or in coastal reefs fringing eroding land, terrestrial runoff can
be the single most significant pressure for selected coral
reefs (Bourke et al., 2002). However, such geospatial models
do not factor in additional indirect effects that may occur
hundreds of kilometers away from areas of eutrophication,
such as increasing frequencies of outbreaks of coral eating
crown-of-thorns starfish populations.
Topography/geomorphology and bathymetry can modify exposure enormously: pollutants are retained for
prolonged periods in poorly flushed embayments and
lagoons with reduced circulation or small water volumes,
whereas they may be rapidly washed away from
a headland surrounded by a deep water body (Hopley
et al., 2007). Around the world, most severely polluted
and hypoxic marine sites are located in shallow and poorly
flushed semienclosed waterbodies such as gulfs, bays,
fjords, or bights (e.g., Kaneohe Bay, the Baltic Sea, Chesapeake Bay, Irish Sea; Diaz, 2001). Bathymetry also modifies exposure by affecting the balance between
sedimentation and resuspension of materials. Materials
are easily washed away from shallow, upper reef slopes,
but accumulate below the reach of surface waves in deeper
areas (Wolanski et al., 2005). Upper reef slopes are also
less affected by turbidity than are deeper areas, where light
becomes limiting for photosynthetic organisms. Exposure
is worst at sites surrounded by a wide, shallow continental
shelf where materials are repeatedly wave resuspended,
whereas pollutants are flushed away into deeper waters if
no shelf retains them. For example, the Great Barrier Reef
is located on a 50–200-km-wide shallow continental shelf
with >2,000 individual coral reefs between the land and
the open ocean. Retention times here are still being
debated, but some estimate them to be up to 300 days for
dissolved materials (Luick et al., 2007). Particulate materials are likely to be retained for even longer periods of
time, as they are repeatedly deposited and resuspended
from the shallow sea floor. So, although nutrient enrichment is less severe on the Great Barrier Reef than in many
other more densely populated regions, symptoms such as
macroalgal dominance and low coral diversity on numerous inshore reefs have been attributed to enhanced terrestrial runoff (van Woesik et al., 1999; Fabricius and De’ath,
2004; Fabricius et al., 2005; De’ath and Fabricius, 2010).
Lastly, biological processes are believed to modify the
resistance and resilience of coral reefs, but many of these
processes are as yet poorly understood. For example,
abundant herbivorous fish strongly control macroalgal
abundances, thereby promoting resilience (Littler and
Littler, 2007). It is also still unresolved to what extent
resistance and resilience are codetermined by biodiversity;
biogeographic regions with low biodiversity have fewer
species to replace the loss of sensitive species and
may be more likely to undergo structural and functional
changes in their communities (Bellwood et al., 2004).
For example in the Caribbean, the loss of the dominant
coral species Acropora palmata and Acropora cervicornis
and the one remaining important algal grazer, Diadema
antillarum, has led to a widespread collapse of reef
ecosystems (Lessios et al., 1984). It is also unknown
whether or not the resistance and resilience of reefs vary
along latitudinal gradients, as reefs in higher latitudes naturally have lower calcification rates, higher macroalgal
biomass, and lower coral biodiversity than do low latitude
reefs. Lastly, regions that are prone to severe or frequent
disturbances (e.g., from coral bleaching, storms, cold
water upwelling, or outbreaks of A. planci) are more likely
to be prone to degradation than rarely disturbed regions.
This is because poor water quality often does not directly
kill the adult coral populations, but retards coral recruitment and hence the speed of recovery from unrelated disturbances. It has also been shown that exposure to one
form of stress may decrease the resilience of an ecosystem
to another stressor (Hughes et al., 2003; Wooldridge et al.,
2005).
In summary, degradation from poor water quality is
most likely to occur on deeper reef slopes, in locations
with weak currents, in places where fish abundances are
low, and in regions that are frequently affected by other
forms of disturbance. In contrast, well-flushed locations
with strong currents, shallow reef crests surrounded by
a deep water body, and reefs inhabited by healthy
populations of fishes are likely to have the highest levels
of resistance and resilience.
Several of these factors are identical to those that determine the resistance and resilience of coral to bleaching
caused by warming oceans. For example, topography, fast
currents, proximity to deep water, and a diverse community with abundant herbivores are considered reliable factors in predicting the likelihood of coral communities
dying as a result of bleaching (West and Salm, 2003) –
as they are in predicting death arising from eutrophication.
One difference is that reefs in shallow waters are – relative
to deeper reef slopes – tolerant of the turbidity associated
with eutrophication but sensitive to bleaching. A better
understanding of the additive or interactive effects
between eutrophication and climate change is clearly
needed.
Conclusions
Although disturbances are a normal and important aspect
of their environment, coral reefs are inherently quite stable
over time, with coral cover and composition often not
changing for many years to decades (Connell, 1997).
After severe acute and short-term disturbances, coral in
shallow, well-lit windward reefs with fast currents can
recover within 10–15 years, if larvae are plentiful
(Connell, 1997). Recovery takes longer – possibly up to
50 years – on deeper reef slopes and in poorly flushed settings such as lagoons and areas with weak water flow and
poor connectivity. Recovery from chronic and humaninduced disturbances that alter the physical environment
is also slower and less commonly observed than recovery
from fast and acute disturbances (Connell et al., 1997). If
widespread reef degradation is to be avoided, it is essential
that the average frequency of severe disturbances does not
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NUTRIENT POLLUTION/EUTROPHICATION
be the single most significant pressure for selected coral
reefs (Bourke et al., 2002). However, such geospatial models
do not factor in additional indirect effects that may occur
hundreds of kilometers away from areas of eutrophication,
such as increasing frequencies of outbreaks of coral eating
crown-of-thorns starfish populations.
