was in the form of fragile branching corals, small foraminifera or other easily transportable sediment components.
Some efficiency in sediment movement may be lost as
the new reef top becomes rougher with new vertical relief
in the form of coral spurs, storm rubble, etc. (Kench and
Brander, 2006). Generally shallow lagoons are likely once
more to become effective sediment sinks. In some circumstances, there may be a surplus that can contribute to
island construction.
Reef island responses
Initial climate change predictions described dire consequences for reef islands, as contours representing extreme
high tide level were merely moved upward on island
shores. However, researchers working directly on the
physical processes on reefs (see Hopley et al., 2007 for discussion) generally agree that rising sea levels of the magnitude projected over the next 100 years will be far less
damaging and will produce substantial reworking of surficial sediments enough to at least maintain the island mass.
Island stability will be aided by a retaining effect of beach
rock and other cemented materials. Additional transported
sediments and higher water levels will allow cyclonic
storms to more effectively emplace shingle ridges to motus,
a very important part of the island building processes (e.g.,
Bayliss-Smith, 1988; Maragos et al., 1973). Sediment
movement will also occur on a more regular basis. Kench
and Brander’s (2006) research on mesotidal Australian
reefs indicated that sediment movement is currently
restricted to less than 50% of the time. On Warraber Reef
in Torres Strait (and other Indo-Pacific reefs) effective sediment movement ceased about 2,000 years ago but a rise in
sea level of only 0.5 m may unlock stored sediments which
will be moved towards an adjacent cay or motu.
Similar responses have been modeled on Raine Island
(northern GBR) by Gourlay and Hacker (1996). They
indicated that the height of the beach berm is determined
by the run-up height of the dominant wave action – at
highest Spring Tide. A berm elevation of 4 m could be
built by small flat waves of 0.5 m height breaking directly
onto a beach at a tide level as low as 2.3 m. They showed
that a small rise in sea level without any corresponding
build up of reef flat level would result in the attainment
of greater berm height under most weather conditions,
i.e., build-up of the island by an amount which would
exceed the amount of increase in water level. On Raine
Island they suggest that with a 0.6 m rise in sea level,
larger 1.6 m waves would increase berm height by a factor
of 0.8 m. Smaller waves of 0.5 m would increase the
height by 1.2 m, i.e., berm height would increase from
an initial 4.0 m to up to 5.2 m.
Whilst the island mass may remain as a habitat for
plants and animals it will be highly disturbed. Deposition
on one part of the shore will be matched by erosion elsewhere as both prevailing wind directions and storm frequencies add to the impacts of sea level rise. Reworking
of older parts of the island may remove a high proportion
of the mature organic soils and vegetation which form the
most favorable habitat including as agricultural land for
human populations. For migratory species such as seabirds and turtles, the reworked island may for a time, still
provide a valuable resource but for permanent residents,
coral reef cays may quickly become uninhabitable.
Other factors influencing reef flat change
The start of this entry presented the optimistic view that
coral reefs may be one of the few ecosystems to benefit
from sea level rise. The discussion that followed
suggested the optimism may be well founded in the light
of renewed upward growth, changed wave action and
increased sediment movement. However, sea level rise is
not the only factor in climate change. Whilst increased
cyclone activity may help add new ridges to reef islands,
other environmental changes such as temperature rise
and ocean acidification will have detrimental affects
resulting in widespread coral mortality.
Sheppard et al. (2005) have described the results of
coral bleaching in reefs of the Seychelles. Prior to the
1998 event fringing reefs had an outer veneer of 0.5 m
high thickets of staghorn corals and massive Porites.
Stripping of this veneer reduced reef roughness and created a pseudo sea level rise that was superimposed over
a regional rise of ca. 5 mm/year. Sheppard et al. modeled
the reef flat conditions of 1994, 2004 and 2014 concluding
that the rate of erosion of Seychelles beaches would double in the next 10 years.
These figures are of great concern for many of the
world’s reefs subjected to rising sea level. However, reef
disintegration as described for the Seychelles may not be
nearly as rapid elsewhere, determined largely by the composition of the reef flat corals. Reef flat surfaces subject to
a small fall in sea level remain within the energetic wave
zone as well as the intertidal zone of highly active
bioerosion. Where branching corals dominate the reef flat,
stripping and pseudo-sea level rise may impact on the
adjacent shore, as in the Seychelles. However, where massive or encrusting corals are dominant even though they
may die in response to environmental change, they
may not be removed, but stay in situ on the reef flat for
5,000 years or more, as illustrated by Hayman Island in
the central GBR (Kan et al. 1997). Here a pseudo sea level
rise is not superimposed on regional sea level behavior.
Summary
Responses of reef flats to sea level rise in the twenty-first
century will likely be many and varied. The response in
each case will depend on specific reef characteristics and
environmental setting: the existing height of the reef relative
to sea level; the local tidal range and rate of sea-level rise;
the existing composition and zonation of coral assemblages,
and hence susceptibility to the exacerbating effect of pseudo
sea-level rise; the changing ecology with changed environmental conditions and disturbance regimes; and propensity
for old and new sediments to be transported by wave action.
