off the colony surface (Hubbard and Pocock, 1972). Corals
exhibit both active and passive removal of sediment particles (Lasker, 1980). In coral colonies, sedimentation stress
increases linearly with the duration and amount of sedimentation: for example, a certain amount of sediment deposited
on the coral for one time unit exerts the same measurable
photophysiological stress as twice the amount deposited
for half the time (Philipp and Fabricius, 2003).
Microbially mediated decline
This is a relatively recent concept, that links together the
co-occurrence of sediment input, dissolved organic matter
(DOM), and nutrient increases, such as would accompany
river discharge (either normal, or augmented by human
activities such as deforestation) with increased bacterial
activity. The augmented microbial processes would then
increase the virulence of disease, alter the dynamics of fine
sediments, and kill corals directly.
Much of the earlier work on sediments and corals paid
little attention to the mineralogy and the size distribution
of the sediments. This bears further investigation, because
mineralogy plays an important role in the potential delivery of toxic and/or deleterious substances to the corals.
For example, the phyllosilicate lattices of clay minerals,
which will be delivered from the weathering of silicate
rocks, are capable of storing (and later releasing) enormous
amounts of dissolved substances, trace metals, and contaminants. Because of the edge charges on the unit cells,
clay minerals can also form aggregates, or flocs; this tendency is enhanced in the presence of organic matter.
Fine-grained carbonate sediments (micrites) lack this storage and agglomeration capabilities; hence, siliciclastic sediments can be predicted as posing greater threats to corals
than the same amount of carbonate sediment (cf. Te, 1997).
Impacts of sediments on corals are strongly influenced
by the type of sediment. Tissue damage under a layer of
sediment increases with increasing organic content and
bacterial activity, and with decreasing grain sizes
(Hodgson, 1990; Weber et al., 2006). Low levels of sedimentation, when combined with “marine snow,” killed
newly settled coral recruits. The same amount of sediment
without the addition of marine snow does not reduce their
short-term survival (Fabricius et al., 2003). Kline et al.
(2006) have shown that increased DOM can cause elevated bacterial activity, leading directly to coral death.
As an example of the sort of research that is needed in
this field, Weber et al. (2006) studied the effects on corals
of sediments differing in grain size, organic content, and
geochemistry. They found that different sediments exerted
greatly contrasting levels of stress in the corals, and that
grain size and organic and nutrient-related sediment properties are key factors determining sedimentation stress in
corals after short-term exposure. Photophysiological
stress was measurable after 36 h of exposure to most of
the silt-sized sediments, and coral recovery was incomplete after 48–96 h recovery time. In contrast, sandy sediment types caused no measurable stress at the same
concentration for the same exposure time. Stress levels
were strongly related to the content of organic matter
and nutrients in the sediments. Anoxia developed on the
sediment surfaces of the nutrient-rich silts, which had
become slimy and smelled of hydrogen sulfide,
suggesting increased bacterial activity. Their findings
may be summarized as follows: silt-sized and nutrient-rich
sediments can stress corals after short exposure, while
sandy sediments or nutrient-poor silts have little effect.
Summary of impacts
In short, the effect of sediments on corals and coral reefs
may involve the effect of nutrients and bacteria carried
by the sediment plume, as much as the sediments themselves. Although, both shading and particle loading are
coral stressors, it would appear that, at least in some cases,
shading suppresses coral growth to a greater extent than
particle loading per se (Anthony, 1999).
There have been several experimental investigations of
the sediment rejecting capabilities of corals (Hubbard and
Pocock, 1972; Bak and Elgershuizen, 1976). In addition,
in both the Caribbean and the Indo-Pacific, there have
been attempts to establish a hierarchy of rejection capabilities among species of corals (Rogers, 1990; StaffordSmith and Ormond, 1992). Some of the earlier attempts
to assess sediment stress have used pulsed loadings of
mostly coarse sediment (sand), which bears little relevance to the typical sediment stress applied to corals unremitting muddy water. Some recent research has, however,
begun to shed more light on this complex problem
(Anthony, 1999; Riegl, 1995). Reefs can, of course, occur
in turbid settings. These tend to be thin accumulations, of
low diversity, but nonetheless they survive (Perry and
Larcombe, 2003). From a management perspective, the
important factor is a change in water quality.
Status of research
There is still a great need for research that would establish
the limits of acceptable sediment input to any given reef.
There may be an indication that Caribbean reefs, at least,
cannot for long survive continued levels of SPM
(Suspended Particulate Matter) in the surrounding water
greater than 10 mg/L (Cortes and Risk, 1985; AbdelSalam and Porter, 1988), but the certainty of this level is
far from established, and legislation based on it is equally
far off. In addition, there is a suggestion that some IndoPacific reefs flourish at SPM levels much higher, in some
cases by shifting their metabolism toward more heterotrophy in murky waters (Anthony, 2000).
There are a number of reasons why research in this field
has not progressed as rapidly as perhaps it should. First of
all, perhaps the most frustrating, is the inability of workers
in the field to agree on common methodology. Sediment
deposition rates are usually estimated using sediment
traps, but almost every published study uses a different
design (trap height, diameter, emplacement above the bottom), and virtually none of them cites Gardner’s work on
IMPACTS OF SEDIMENT ON CORAL REEFS
579
exhibit both active and passive removal of sediment particles (Lasker, 1980). In coral colonies, sedimentation stress
increases linearly with the duration and amount of sedimentation: for example, a certain amount of sediment deposited
on the coral for one time unit exerts the same measurable
photophysiological stress as twice the amount deposited
for half the time (Philipp and Fabricius, 2003).
