greater stress and damage to corals than do sediments that
are poor in organic matter (Weber et al., 2006). Fine and
organically enriched sediments may support different bacterial communities and metabolic processes, further altering the microenvironment on the reefs biofilm surfaces.
Fine sediments may also bind with marine snow aggregates, and this additional organic material is highly damaging to coral recruits (Fabricius et al., 2003). At very
high levels of pollution, the decomposition of organically
enriched sediments can lead to hypoxia in poorly flushed
and highly stratified locations, further damaging benthos.
Not only the amount of sedimentation and its geochemical
and physical properties, but also its biological properties
and organic contents are ecologically relevant and determine the extent of sedimentation damage in coral reefs.
Corals are most sensitive to sedimentation during their
recruitment stage, and reduced coral recruitment is one of
the most deleterious effects of eutrophication on coral
reefs. Coral settlement is sparse on sediment-covered surfaces, and the tolerance of coral recruits to sediment is at
least one order of magnitude lower than that of adult corals
(Fabricius, 2005). Newly settled corals may be killed by
exposure to 12 mg cm À2 day À1 for <48 h if the sediment
is bound by marine snow and rich in organic contents, but
can tolerate such levels if the sediments are low in organic
content (Fabricius et al., 2003). These and similar data
demonstrate the critical role of organic sediment enrichment and the interactions of nutrients and sediments on
the effects on coral reefs.
Certain species of crustose coralline red algae are
essential for coral settlement. While high nutrient levels
do not greatly alter the physiology of crustose coralline
algae, high sedimentation rates are related to low coralline
algal cover in coral reefs (Kendrick, 1991; Fabricius and
De’ath, 2001). Some coralline red algae survive burial
under coarse inorganic sediments for days to weeks, but
their survival rates rapidly decline if the sediments contain
traces of herbicides (Harrington et al., 2005). Turf algae
often outcompete coralline algae, and by trapping sediments they can make the substratum unsuitable for coralline algae and for coral settlement (Birrell et al., 2005).
Factors determining the vulnerability of a specific
coral reef to eutrophication
Effective coastal management requires an understanding
of the factors that determine the vulnerability of specific
coral reefs to eutrophication. Vulnerability may vary
depending on the reef’s exposure, resistance, and
resilience.
The degree of exposure of an organism or ecosystem to
a stressor is typically a function of the amount (= concentration, level, or load) of the stressor and the length of time
it is in contact with the stressor. A reef may be exposed to
a high level of a stressor for a short period of time or to
lower levels for longer periods. When quantifying exposure levels, it is important to determine peak concentrations (potentially leading to damage or mortality within
hours to days), as well as to quantify long-term mean (or
median) concentrations and the duration of exposure.
The resistance of an ecosystem is defined as the ease or
difficulty with which it changes when exposed to
a disturbance (Pimm, 1984). Resistance can arise from
prior exposure history, environmental factors, or biological factors (Done, 1999; West and Salm, 2003). For example, a coral reef that has developed in a naturally turbid
setting and is dominated by corals of the turbidity-tolerant
genera Turbinaria and massive Porites is more resistant to
change from exposure to nutrient pollution than an
Acropora-dominated reef that has developed in clear
waters. Ecological resilience is defined here as the time
it takes for an ecosystem to return to its equilibrium point
after being disturbed (Nyström et al., 2008).
A review of some of the better-described case studies in
the literature allows to qualitatively identify the main factors that predict the exposure, resistance, and resilience of
specific reefs to degradation. Currents are extremely
important properties of coral reefs and are key predictors
of exposure, resistance, and resilience at both local and
regional scales. At local scales, current-swept reef fronts,
flanks and channels, and reef crests with moderate wave
exposure are the least likely to retain pollutants; they support high recruitment and have the fastest coral growth
(Sebens, 1991). Ideally, wave exposure is sufficient to
remove sediment deposits without causing frequent coral
breakage. As fast currents and waves also facilitate
macroalgal growth, competition between corals and
macroalgae is intense if nutrient levels are high. At
regional scales, currents determine where and how far pollutants are being transported. Where currents are predominantly tidal, a very low or very high tidal range
diminishes the ability of reefs to withstand pollution, by
affecting circulation. Currents that create a strong connectivity to source populations of pelagic larvae also improve
the resilience of a reef, by decreasing the time of recovery
from a disturbance.
The exposure of a coral reef is furthermore well
predicted simply based on spatial factors: the closer downstream a coral reef is to a pollution source and the higher
the mean annual load of this source, the greater is its exposure to a pollutant (Bryant et al., 1998). Geospatial models
of the global scale of pollution around coral reefs have
been developed based on the distance of reefs to pollution
sources (Bryant et al., 1998). These models estimate that
22% of all coral reefs worldwide are classified as at high
or medium threat from inland pollution and soil erosion.
The global models also classify 12% of reefs at threat from
marine pollution (distance from ports, oil tanks, oil wells,
and shipping areas) and 30% of reefs as threatened from
coastal development such as cities, mines, and resorts
(Bryant et al., 1998). At regional scales, the percentage of
reefs at risk is a direct function of the extent of land clearing,
and up to 50% of reefs is at risk in the countries with the most
widespread land clearing (Bryant et al., 1998). At local
scales, such as downstream from well-defined point sources
NUTRIENT POLLUTION/EUTROPHICATION
727
are poor in organic matter (Weber et al., 2006). Fine and
organically enriched sediments may support different bacterial communities and metabolic processes, further altering the microenvironment on the reefs biofilm surfaces.