Topography/geomorphology and bathymetry can modify exposure enormously: pollutants are retained for
prolonged periods in poorly flushed embayments and
lagoons with reduced circulation or small water volumes,
whereas they may be rapidly washed away from
a headland surrounded by a deep water body (Hopley
et al., 2007). Around the world, most severely polluted
and hypoxic marine sites are located in shallow and poorly
flushed semienclosed waterbodies such as gulfs, bays,
fjords, or bights (e.g., Kaneohe Bay, the Baltic Sea, Chesapeake Bay, Irish Sea; Diaz, 2001). Bathymetry also modifies exposure by affecting the balance between
sedimentation and resuspension of materials. Materials
are easily washed away from shallow, upper reef slopes,
but accumulate below the reach of surface waves in deeper
areas (Wolanski et al., 2005). Upper reef slopes are also
less affected by turbidity than are deeper areas, where light
becomes limiting for photosynthetic organisms. Exposure
is worst at sites surrounded by a wide, shallow continental
shelf where materials are repeatedly wave resuspended,
whereas pollutants are flushed away into deeper waters if
no shelf retains them. For example, the Great Barrier Reef
is located on a 50–200-km-wide shallow continental shelf
with >2,000 individual coral reefs between the land and
the open ocean. Retention times here are still being
debated, but some estimate them to be up to 300 days for
dissolved materials (Luick et al., 2007). Particulate materials are likely to be retained for even longer periods of
time, as they are repeatedly deposited and resuspended
from the shallow sea floor. So, although nutrient enrichment is less severe on the Great Barrier Reef than in many
other more densely populated regions, symptoms such as
macroalgal dominance and low coral diversity on numerous inshore reefs have been attributed to enhanced terrestrial runoff (van Woesik et al., 1999; Fabricius and De’ath,
2004; Fabricius et al., 2005; De’ath and Fabricius, 2010).
Lastly, biological processes are believed to modify the
resistance and resilience of coral reefs, but many of these
processes are as yet poorly understood. For example,
abundant herbivorous fish strongly control macroalgal
abundances, thereby promoting resilience (Littler and
Littler, 2007). It is also still unresolved to what extent
resistance and resilience are codetermined by biodiversity;
biogeographic regions with low biodiversity have fewer
species to replace the loss of sensitive species and
may be more likely to undergo structural and functional
changes in their communities (Bellwood et al., 2004).
For example in the Caribbean, the loss of the dominant
coral species Acropora palmata and Acropora cervicornis
and the one remaining important algal grazer, Diadema
antillarum, has led to a widespread collapse of reef
ecosystems (Lessios et al., 1984). It is also unknown
whether or not the resistance and resilience of reefs vary
along latitudinal gradients, as reefs in higher latitudes naturally have lower calcification rates, higher macroalgal
biomass, and lower coral biodiversity than do low latitude
reefs. Lastly, regions that are prone to severe or frequent
disturbances (e.g., from coral bleaching, storms, cold
water upwelling, or outbreaks of A. planci) are more likely
to be prone to degradation than rarely disturbed regions.
This is because poor water quality often does not directly
kill the adult coral populations, but retards coral recruitment and hence the speed of recovery from unrelated disturbances. It has also been shown that exposure to one
form of stress may decrease the resilience of an ecosystem
to another stressor (Hughes et al., 2003; Wooldridge et al.,
2005).
In summary, degradation from poor water quality is
most likely to occur on deeper reef slopes, in locations
with weak currents, in places where fish abundances are
low, and in regions that are frequently affected by other
forms of disturbance. In contrast, well-flushed locations
with strong currents, shallow reef crests surrounded by
a deep water body, and reefs inhabited by healthy
populations of fishes are likely to have the highest levels
of resistance and resilience.
Several of these factors are identical to those that determine the resistance and resilience of coral to bleaching
caused by warming oceans. For example, topography, fast
currents, proximity to deep water, and a diverse community with abundant herbivores are considered reliable factors in predicting the likelihood of coral communities
dying as a result of bleaching (West and Salm, 2003) –
as they are in predicting death arising from eutrophication.
One difference is that reefs in shallow waters are – relative
to deeper reef slopes – tolerant of the turbidity associated
with eutrophication but sensitive to bleaching. A better
understanding of the additive or interactive effects
between eutrophication and climate change is clearly
needed.
Conclusions
Although disturbances are a normal and important aspect
of their environment, coral reefs are inherently quite stable
over time, with coral cover and composition often not
changing for many years to decades (Connell, 1997).
After severe acute and short-term disturbances, coral in
shallow, well-lit windward reefs with fast currents can
recover within 10–15 years, if larvae are plentiful
(Connell, 1997). Recovery takes longer – possibly up to
50 years – on deeper reef slopes and in poorly flushed settings such as lagoons and areas with weak water flow and
poor connectivity. Recovery from chronic and humaninduced disturbances that alter the physical environment
is also slower and less commonly observed than recovery
from fast and acute disturbances (Connell et al., 1997). If
widespread reef degradation is to be avoided, it is essential
that the average frequency of severe disturbances does not
728
NUTRIENT POLLUTION/EUTROPHICATION