Microbially mediated decline
This is a relatively recent concept, that links together the
co-occurrence of sediment input, dissolved organic matter
(DOM), and nutrient increases, such as would accompany
river discharge (either normal, or augmented by human
activities such as deforestation) with increased bacterial
activity. The augmented microbial processes would then
increase the virulence of disease, alter the dynamics of fine
sediments, and kill corals directly.
Much of the earlier work on sediments and corals paid
little attention to the mineralogy and the size distribution
of the sediments. This bears further investigation, because
mineralogy plays an important role in the potential delivery of toxic and/or deleterious substances to the corals.
For example, the phyllosilicate lattices of clay minerals,
which will be delivered from the weathering of silicate
rocks, are capable of storing (and later releasing) enormous
amounts of dissolved substances, trace metals, and contaminants. Because of the edge charges on the unit cells,
clay minerals can also form aggregates, or flocs; this tendency is enhanced in the presence of organic matter.
Fine-grained carbonate sediments (micrites) lack this storage and agglomeration capabilities; hence, siliciclastic sediments can be predicted as posing greater threats to corals
than the same amount of carbonate sediment (cf. Te, 1997).
Impacts of sediments on corals are strongly influenced
by the type of sediment. Tissue damage under a layer of
sediment increases with increasing organic content and
bacterial activity, and with decreasing grain sizes
(Hodgson, 1990; Weber et al., 2006). Low levels of sedimentation, when combined with “marine snow,” killed
newly settled coral recruits. The same amount of sediment
without the addition of marine snow does not reduce their
short-term survival (Fabricius et al., 2003). Kline et al.
(2006) have shown that increased DOM can cause elevated bacterial activity, leading directly to coral death.
As an example of the sort of research that is needed in
this field, Weber et al. (2006) studied the effects on corals
of sediments differing in grain size, organic content, and
geochemistry. They found that different sediments exerted
greatly contrasting levels of stress in the corals, and that
grain size and organic and nutrient-related sediment properties are key factors determining sedimentation stress in
corals after short-term exposure. Photophysiological
stress was measurable after 36 h of exposure to most of
the silt-sized sediments, and coral recovery was incomplete after 48–96 h recovery time. In contrast, sandy sediment types caused no measurable stress at the same
concentration for the same exposure time. Stress levels
were strongly related to the content of organic matter
and nutrients in the sediments. Anoxia developed on the
sediment surfaces of the nutrient-rich silts, which had
become slimy and smelled of hydrogen sulfide,
suggesting increased bacterial activity. Their findings
may be summarized as follows: silt-sized and nutrient-rich
sediments can stress corals after short exposure, while
sandy sediments or nutrient-poor silts have little effect.
Summary of impacts
In short, the effect of sediments on corals and coral reefs
may involve the effect of nutrients and bacteria carried
by the sediment plume, as much as the sediments themselves. Although, both shading and particle loading are
coral stressors, it would appear that, at least in some cases,
shading suppresses coral growth to a greater extent than
particle loading per se (Anthony, 1999).
There have been several experimental investigations of
the sediment rejecting capabilities of corals (Hubbard and
Pocock, 1972; Bak and Elgershuizen, 1976). In addition,
in both the Caribbean and the Indo-Pacific, there have
been attempts to establish a hierarchy of rejection capabilities among species of corals (Rogers, 1990; StaffordSmith and Ormond, 1992). Some of the earlier attempts
to assess sediment stress have used pulsed loadings of
mostly coarse sediment (sand), which bears little relevance to the typical sediment stress applied to corals unremitting muddy water. Some recent research has, however,
begun to shed more light on this complex problem
(Anthony, 1999; Riegl, 1995). Reefs can, of course, occur
in turbid settings. These tend to be thin accumulations, of
low diversity, but nonetheless they survive (Perry and
Larcombe, 2003). From a management perspective, the
important factor is a change in water quality.
Status of research
There is still a great need for research that would establish
the limits of acceptable sediment input to any given reef.
There may be an indication that Caribbean reefs, at least,
cannot for long survive continued levels of SPM
(Suspended Particulate Matter) in the surrounding water
greater than 10 mg/L (Cortes and Risk, 1985; AbdelSalam and Porter, 1988), but the certainty of this level is
far from established, and legislation based on it is equally
far off. In addition, there is a suggestion that some IndoPacific reefs flourish at SPM levels much higher, in some
cases by shifting their metabolism toward more heterotrophy in murky waters (Anthony, 2000).
There are a number of reasons why research in this field
has not progressed as rapidly as perhaps it should. First of
all, perhaps the most frustrating, is the inability of workers
in the field to agree on common methodology. Sediment
deposition rates are usually estimated using sediment
traps, but almost every published study uses a different
design (trap height, diameter, emplacement above the bottom), and virtually none of them cites Gardner’s work on
IMPACTS OF SEDIMENT ON CORAL REEFS
579