Signs of Change
Pollution
The major form of pollution on coral reefs is nutrient
enrichment, primarily linked to human waste, but also to
agricultural runoff. Recent decades have seen burgeoning
human populations in coastal areas close to reefs, and rapid
urbanization of many societies. Many countries, particularly
the poorer nations, have failed to develop sewage treatment
systems able to cope with growing urban areas, and vast
amounts of sewage enter coastal zones through the drainage
system or are directly piped into nearshore waters. Even
away from urban centers, coastal development has occurred
at a considerable pace. Tourism in particular has led to this
kind of development. While it is smaller scale, it actively
seeks out sites immediately adjacent to the coastline, and
in many cases there is again little or no sewage treatment. In
addition to sewage pollution, agricultural development in
many countries has entailed the ennchment of the land with
nutrients in the form of artificial fertilizers, many of which
enter the drainage system and are carried to coastal waters.
The impacts of eutrophication on reefs are complex.
Typically there are changes in the community structure.
Algae flourish in situations of high nutrient loads, and
can overgrow and kill corals or prevent new settlement of
coral larvae. Algal blooms are also sometimes observed in
the water column, with the further effect of reducing light
levels for the corals below. Among the rest of the reef
community, particle-feeding organisms such as sponges
and zoanthids may become more common, and planktoneating and herbivorous fish may increase. More subtle
changes, such as a reduction in the reproductive and
growth capacity of particular species may also result, but
may not be immediately apparent. These changes may
reduce the reef's resilience to further changes or its ability
to recover from events such as hurricanes. They may also
affect the sustainable yield of reef fisheries. In extreme
conditions, nutrient pollution alone is sufficient to kill a
reef and lead to its replacement by algal communities.
Toxic pollutants receive less attention and have been
less well studied. Oil is perhaps the most common pollutant in many remote areas. Spills have occurred widely
in coral reef regions. Continuous chronic oil pollution
results from minor spills or deliberate discharges of oil.
These occur in oil drilling areas such as the Arabian Gulf,
and in certain localities such as the Straits of Malacca, the
Straits of Hormuz, the Gulf of Aden and the approaches to
the Panama Canal, where vessels often discharge ballast
water or clean out their tanks. Other toxic chemicals are
released by a large range of industries, including tailings
from mining. The effects of oil pollution on corals are
varied. In some cases polluted reefs have shown slightly
higher mortality rates than unaffected reefs, and the
reproductive potential of surviving corals is significantly
reduced. A major spill in Panama led to a decrease in total
coral cover of 50-75 percent in shallow water reefs.
Sedimentation
Reef corals are highly sensitive to the impacts of sediments.
As sediments hang suspended in the water column they
block light from the reef below, preventing the growth or
even survival of corals, particularly in deeper waters. As
the sediments settle they may smother corals. While corals
are able to remove sediments, secreting mucus and then
sloughing this off into the water, such an activity uses
energy and nutrients. In the long term the smothering effects
of sediments weakens many corals, reducing growth or
reproductive potential and leaving them less able to compete
with other benthic organisms such as algae or filter feeders.
Even as sediments settle they create a surface which is
inimical to new coral settlement and growth, while light
sediments are often resuspended by wave action and hence
may remain in the system for considerable lengths of time.
Increasing levels of sediment in coastal environments
in recent times can be linked to coastal development,
dredging, land reclamation, and also distant terrestrial
activities, including deforestation and poor agricultural
practices. In many cases the intTuences of sedimentation
and pollution are combined, and the differential impact of
each is somewhat difficult to determine. Mine tailings
in Papua New Guinea are reported to have degraded or
destroyed significant areas of reef around Bougainville, an
area of little or no nutrient impacts. However, even here
these impacts may have been exacerbated by toxic compounds present in the tailings themselves.
Unsustainable fishing
Reefs are highly productive ecosystems. It is possible to
harvest some of this productivity for human consumption,
without compromising the overall functioning of the reef
ecosystem, and humans have utilized and depended on
reefs in this way for millennia. For many traditional cultures the relationship between local populations and their
adjacent coral reefs can best be seen as an ecological one,
where humans play a role as a part of the reef ecosystem.
Just as with other fisheries, there are limits to this
utilization. Beyond these limits fish stocks decline and
catch per unit of effort begins to fall. Fisheries scientists
regularly talk in terms of maximum sustainable yields,
although exact figures are difficult to ascertain and may
be highly variable over space and time. A more cautious
statistic, which is probably more appropriate for the
management of poorly understood coral reef fisheries, is
that of maximum economic yield. This is defined as that
yield which gives the highest possible economic return for
the effort expended. This represents a lower total catch
for the simple reason that catch per unit of effort begins to
decline as fish stocks begin to be impacted and before the
maximum sustainable yield is reached.
