turbidity from sediments and phytoplankton
reduced light penetration to corals (Figure 1). High variance in extension rates was observed on polluted Java Sea
reefs. They found that extension rates on polluted nearshore
reefs were high, and could not be statistically distinguished
from extension rates from cleaner offshore reefs, not
576
IMPACTS OF SEDIMENT ON CORAL REEFS