et al. (1994) and Sammarco and Risk (1999), using stable
isotopic tracers of metabolic activity, have shown that nearshore corals of at least some species can obtain much of
their nutrition from terrestrial sources. Analysis of d
13
C
and d
15
N of P. lobata coral tissue of Indo-Pacific reefs subject to combined sedimentation and eutrophication showed
the influence of both enhanced algal photosynthesis, fueled
by increased inorganic nutrient availability, and heterotrophy, fueled by increased dissolved or particulate organic
matter availability. The relative enhancement of these different processes, in differing proportions, depends on the
relative influences of turbidity, organic matter loading,
and inorganic nutrient loading.
Recent research on the fringing reef of Molokai,
Hawai’i by the USGS has set the bar for sediment research
in coral environments. As a selection from this large body
of work, Bothner et al. (2006) employed very sophisticated sediment traps, and were able to conclude that sediment input to the reef was episodic, driven by storms.
What is more, sediment traps were found not to be good
indicators of net sediment accumulation. Presto et al.
(2006) studied the temporal and spatial variability of flow
in suspended sediments on the reef flat, and found that
most of the sediment was transported alongshore. Regions
where sediment moved offshore were regions of low coral
cover. Calhoun and Field (2008) studied the composition
of grain types from the beaches and reef environments,
and were able to deduce transport directions. And finally,
much of this research is summarized in the beautifully
illustrated USGS report (Field et al., 2008).
Guidelines – how much sedimentation is too much?
In short, development of an integrated set of guidelines for
sediment resistance of reefs has been hampered by lack of
agreement on methodology, lack of precision when dealing with the sediments themselves, the confounded effects
of increased sediment input and increased nutrients, and
the plasticity of coral behavior.
It is highly unlikely that there can ever be one set of regulations, or guidelines, developed for all reefs that could
control impacts of sediment (cf. Pastorok and Bilyard,
1985), nor is a global set of water quality guidelines necessarily desirable. It would seem that most Caribbean reefs
have difficulty surviving SPM loads that are consistently
in excess of 10 mg/L, and indeed it would seem appropriate to incorporate that level into regulatory guidelines.
Reefs in other locales, for example, the northern Great
Barrier Reef, will have adapted and evolved to survive
very different conditions (Hopley et al., 1990, 2007).
Corals near the mouth of the Daintree River flourish under
conditions that would slay a Caribbean reef (cf. Perry and
Larcombe, 2003). Within each region, corals have adapted
to long-term regional variation in normal sedimentation
rates and patterns. High-ambient sedimentation levels are
not inherently stressful; rather, it is the change in the rates
of sedimentation that causes stress for coral reefs
(Fabricius, 2005; Hopley et al., 2007).
Even recognizing that change in sedimentation rate,
rather than absolute sedimentation rate, is crucial for coral
reefs (cf. Dikou and van Woesik, 2006), it is unlikely that
a single critical value for percentage increase in sedimentation can be identified for all reefs. Sedimentation in
many environments is highly episodic, and corals living
in areas of high ambient sedimentation can probably tolerate a higher variance in sedimentation rate than those living in clear-water, low-sedimentation settings. It is
highly likely that separate guidelines will have to be developed for each reef province, and we are a very long way
from being able to produce those guidelines.
Sclerochronology: the two-edged sword
Sclerochronology is the study of past environmental conditions by way of the study of the records in skeletons;
corals and bivalves are the most commonly used. By
“two-edged sword” here it is meant that, in many cases,
corals are capable of recording the timing and extent of
the very stresses that are killing them.
Coral skeletons record the environmental conditions
under which the coral has grown, and in particular can provide an annually resolved record of how those conditions
have changed through the life of the coral. Variables
responding to sediment that can be recorded in coral skeletons include skeletal density, extension and calcification
rates, quantity and mineralogy of insoluble residue, stable
isotopes of C and O, and trace element geochemistry. Insoluble residue mineralogy and trace element geochemistry are
particularly useful for studying the effects of sedimentation
on reefs, especially determining the source of sediments.
Trace element studies in coral skeletons have been particularly useful in recording sedimentation from natural terrestrial runoff (often enhanced by land clearing for
agriculture or forestry), urban runoff including sewage pollution, and mine tailings. The discussion below focuses on
trace element analyses of coral skeletons to reflect quantity
and type of sedimentation. Trace elements differ in the
extent to which they are incorporated into coral skeletons
as a function of their valence state and atomic radius. Cations with valence similar to Calcium, such as Strontium
and the Lanthanides are most readily incorporated into the
aragonite crystal lattice, explaining why Sr/Ca ratios can
be consistently recorded in coral skeletons at mmol/mol
levels (see Gagan et al., 2000 for review). Ratios of Mg/
Ca, Sr/Ca, and U/Ca respond to sea-surface temperature
consistently, but with varying sensitivity (Fallon et al.,
2003). These elements are useful in sediment studies for
chronological control (in seasonal settings) and for records
of changing oceanographic conditions at seasonal to
decadal time-scales. Trace elements offer advantages over
d
18
O and fluorescent bands in responding to sea-surface
temperature independent of salinity or fluvial discharge,
but are most powerful as recorders of climate variation
when combined with d
18
O (Gagan et al., 2000). Other trace
elements reflect natural and anthropogenic sedimentation
directly (see below).