Fine sediments may also bind with marine snow aggregates, and this additional organic material is highly damaging to coral recruits (Fabricius et al., 2003). At very
high levels of pollution, the decomposition of organically
enriched sediments can lead to hypoxia in poorly flushed
and highly stratified locations, further damaging benthos.
Not only the amount of sedimentation and its geochemical
and physical properties, but also its biological properties
and organic contents are ecologically relevant and determine the extent of sedimentation damage in coral reefs.
Corals are most sensitive to sedimentation during their
recruitment stage, and reduced coral recruitment is one of
the most deleterious effects of eutrophication on coral
reefs. Coral settlement is sparse on sediment-covered surfaces, and the tolerance of coral recruits to sediment is at
least one order of magnitude lower than that of adult corals
(Fabricius, 2005). Newly settled corals may be killed by
exposure to 12 mg cm À2 day À1 for <48 h if the sediment
is bound by marine snow and rich in organic contents, but
can tolerate such levels if the sediments are low in organic
content (Fabricius et al., 2003). These and similar data
demonstrate the critical role of organic sediment enrichment and the interactions of nutrients and sediments on
the effects on coral reefs.
Certain species of crustose coralline red algae are
essential for coral settlement. While high nutrient levels
do not greatly alter the physiology of crustose coralline
algae, high sedimentation rates are related to low coralline
algal cover in coral reefs (Kendrick, 1991; Fabricius and
De’ath, 2001). Some coralline red algae survive burial
under coarse inorganic sediments for days to weeks, but
their survival rates rapidly decline if the sediments contain
traces of herbicides (Harrington et al., 2005). Turf algae
often outcompete coralline algae, and by trapping sediments they can make the substratum unsuitable for coralline algae and for coral settlement (Birrell et al., 2005).
Factors determining the vulnerability of a specific
coral reef to eutrophication
Effective coastal management requires an understanding
of the factors that determine the vulnerability of specific
coral reefs to eutrophication. Vulnerability may vary
depending on the reef’s exposure, resistance, and
resilience.
The degree of exposure of an organism or ecosystem to
a stressor is typically a function of the amount (= concentration, level, or load) of the stressor and the length of time
it is in contact with the stressor. A reef may be exposed to
a high level of a stressor for a short period of time or to
lower levels for longer periods. When quantifying exposure levels, it is important to determine peak concentrations (potentially leading to damage or mortality within
hours to days), as well as to quantify long-term mean (or
median) concentrations and the duration of exposure.
The resistance of an ecosystem is defined as the ease or
difficulty with which it changes when exposed to
a disturbance (Pimm, 1984). Resistance can arise from
prior exposure history, environmental factors, or biological factors (Done, 1999; West and Salm, 2003). For example, a coral reef that has developed in a naturally turbid
setting and is dominated by corals of the turbidity-tolerant
genera Turbinaria and massive Porites is more resistant to
change from exposure to nutrient pollution than an
Acropora-dominated reef that has developed in clear
waters. Ecological resilience is defined here as the time
it takes for an ecosystem to return to its equilibrium point
after being disturbed (Nyström et al., 2008).
A review of some of the better-described case studies in
the literature allows to qualitatively identify the main factors that predict the exposure, resistance, and resilience of
specific reefs to degradation. Currents are extremely
important properties of coral reefs and are key predictors
of exposure, resistance, and resilience at both local and
regional scales. At local scales, current-swept reef fronts,
flanks and channels, and reef crests with moderate wave
exposure are the least likely to retain pollutants; they support high recruitment and have the fastest coral growth
(Sebens, 1991). Ideally, wave exposure is sufficient to
remove sediment deposits without causing frequent coral
breakage. As fast currents and waves also facilitate
macroalgal growth, competition between corals and
macroalgae is intense if nutrient levels are high. At
regional scales, currents determine where and how far pollutants are being transported. Where currents are predominantly tidal, a very low or very high tidal range
diminishes the ability of reefs to withstand pollution, by
affecting circulation. Currents that create a strong connectivity to source populations of pelagic larvae also improve
the resilience of a reef, by decreasing the time of recovery
from a disturbance.
The exposure of a coral reef is furthermore well
predicted simply based on spatial factors: the closer downstream a coral reef is to a pollution source and the higher
the mean annual load of this source, the greater is its exposure to a pollutant (Bryant et al., 1998). Geospatial models
of the global scale of pollution around coral reefs have
been developed based on the distance of reefs to pollution
sources (Bryant et al., 1998). These models estimate that
22% of all coral reefs worldwide are classified as at high
or medium threat from inland pollution and soil erosion.
The global models also classify 12% of reefs at threat from
marine pollution (distance from ports, oil tanks, oil wells,
and shipping areas) and 30% of reefs as threatened from
coastal development such as cities, mines, and resorts
(Bryant et al., 1998). At regional scales, the percentage of
reefs at risk is a direct function of the extent of land clearing,
and up to 50% of reefs is at risk in the countries with the most
widespread land clearing (Bryant et al., 1998). At local
scales, such as downstream from well-defined point sources
NUTRIENT POLLUTION/EUTROPHICATION
727