CLIMATE CHANGE: IMPACT OF SEA LEVEL RISE ON REEF FLAT ZONATION AND PRODUCTIVITY
213
Some efficiency in sediment movement may be lost as
the new reef top becomes rougher with new vertical relief
in the form of coral spurs, storm rubble, etc. (Kench and
Brander, 2006). Generally shallow lagoons are likely once
more to become effective sediment sinks. In some circumstances, there may be a surplus that can contribute to
island construction.
Reef island responses
Initial climate change predictions described dire consequences for reef islands, as contours representing extreme
high tide level were merely moved upward on island
shores. However, researchers working directly on the
physical processes on reefs (see Hopley et al., 2007 for discussion) generally agree that rising sea levels of the magnitude projected over the next 100 years will be far less
damaging and will produce substantial reworking of surficial sediments enough to at least maintain the island mass.
Island stability will be aided by a retaining effect of beach
rock and other cemented materials. Additional transported
sediments and higher water levels will allow cyclonic
storms to more effectively emplace shingle ridges to motus,
a very important part of the island building processes (e.g.,
Bayliss-Smith, 1988; Maragos et al., 1973). Sediment
movement will also occur on a more regular basis. Kench
and Brander’s (2006) research on mesotidal Australian
reefs indicated that sediment movement is currently
restricted to less than 50% of the time. On Warraber Reef
in Torres Strait (and other Indo-Pacific reefs) effective sediment movement ceased about 2,000 years ago but a rise in
sea level of only 0.5 m may unlock stored sediments which
will be moved towards an adjacent cay or motu.
Similar responses have been modeled on Raine Island
(northern GBR) by Gourlay and Hacker (1996). They
indicated that the height of the beach berm is determined
by the run-up height of the dominant wave action – at
highest Spring Tide. A berm elevation of 4 m could be
built by small flat waves of 0.5 m height breaking directly
onto a beach at a tide level as low as 2.3 m. They showed
that a small rise in sea level without any corresponding
build up of reef flat level would result in the attainment
of greater berm height under most weather conditions,
i.e., build-up of the island by an amount which would
exceed the amount of increase in water level. On Raine
Island they suggest that with a 0.6 m rise in sea level,
larger 1.6 m waves would increase berm height by a factor
of 0.8 m. Smaller waves of 0.5 m would increase the
height by 1.2 m, i.e., berm height would increase from
an initial 4.0 m to up to 5.2 m.
Whilst the island mass may remain as a habitat for
plants and animals it will be highly disturbed. Deposition
on one part of the shore will be matched by erosion elsewhere as both prevailing wind directions and storm frequencies add to the impacts of sea level rise. Reworking
of older parts of the island may remove a high proportion
of the mature organic soils and vegetation which form the
most favorable habitat including as agricultural land for
human populations. For migratory species such as seabirds and turtles, the reworked island may for a time, still
provide a valuable resource but for permanent residents,
coral reef cays may quickly become uninhabitable.
Other factors influencing reef flat change
The start of this entry presented the optimistic view that
coral reefs may be one of the few ecosystems to benefit
from sea level rise. The discussion that followed
suggested the optimism may be well founded in the light
of renewed upward growth, changed wave action and
increased sediment movement. However, sea level rise is
not the only factor in climate change. Whilst increased
cyclone activity may help add new ridges to reef islands,
other environmental changes such as temperature rise
and ocean acidification will have detrimental affects
resulting in widespread coral mortality.
Sheppard et al. (2005) have described the results of
coral bleaching in reefs of the Seychelles. Prior to the
1998 event fringing reefs had an outer veneer of 0.5 m
high thickets of staghorn corals and massive Porites.
Stripping of this veneer reduced reef roughness and created a pseudo sea level rise that was superimposed over
a regional rise of ca. 5 mm/year. Sheppard et al. modeled
the reef flat conditions of 1994, 2004 and 2014 concluding
that the rate of erosion of Seychelles beaches would double in the next 10 years.
These figures are of great concern for many of the
world’s reefs subjected to rising sea level. However, reef
disintegration as described for the Seychelles may not be
nearly as rapid elsewhere, determined largely by the composition of the reef flat corals. Reef flat surfaces subject to
a small fall in sea level remain within the energetic wave
zone as well as the intertidal zone of highly active
bioerosion. Where branching corals dominate the reef flat,
stripping and pseudo-sea level rise may impact on the
adjacent shore, as in the Seychelles. However, where massive or encrusting corals are dominant even though they
may die in response to environmental change, they
may not be removed, but stay in situ on the reef flat for
5,000 years or more, as illustrated by Hayman Island in
the central GBR (Kan et al. 1997). Here a pseudo sea level
rise is not superimposed on regional sea level behavior.
Summary
Responses of reef flats to sea level rise in the twenty-first
century will likely be many and varied. The response in
each case will depend on specific reef characteristics and
environmental setting: the existing height of the reef relative
to sea level; the local tidal range and rate of sea-level rise;
the existing composition and zonation of coral assemblages,
and hence susceptibility to the exacerbating effect of pseudo
sea-level rise; the changing ecology with changed environmental conditions and disturbance regimes; and propensity
for old and new sediments to be transported by wave action.
CLIMATE CHANGE: IMPACT OF SEA LEVEL RISE ON REEF FLAT ZONATION AND PRODUCTIVITY
213