IMPACTS OF SEDIMENT ON CORAL REEFS
581
isotopic tracers of metabolic activity, have shown that nearshore corals of at least some species can obtain much of
their nutrition from terrestrial sources. Analysis of d
13
C
and d
15
N of P. lobata coral tissue of Indo-Pacific reefs subject to combined sedimentation and eutrophication showed
the influence of both enhanced algal photosynthesis, fueled
by increased inorganic nutrient availability, and heterotrophy, fueled by increased dissolved or particulate organic
matter availability. The relative enhancement of these different processes, in differing proportions, depends on the
relative influences of turbidity, organic matter loading,
and inorganic nutrient loading.
Recent research on the fringing reef of Molokai,
Hawai’i by the USGS has set the bar for sediment research
in coral environments. As a selection from this large body
of work, Bothner et al. (2006) employed very sophisticated sediment traps, and were able to conclude that sediment input to the reef was episodic, driven by storms.
What is more, sediment traps were found not to be good
indicators of net sediment accumulation. Presto et al.
(2006) studied the temporal and spatial variability of flow
in suspended sediments on the reef flat, and found that
most of the sediment was transported alongshore. Regions
where sediment moved offshore were regions of low coral
cover. Calhoun and Field (2008) studied the composition
of grain types from the beaches and reef environments,
and were able to deduce transport directions. And finally,
much of this research is summarized in the beautifully
illustrated USGS report (Field et al., 2008).
Guidelines – how much sedimentation is too much?
In short, development of an integrated set of guidelines for
sediment resistance of reefs has been hampered by lack of
agreement on methodology, lack of precision when dealing with the sediments themselves, the confounded effects
of increased sediment input and increased nutrients, and
the plasticity of coral behavior.
It is highly unlikely that there can ever be one set of regulations, or guidelines, developed for all reefs that could
control impacts of sediment (cf. Pastorok and Bilyard,
1985), nor is a global set of water quality guidelines necessarily desirable. It would seem that most Caribbean reefs
have difficulty surviving SPM loads that are consistently
in excess of 10 mg/L, and indeed it would seem appropriate to incorporate that level into regulatory guidelines.
Reefs in other locales, for example, the northern Great
Barrier Reef, will have adapted and evolved to survive
very different conditions (Hopley et al., 1990, 2007).
Corals near the mouth of the Daintree River flourish under
conditions that would slay a Caribbean reef (cf. Perry and
Larcombe, 2003). Within each region, corals have adapted
to long-term regional variation in normal sedimentation
rates and patterns. High-ambient sedimentation levels are
not inherently stressful; rather, it is the change in the rates
of sedimentation that causes stress for coral reefs
(Fabricius, 2005; Hopley et al., 2007).
Even recognizing that change in sedimentation rate,
rather than absolute sedimentation rate, is crucial for coral
reefs (cf. Dikou and van Woesik, 2006), it is unlikely that
a single critical value for percentage increase in sedimentation can be identified for all reefs. Sedimentation in
many environments is highly episodic, and corals living
in areas of high ambient sedimentation can probably tolerate a higher variance in sedimentation rate than those living in clear-water, low-sedimentation settings. It is
highly likely that separate guidelines will have to be developed for each reef province, and we are a very long way
from being able to produce those guidelines.
Sclerochronology: the two-edged sword
Sclerochronology is the study of past environmental conditions by way of the study of the records in skeletons;
corals and bivalves are the most commonly used. By
“two-edged sword” here it is meant that, in many cases,
corals are capable of recording the timing and extent of
the very stresses that are killing them.
Coral skeletons record the environmental conditions
under which the coral has grown, and in particular can provide an annually resolved record of how those conditions
have changed through the life of the coral. Variables
responding to sediment that can be recorded in coral skeletons include skeletal density, extension and calcification
rates, quantity and mineralogy of insoluble residue, stable
isotopes of C and O, and trace element geochemistry. Insoluble residue mineralogy and trace element geochemistry are
particularly useful for studying the effects of sedimentation
on reefs, especially determining the source of sediments.
Trace element studies in coral skeletons have been particularly useful in recording sedimentation from natural terrestrial runoff (often enhanced by land clearing for
agriculture or forestry), urban runoff including sewage pollution, and mine tailings. The discussion below focuses on
trace element analyses of coral skeletons to reflect quantity
and type of sedimentation. Trace elements differ in the
extent to which they are incorporated into coral skeletons
as a function of their valence state and atomic radius. Cations with valence similar to Calcium, such as Strontium
and the Lanthanides are most readily incorporated into the
aragonite crystal lattice, explaining why Sr/Ca ratios can
be consistently recorded in coral skeletons at mmol/mol
levels (see Gagan et al., 2000 for review). Ratios of Mg/
Ca, Sr/Ca, and U/Ca respond to sea-surface temperature
consistently, but with varying sensitivity (Fallon et al.,
2003). These elements are useful in sediment studies for
chronological control (in seasonal settings) and for records
of changing oceanographic conditions at seasonal to
decadal time-scales. Trace elements offer advantages over
d
18
O and fluorescent bands in responding to sea-surface
temperature independent of salinity or fluvial discharge,
but are most powerful as recorders of climate variation
when combined with d
18
O (Gagan et al., 2000). Other trace
elements reflect natural and anthropogenic sedimentation
directly (see below).
IMPACTS OF SEDIMENT ON CORAL REEFS
581