Worldwide, reef fish stocks are regularly harvested
Pollution
The major form of pollution on coral reefs is nutrient
enrichment, primarily linked to human waste, but also to
agricultural runoff. Recent decades have seen burgeoning
human populations in coastal areas close to reefs, and rapid
urbanization of many societies. Many countries, particularly
the poorer nations, have failed to develop sewage treatment
systems able to cope with growing urban areas, and vast
amounts of sewage enter coastal zones through the drainage
system or are directly piped into nearshore waters. Even
away from urban centers, coastal development has occurred
at a considerable pace. Tourism in particular has led to this
kind of development. While it is smaller scale, it actively
seeks out sites immediately adjacent to the coastline, and
in many cases there is again little or no sewage treatment. In
addition to sewage pollution, agricultural development in
many countries has entailed the ennchment of the land with
nutrients in the form of artificial fertilizers, many of which
enter the drainage system and are carried to coastal waters.
The impacts of eutrophication on reefs are complex.
Typically there are changes in the community structure.
Algae flourish in situations of high nutrient loads, and
can overgrow and kill corals or prevent new settlement of
coral larvae. Algal blooms are also sometimes observed in
the water column, with the further effect of reducing light
levels for the corals below. Among the rest of the reef
community, particle-feeding organisms such as sponges
and zoanthids may become more common, and planktoneating and herbivorous fish may increase. More subtle
changes, such as a reduction in the reproductive and
growth capacity of particular species may also result, but
may not be immediately apparent. These changes may
reduce the reef's resilience to further changes or its ability
to recover from events such as hurricanes. They may also
affect the sustainable yield of reef fisheries. In extreme
conditions, nutrient pollution alone is sufficient to kill a
reef and lead to its replacement by algal communities.
Toxic pollutants receive less attention and have been
less well studied. Oil is perhaps the most common pollutant in many remote areas. Spills have occurred widely
in coral reef regions. Continuous chronic oil pollution
results from minor spills or deliberate discharges of oil.
These occur in oil drilling areas such as the Arabian Gulf,
and in certain localities such as the Straits of Malacca, the
Straits of Hormuz, the Gulf of Aden and the approaches to
the Panama Canal, where vessels often discharge ballast
water or clean out their tanks. Other toxic chemicals are
released by a large range of industries, including tailings
from mining. The effects of oil pollution on corals are
varied. In some cases polluted reefs have shown slightly
higher mortality rates than unaffected reefs, and the
reproductive potential of surviving corals is significantly
reduced. A major spill in Panama led to a decrease in total
coral cover of 50-75 percent in shallow water reefs.
Sedimentation
Reef corals are highly sensitive to the impacts of sediments.
As sediments hang suspended in the water column they
block light from the reef below, preventing the growth or
even survival of corals, particularly in deeper waters. As
the sediments settle they may smother corals. While corals
are able to remove sediments, secreting mucus and then
sloughing this off into the water, such an activity uses
energy and nutrients. In the long term the smothering effects
of sediments weakens many corals, reducing growth or
reproductive potential and leaving them less able to compete
with other benthic organisms such as algae or filter feeders.
Even as sediments settle they create a surface which is
inimical to new coral settlement and growth, while light
sediments are often resuspended by wave action and hence
may remain in the system for considerable lengths of time.
Increasing levels of sediment in coastal environments
in recent times can be linked to coastal development,
dredging, land reclamation, and also distant terrestrial
activities, including deforestation and poor agricultural
practices. In many cases the intTuences of sedimentation
and pollution are combined, and the differential impact of
each is somewhat difficult to determine. Mine tailings
in Papua New Guinea are reported to have degraded or
destroyed significant areas of reef around Bougainville, an
area of little or no nutrient impacts. However, even here
these impacts may have been exacerbated by toxic compounds present in the tailings themselves.
Unsustainable fishing
Reefs are highly productive ecosystems. It is possible to
harvest some of this productivity for human consumption,
without compromising the overall functioning of the reef
ecosystem, and humans have utilized and depended on
reefs in this way for millennia. For many traditional cultures the relationship between local populations and their
adjacent coral reefs can best be seen as an ecological one,
where humans play a role as a part of the reef ecosystem.
Just as with other fisheries, there are limits to this
utilization. Beyond these limits fish stocks decline and
catch per unit of effort begins to fall. Fisheries scientists
regularly talk in terms of maximum sustainable yields,
although exact figures are difficult to ascertain and may
be highly variable over space and time. A more cautious
statistic, which is probably more appropriate for the
management of poorly understood coral reef fisheries, is
that of maximum economic yield. This is defined as that
yield which gives the highest possible economic return for
the effort expended. This represents a lower total catch
for the simple reason that catch per unit of effort begins to
decline as fish stocks begin to be impacted and before the
maximum sustainable yield is reached.
Worldwide, reef fish stocks are regularly harvested